Plasma doi: 10.3390/plasma9030037

Authors: Oleksandr Galmiz Bernard Gitura Kimani Mária Peťková Styliani Roufou Vasilis Valdramidis Zdenko Machala

Biofilm formation on material surfaces represents a major challenge in many technological and biomedical applications, particularly in confined geometries such as tubes and channels. In this study, the antibacterial performance of a surface dielectric barrier discharge system with liquid electrodes was investigated to develop a plasma-assisted approach for inactivating bacterial biofilms on internal surfaces. The discharge operated along the inner surface of the dielectric tubes as it was gradually filled with water. This configuration created a dynamic plasma–liquid environment in which plasma filaments propagated along the treated surface, while reactive species were produced simultaneously in both the gas and liquid phases. The formation of reactive oxygen and nitrogen species in plasma-treated water was characterized, and its antimicrobial activity was evaluated against planktonic Escherichia coli CCM 3954. The effectiveness of both plasma-activated water and direct plasma exposure in inactivating E. coli biofilms grown on the inner surfaces of polymer tubes was also investigated. The results demonstrate that the investigated surface dielectric barrier discharge configuration enables simultaneous direct plasma treatment and in situ generation of plasma-activated water. This approach shows promise for plasma-assisted decontamination of tubular materials and confined systems, where conventional disinfection methods may be limited.

Plasma doi: 10.3390/plasma9030036

Authors: Jiaxinyi Huang Zhuo Liu Aiguo Tan

Experimental measurements and two-dimensional axisymmetric fluid simulations are performed to study coaxial argon dielectric barrier discharge. Oscilloscope measurements capture voltage–charge waveforms and Lissajous figures to resolve cycle-integrated electrical characteristics. Continuous-integration optical emission spectroscopy without phase resolution is used to qualitatively verify metastable argon. As peak voltage rises from 5 kV to 13 kV, the simulated volume-averaged electron temperature displays a pronounced N-shaped trend: it peaks at 9 kV, falls abnormally between 10 kV and 11 kV, and recovers at higher voltages. This non-monotonic variation arises from intra-cycle self-shielding by dielectric surface charges together with power-broadening driven by discharge spatial expansion. Monotonically increasing equivalent capacitance confirms continuous surface charge accumulation, and the simulated inward shift of the high-electron-temperature region validates the emergence of surface-charge-induced reverse electric fields. The discharge maintains a steady filamentary regime across all tested conditions. Because microdischarge filaments occupy only a small portion of the gap, the volume-averaged electron density from simulations is far lower than the peak density within individual streamers. This work elucidates the mechanism underlying the anomalous electron temperature drop at moderate voltages and offers guidance for controlling atmospheric-pressure filamentary DBD.

Plasma doi: 10.3390/plasma9030035

Authors: Yang Liu Ruizhi Zhang Xi Chen Xinpei Lu Lanlan Nie

This study proposes a self-condensing water-electrode plasma device in which ambient water vapor is condensed on a cooled needle-tip electrode within a strong electric-field region, forming continuously renewed water droplets, Taylor cones, or water-film interfaces that participate in the discharge process. The effects of ambient humidity, needle-tip temperature, and applied voltage on the self-condensation behavior were investigated. The discharge modes at different applied voltages and discharge gaps, the generation of reactive species, and the inactivation efficacy against Staphylococcus aureus were also analyzed. The results showed that increasing ambient humidity, decreasing the needle-tip temperature, and increasing the applied voltage all promoted water condensation at the needle tip. The electric field shortened the droplet formation time and reduced the droplet detachment size. As the applied voltage increased, the device sequentially underwent water condensation, electrospray, stable Taylor-cone, water-film discharge, and bare-electrode discharge stages, while the boundary voltage of each stage increased with the discharge gap. The ozone concentration remained below the instrument’s limit of detection during discharge with the self-condensing water electrode, whereas hydroxyl-radical generation was significantly enhanced. Compared with a conventional metal electrode, the self-condensing water electrode exhibited greater inactivation of S. aureus at the same applied voltage, with a more pronounced advantage under low-voltage conditions. These findings demonstrate that introducing a self-condensing water interface can regulate the local discharge morphology and reactive-species composition, providing a new strategy for developing low-temperature plasma sterilization technologies with low ozone production and high bactericidal activity.

Plasma doi: 10.3390/plasma9030034

Authors: Pierpaolo Iovane Sabrina Portofino Carmela Borriello Giuseppe Pandolfi Anna De Girolamo Del Mauro Nicola Fedele Sergio Galvagno

Titanium alloy Grade 5 (Ti6Al4V) is a widely used material for aerospace components and biomedical implants due to its excellent combination of strength, low density, corrosion resistance and biocompatibility. Additive manufacturing (AM) technologies, such as Laser Powder Bed Fusion (LPBF) and Electron Beam Melting (EBM), could contribute to the manufacturing of high-performance Ti6Al4V components with complex geometries at reduced waste material and costs, but require powders with controlled morphology and flowability. Plasma Spheroidization is a key process for producing spherical titanium alloy powders suitable for AM; however, Direct Current (DC) thermal plasma is generally optimized for fine particle sizes. This study investigated the spheroidization of irregular Ti6Al4V powders (45–106 µm) through the optimization of DC thermal plasma processing parameters with the aim of extending its applicability to coarser powders. Particular attention was dedicated to particle size classification before and after plasma treatment. Pre-treatment sieving was found to be the most effective strategy for enhancing spheroidization and powder quality, while post-treatment classification improved the homogeneity of the final product. The resulting products were characterized by X-ray diffraction, morphological analysis, and flowability testing. High circularity powders (>0.7) with improved flowability (<30 s/50 g) were successfully obtained. The results demonstrate the potential of DC thermal plasma processing for producing coarser Ti6Al4V powders suitable for AM applications.

Plasma doi: 10.3390/plasma9030033

Authors: Giuseppe A. Rattá Boudewijn Ph. van Milligen Víctor Ángel Fuentes Blas Mauricio Samper Mauro Jurado Alejandro González-Ganzábal The TJ-II Curated Database Team The TJ-II Curated Database Team

In the TJ-II stellarator, a finite net toroidal plasma current can modify the rotational transform profile and shift the radial position of low-order rational surfaces. Because these surfaces can influence edge gradients, turbulence and confinement in low-shear configurations, estimating the current waveform is relevant for the preparation and interpretation of scenarios programmed with the ohmic heating (OH) coil. This work compares two data-driven estimators using 87 selected OH-programmed discharges from the fixed magnetic configuration 100_44_64, divided into 50 training, 20 validation and 17 test cases. The first estimator uses MultiGene Genetic Programming (MGGP) to obtain explicit equations. The second is a nonlinear autoregressive recurrent neural network with exogenous inputs (NARX-RNN), implemented with long short-term memory (LSTM) branches. The symbolic equations and their settings were selected using the training and validation discharges, while the test set was used only for final evaluation. On the original 17-discharge test partition, the general OH-normalized equation achieved an overall root-mean-square error (RMSE) of 0.4898 kA, whereas the compact equation achieved 0.5700 kA with substantially lower expression complexity. After routine signal quality control, two test records were excluded, and the final comparison used 15 discharges. On this set, the mean discharge-wise RMSE was 0.350 kA for the NARX-RNN, 0.515 kA for the general symbolic equation and 0.576 kA for the compact equation. The NARX-RNN also gave a median RMSE of 0.230 kA. Whether this current error level is sufficient for positioning a particular rational surface depends on the local rotational transform response and magnetic shear, so no universal current error threshold for edge control is assigned here. The symbolic models were less accurate on average but provided explicit relations involving OH amplitude, previous current information, electron cyclotron resonance heating, fuelling and wall-conditioning variables.

Plasma doi: 10.3390/plasma9030032

Authors: Tatjana Maravic Roberto Montalbetti Tijana Lainovic Diego D’Urso Claudia Mazzitelli Uros Josic Vittorio Checchi Romolo Laurita Matteo Gherardi Lorenzo Breschi

Endogenous matrix metalloproteinases (MMPs) are activated in dentin during carious lesion progression and restorative procedures, degrading the tooth-restoration interface and contributing to restoration failure. This study investigated by means of in situ zymography whether cold atmospheric plasma activation (PA) of distilled water (DW) and phosphate-buffered saline (PBS) modulates endogenous dentinal MMP activity. A Dielectric Barrier Discharge-rod source generated PA liquids, treating DW and PBS for 22 min. Chemical characterization demonstrated that PADW yielded 9.61 mg/L H2O2, 18.3 mg/L NO2−, 375.70 mg/L NO3−, and a pH of 3.2. PAPBS yielded 9.79 mg/L H2O2, 36.01 mg/L NO2−, 505.74 mg/L NO3−, and a pH of 7.13. Both liquids served as 1 min dentin pretreatments in a simulated restorative procedure using a universal adhesive and resin composite, tested after 24 h. MMP activity was assessed via in situ zymography with fluorescein-conjugated gelatin and confocal microscopy. Data were statistically analyzed (p < 0.05). PADW increased dentinal enzymatic activity, while PAPBS reduced it (p < 0.05). Non-activated PBS elicited higher baseline MMP activity than non-activated DW. The divergent responses likely reflect differences in RONS composition, pH, and initial ionic content between the two liquids. The precise mechanism underlying PA liquid interactions with dentinal MMPs warrants further investigation.

Plasma doi: 10.3390/plasma9030031

Authors: Songlin Liu Lianjun Shi Guilan Liu Wei Xiao

The conversion of carbon dioxide (CO2) driven by microwave plasma has garnered extensive attention due to its capability to recycle carbon resources and mitigate the greenhouse effect. However, the existing microwave plasma technologies suffer from cumbersome system setups and relatively low energy efficiency. This work investigates CO2 conversion using a portable atmospheric microwave plasma source, which requires low plasma-generating power. When the working gas is a mixture of CO2 and Ar, the minimum plasma-generating power required for the proposed portable microwave plasma source is only 50 W. A Fourier transform infrared spectrometer is used to quantify both the CO2 and CO concentrations, and the C2 Swan bands in the plasma are measured to determine the gas temperature. The maximum CO2 conversion rate is approximately 12.7%. Furthermore, the maximum energy efficiency reaches 87.9%, which exceeds that of most state-of-the-art atmospheric microwave plasma configurations. Owing to its simplicity of operation, portability, and high energy efficiency, this method is well-suited for distributed CO2 conversion systems.

Plasma doi: 10.3390/plasma9030030

Authors: Abdulaziz H. Alharbi Ibrahem S. Elkamash

Arbitrary-amplitude nonlinear periodic electrostatic waves and the wave-breaking limit are considered in a one-dimensional relativistic electronegative plasma consisting of positive ions, negative ions, and a relativistically degenerate electron-fluid background. A cold, inertial fluid description is adopted for the ions, while the electrons are described by a relativistic Fermi–Dirac equation of state, which provides the required restoring physics through degeneracy rather than ordinary thermal pressure. After transforming to a travelling coordinate system, we reduce the general multicomponent plasma dynamical system to a pseudopotential energy form with a constant of motion. We then determine the allowed potential range, the associated asymmetric pseudopotentials, and the wave-breaking electric field for both linear and nonlinear waves on each of the two admissible branches. We find that arbitrary-amplitude nonlinear periodic waves are intrinsically asymmetric and that the maximum field strength is set by the effective charge-density boundary of each plasma species along the field direction. Parametric analysis suggests that the critical minimum field strength required for nonlinear wave formation may be controlled by increasing either the negative-ion mass ratio, the wave propagation speed, or the negative-ion concentration; however, each of these changes produces a distinct modification of the pseudopotential geometry. This illustrates that relativistic and interspecies effects are not merely responsible for quantitative deviations from the non-relativistic and single-species cases, but instead completely reshape the nonlinear phase space governing the persistence, deformation, and breaking of periodic electrostatic waves.

Plasma doi: 10.3390/plasma9030029

Authors: Rut Morales Crespo Encarnación Muñoz Serrano María Simón Lora

This article analyses the transition from the space-charge-limited (SCL) regime to the inverse sheath regime of a thermionic emissive probe, considering ionisation and collisions as presheath mechanisms. We show that, before the fully developed inverse sheath mode is reached, an intermediate regime appears, characterised by the formation of a virtual cathode structure with ion confinement and probe potentials above the plasma potential. The inverse sheath mode is reached when the minimum potential of the virtual cathode reaches the plasma potential. At this point, a monotonic sheath potential profile above the plasma potential is produced, together with a flat, non-accelerating presheath. The findings of this work can help optimise the performance of emissive probes in plasma diagnostics and control plasma–surface interactions in devices such as fusion reactors and electric propulsion systems, thereby mitigating sputtering and material erosion.

Plasma doi: 10.3390/plasma9030028

Authors: Odutayo R. Rufai Ayooluwa O. Odufowora

We examine energetic electron distributions in the region of the diffuse aurora using a two-dimensional bi-Maxwellian model, applied to electron flux data at a single geomagnetic equator (L = 6.5). Electron flux is analyzed as a function of energy and pitch angle and transformed into velocity space to reconstruct the distribution function. An unweighted log-space least-squares fit of the bi-Maxwellian model to the reconstructed distribution yields a reduced residual measure of χv2=1.000053, a mean absolute residual of |Δlog10f|=0.0469 dex, an anisotropy factor, AT=0.9886±0.0016, and no statistically significant bulk drift. These results show that, at this location, the bi-Maxwellian model reproduces the observed velocity-space structure with good quantitative accuracy and reveals a quasi-isotropic, near-equilibrium electron population.

Plasma doi: 10.3390/plasma9030027

Authors: Lingyun Yu Alice Martinet Linus Hübner Lars Boeckmann Steffen Emmert Sander Bekeschus

Reactive oxygen and nitrogen species (RONS) generated by medical gas plasmas are considered major mediators of plasma-induced biological effects. This includes the atmospheric pressure argon plasma jet kINPen routinely used in clinical applications. The jet’s biomedical action has been shown to be tailored by modifying its feed gas. However, a systematic comparison of how feed gas composition and humidity shape plasma chemistry remains lacking, which would shift application-specific plasma chemistries from guessing to designing. In this study, we systematically investigated, compared, and statistically related 65 individual feed gas conditions of the kINPen argon plasma jet by increasing O2, N2, and combined O2 + N2 admixtures under dry and humidified conditions. Plasma gas phases were assessed using optical emission spectroscopy and reactive species produced in liquid via hydrogen peroxide, nitrite, and nitrate quantification. O2-containing admixtures generally reduced overall plasma emission and liquid-phase RONS accumulation, whereas N2-containing admixtures preferentially enhanced nitrogen-associated emission features. Water vapor addition via the admixture gas stream acted as an important secondary tuning parameter, exerting the strongest effects under combined O2 + N2 conditions. Multivariate analyses confirmed clear separation of chemistry profiles according to feed gas composition and humidity, while correlation and regression analyses identified several condition-dependent relationships between gas-phase emissions and liquid-phase reaction products. These data provide a comprehensive characterization of kINPen plasma chemistry under controlled feed gas modification and establish a reference framework for tailoring plasma-derived reactive species profiles in future plasma biology and medicine studies.

Plasma doi: 10.3390/plasma9030026

Authors: Alexander S. Metel Marina A. Volosova Enver S. Mustafaev Yury A. Melnik Sergey N. Grigoriev

Expanding the operating pressure range of hollow cathode glow discharge to the region of 0.01–0.1 Pa makes it possible to use the discharge plasma in a number of technological processes that were previously not feasible, because pressure always exceeded p = 1 Pa. Implantation of nitrogen ions by means of application of 40 kV pulses to a steel workpiece immersed in plasma at p < 0.1 Pa allows production of a 40 µm thick surface layer with hardness of 13 GPa exceeding by 6.5 times the hardness of the bulk and a decrease in the processing time by an order of magnitude. Nitriding a steel workpiece at p = 0.1 Pa allows a substantial increase in the nitriding rate. The use of a titanium workpiece as a discharge anode with surface area not exceeding a critical value allows it to melt due to heating to the melting point of 1670 °C by electrons accelerated in the positive anode fall of potential.

Plasma doi: 10.3390/plasma9030025

Authors: Henrik Wollner Stanislawa Hausmann Gisela Ohms

Carbon fiber reinforced epoxy resin composites were manufactured using the vacuum infusion technique. Composite samples were subjected to impact and then exposed to various weathering conditions. Static and dynamic mechanical tests were performed to evaluate the effect of an additional process step, a plasma treatment of the carbon fiber fabric, before the composite is manufactured. Scanning electron microscopy and thermal analysis were used to get further information on the degree of damage after weathering. Treating the reinforcing carbon fibers with air plasma resulted in improved strength values and fatigue behavior of the epoxy resin composite. This performance enhancement persisted even after low-energy mechanical stress and subsequent weathering.

Plasma doi: 10.3390/plasma9030024

Authors: Dariusz Korzec Florian Hoppenthaler Simona Lerach

Among numerous atmospheric-pressure plasma jets (APPJs), high industrial acceptability has been reached for the ones based on high-voltage, low-current, vortex-stabilized arc, typically operated with kHz DC-pulses. This review explores the interrelations between the “process” in a chemical–physical sense and “process”, or to better differentiate, “processing” in the sense of technological treatment, with respect to such APPJs. The mutual dependence of the processing requirements (e.g., high processing speed, compatibility with robotic processing, low total cost of ownership, reliability, and long service intervals) and the physical and chemical processes in the plasma jet are analyzed. The focus is on the hybrid character of the produced plasma, comprising a non-equilibrium arc and a diffuse plasma. Different operation modes of the gliding arc discharge (GAD) are discussed. The reviewed chemical processes are the generation of reactive oxygen–nitrogen species (RONS), oxidation and reduction reactions, and interactions with vapors, solids, and liquids. The considered processing examples are established applications, such as surface activation, cleaning, oxide reduction, film removal, and coating, as well as emerging applications for sterilization and plasma-activated water (PAW) production.

Plasma doi: 10.3390/plasma9030023

Authors: Lou-Chuang Lee Kun-Han Lee Hau-Kun Jhuang Dong-Dong Ni

This paper presents a conducting-channel model aimed at elucidating the generation of high-energy particles within a plasma chamber. Initially, the chamber is charged with neutral hydrogen gas at a density of approximately ~3.3 × 1022/m3, equivalent to 1 torr at 300 K under ideal gas conditions. A Townsend discharge (dark discharge), driven by an externally imposed electric potential (500–1000 V) across the cathode and anode, is utilized to induce partial ionization of the hydrogen gas. Once a stable conducting channel with a high conductivity is established, a low electric potential (e.g., 100–500 V) is introduced to sustain the current in the conducting channel. Our investigation then delves into the impact of a high-emissivity cathode, such as lanthanum hexaboride (LaB6), on an arc discharge. We develop a theoretical model of the conducting channel that may emerge under these conditions. As the cathode surface heats, thermionic electrons form a localized layer of negative charge density outside the cathode, leading to an electric potential dip. Our multi-fluid simulations reveal the emergence of an electron-ion two-stream instability owing to the high-density electron layer, leading to the appearance of multiple potential peaks and dips, each measuring several to tens of kV. We delineate a set of conditions conducive to the formation of these potential peaks and dips within the conducting channel. Our proposed scenario furnishes a framework for elucidating electron and ion acceleration within a weakly ionized plasma chamber.

Plasma doi: 10.3390/plasma9020022

Authors: Nian Tang Hanyue Zhao Dongwei Sun

With increasingly stringent restrictions on SF6 greenhouse gas emissions, C4F7N-based gas mixtures have attracted considerable attention as promising alternatives for high-voltage circuit breakers; however, their relatively weaker arc-quenching capability poses significant challenges for interruption chamber design at high voltage levels. In this study, a 3.5% C4F7N/83.5% CO2/13% O2 gas mixture was used as the arc-extinguishing medium in a 550 kV environmentally friendly gas circuit breaker. Based on a magnetohydrodynamic (MHD) model considering PTFE nozzle ablation effects, systematic optimization studies were conducted on key structural parameters of the puffer-type interruption chamber, including the exhaust hole diameter, nozzle throat diameter and length, arcing contact diameter, and downstream expansion angle. Simulations under arcing times of 9.9 ms and 11.4 ms were performed to evaluate chamber pressure, axial temperature, extinction peak voltage, and post-arc conductance characteristics. The results indicate that extending the nozzle throat straight section to 70 mm, enlarging the exhaust hole, and increasing the moving contact radius can effectively enhance pressure buildup, reduce arc-core temperature, and improve dielectric recovery capability. Under the 11.4 ms arcing condition, the optimized structure achieved an extinction peak voltage of 6972.4 V and a G200 value of 0.731 ms, demonstrating substantially improved interruption performance. These findings reveal the synergistic relationship between arcing time and structural parameters and provide theoretical guidance for the engineering design of environmentally friendly high-voltage gas circuit breakers.

Plasma doi: 10.3390/plasma9020021

Authors: Satyendra Nath Barman Kingkar Talukdar

In this study, we investigated the existence and properties of solitons in an unmagnetized plasma composed of positive ions, negative ions, negatively charged dust grains, non-thermal electrons and non-extensive positrons. We have conducted our study on this complex plasma model because it moves away from simplistic and idealized plasma models. Also, a study of solitons has not previously been conducted on this complex plasma model. Through the Sagdeev potential method, we have derived the energy integral and investigated the variation in the Sagdeev potential for different values of the parameters that are involved in our plasma model. We have found that the non-thermal parameter (β) and the non-extensive parameter (q) significantly influence the features of the solitons. The features of the solitons are also found to be influenced by the Mach number (M), the negative-ion-to-positive-ion mass ratio (Ω), the positron-to-positive-ion density ratio (δp), the electron-to-positron temperature ratio (σp), the dust charge density ratio (δd) and the negative-ion-to-positive-ion density ratio (δ_). The results from our study can be useful in investigating plasma in astrophysical environments, such as cometary tails and interstellar clouds.

Plasma doi: 10.3390/plasma9020020

Authors: Fangcheng Qiu Xin Zhang Shuai Jiang Huilin Zhou Lin Wang Yufeng Song Jian Huang Xin Zheng Ronghai Liu Xuekai Pei

In the renewable energy-driven “green electricity–green hydrogen–green ammonia” pathway, the development of low-temperature and low-energy-consumption ammonia synthesis technologies is of great significance. In this work, a plasma-catalytic ammonia synthesis system was established using a coaxial dielectric barrier discharge (DBD) reactor. The effects of different catalysts, including Ag, Cu, γ-Al2O3, BaTiO3 and Co/BaTiO3, Ni/BaTiO3 on ammonia synthesis performance were systematically investigated. The reaction process was analyzed using voltage–current waveforms, Lissajous figures, and optical emission spectroscopy (OES). The results show that different catalytic systems have a significant influence on ammonia synthesis performance, with the promotional effect ranked as follows: Ni/BaTiO3 > Co/BaTiO3 > BaTiO3 > Ag > γ-Al2O3 > Cu. Among them, Ni/BaTiO3 exhibited the best performance. Under the conditions of N2:H2 = 1:1 and a gas flow rate of 2.5 L/min, the NH3 synthesis rate reached 259.48 μmol/min, and the maximum energy efficiency reached 1.40 g-NH3/kWh. Catalyst characterization results indicate that the BaTiO3 support maintained a stable crystal structure, while the loaded metal species were highly dispersed and uniformly distributed on the support surface, which is beneficial for the adsorption and conversion of reactive species on the catalyst surface. Discharge characteristic analysis shows that the introduction of BaTiO3 enhanced the local electric field and improved the uniformity of micro-discharges, while the further incorporation of metal active components strengthened the micro-discharge behavior. OES results reveal that the intensities of characteristic emission lines, such as NH, N2+, and Hα, were significantly enhanced in the Ni/BaTiO3 system, facilitating the formation and conversion of NHx intermediates. The superior performance of Ni/BaTiO3 is attributed to the coupling between BaTiO3-induced dielectric enhancement and Ni-promoted surface hydrogenation and NH3 desorption. This work provides mechanistic insight into catalyst-dependent DBD plasma-catalytic ammonia synthesis and offers an experimental basis for the further optimization of plasma-based ammonia production.

Plasma doi: 10.3390/plasma9020019

Authors: Chen Ma Li Yao Jialu Liu Feng He

In this work, based on a half-bridge circuit and pulse transformer, a miniaturized and low-cost microsecond high-voltage pulsed power supply for the atmospheric pressure plasma jet (APPJ) is designed. Because of the low bus voltage of the half-bridge circuit, low-voltage switches can be chosen by the power supply. The characteristics of the output high voltage of the power supply are studied. The experimental results show that uni-polar and bi-polar pulses can be generated by the power supply. The high-voltage pulses have good consistency at different frequencies, and the amplitude of the high-voltage pulse varies approximately linearly with the bus voltage. A needle-ring plasma jet device was driven by the uni-polar pulse of this supply, and the single discharge current pulse can be obtained at the rising edge and falling edge of the high-voltage pulse, respectively. The effects of voltage pulse on APPJ and the characteristics of jet are also investigated. The results show that the plasma jet is only formed at the rising edge of the voltage pulse. The jet length is almost unaffected by the pulse frequency, whereas the normalized intensity of most species increases with frequency linearly.

Plasma doi: 10.3390/plasma9020018

Authors: Vasily Kozhevnikov Andrey Kozyrev Dmitry Sorokin Victor Tarasenko Dmitry Beloplotov Eugene Baksht Mikhail Lomaev

This paper introduces a deep learning-based methodology for reconstructing particle beam energy spectra from experimental attenuation curves. This task involves solving a classic ill-posed inverse problem for a Fredholm integral equation of the first kind. Unlike traditional Arsenin–Tikhonov regularization, the proposed framework utilizes two coupled neural networks for spectrum approximation and adaptive kernel correction. This approach explicitly accounts for measurement uncertainties in the experimental data. As a mesh-free technique, it operates directly on raw sparse experimental datasets without preprocessing. Validation using data from subnanosecond electron beams in gas-filled and vacuum diodes demonstrates that the method successfully resolves non-trivial two-peak spectral structures. In particular, it reliably identifies populations of “anomalous” high-energy electrons that are often obscured by classical regularization artifacts.

Plasma doi: 10.3390/plasma9020017

Authors: Jeong-Hae Choi Seung-Ah Park Hyun-Young Lee Wonkyu Hong Jaehong Kim Jin-Woo Hong Gyoo-Cheon Kim

Recently, an anti-inflammatory effect of no-ozone cold plasma (NCP) has been reported, but the direct use of NCP for treating muscle inflammation is very difficult since NCP is a form of gas. In this study, we tested whether the anti-inflammatory effect of the NCP could be delivered through conductive metal needles to reduce muscle inflammation. C2C12 mouse muscle cells were treated with TNFα to induce muscle inflammation and then treated with NCP and a conductive metal needle separately or in combination. The effects of NCP and a needle were monitored by performing RT-PCR and Western blot analysis. As a result, NCP effectively suppressed the TNFα-mediated expression of the TNFα, IL1β, and FasL genes, but this effect weakened as the distance between the cells and the NCP increased. On the other hand, treatment of cells with a plasma-needle (PN) had an anti-inflammatory effect regardless of distance, and the anti-inflammatory effect of the PN was maintained under conditions where the gas flow of NCP was not delivered to the cells. It is believed that the PN-mediated activation of media plays a pivotal role in the anti-inflammatory effect of the PN. Finally, this study also showed that electroacupuncture can inhibit TNFα-induced inflammatory gene expression in a manner like a PN. Taken together, the results of this study demonstrate that the anti-inflammatory effect of NCP can be delivered through metal needles, suggesting that PN may be useful for treating inflammatory muscle pain.

Plasma doi: 10.3390/plasma9020016

Authors: Ke Wang Yuying Shi Bochen Li Yiheng Zhang Suoyun Yang Xianping Zhao

High-voltage self-blast circuit breakers feature complex gas flow field dynamics during the arc interruption process due to the multiple gas chambers and valves in the interrupter. The structure of key interrupter components and the characteristics of the operating mechanism significantly influence the gas flow field behavior, thereby affecting the breaking performance. The C4F7N gas mixture is currently the most promising alternative to SF6. However, the influence mechanisms of various factors on its breaking performance remain unclear, which limits the design of C4F7N-based self-blast interrupter chambers. This paper investigates the impact of nozzle throat length and mechanism stroke on the breaking performance of a 126 kV double-motion self-blast circuit breaker prototype by establishing a magnetohydrodynamic (MHD) arc model for C4F7N gas mixtures. The results indicate that a longer throat length can enhance the pressure-buildup capability in the expansion chamber to some extent, but its effect on short arcing times is limited, whereas it has a more pronounced influence on medium and long arcing times. However, it also impedes arc energy dissipation, potentially reducing the breaking capability for short and medium arcing times while improving performance for long arcing times. A larger mechanism stroke not only ensures a greater contact gap at current zero for long arcing times but also accelerates the gas flow velocity between the contacts, facilitating arc energy dissipation and enhancing the thermal interruption performance.

Plasma doi: 10.3390/plasma9020015

Authors: Luutzen Franciscus Ate Wymenga Jan van Turnhout Mohamad Ghaffarian Niasar Henk van Zeijl Willem Dirk van Driel Guoqi Zhang

Low-temperature atmospheric plasma (LTP) is widely used in industrial processes, such as disinfection, surface modification and wastewater treatment. The dielectric barrier discharge (DBD) is regarded as one of the most robust and reliable methods for generating LTP in ambient air. Compared to conventional AC excitation, pulsed powering offers several advantages (i.e., lower energy use and heat production). The present trend is to use short and fast pulses (in the nano- and picosecond range). In this review, the key design parameters of a DBD (barrier thickness, relative permittivity and gap distance) are discussed. Material-specific phenomena like surface charging and degradation are analyzed. The complex interactions between the pulse source and DBD are examined. By mapping the interdependencies, this review aims to support the rational design and optimization of pulsed DBD systems, and to facilitate their broader industrial use.

Plasma doi: 10.3390/plasma9020014

Authors: Mahmood Nasser

A hybrid DC–RF inductively coupled plasma (ICP) driven by a single-turn internal antenna was experimentally investigated to quantify magnetic confinement effects in low-pressure argon discharges. Superposition of a dc current on the RF antenna generated an azimuthal magnetic field that modified electron transport and reduced cross-field diffusion in the near-antenna region. Spatially resolved measurements of plasma density, electron temperature, plasma potential, and magnetic-field components were obtained using Langmuir, emissive, and B-dot probes. Increasing the dc current enhanced electron confinement and increased the plasma density by up to an order of magnitude at low RF power, together with improved radial and axial uniformity. A semi-empirical diffusion model incorporating electron-temperature-dependent ambipolar transport reproduced the measured ion-density distributions, ni(R,Z), within ±15%. The results support the interpretation that the discharge behaviour is governed by the coupled effects of localized magnetic confinement and inductive power deposition, and show that hybrid DC–RF excitation provides an effective route to denser and more spatially extended plasmas under low-pressure conditions.

Plasma doi: 10.3390/plasma9020013

Authors: Mohmmad Al-Dweikat Moath Bani Fayyad Hana Rababah Qirong Wu

Piezoelectric actuators enable ultra-fast switching due to their microsecond-scale response and high acceleration capability. This study experimentally investigates arc behavior in air, vacuum, and nitrogen using round and flat contacts driven by an amplified piezoelectric actuator. Unlike prior work focused mainly on actuation dynamics, this study provides a multi-medium comparison and investigates the coupled effects of drive operating time and contact geometry on arc characteristics. Arc tests were conducted using a capacitor discharge platform, with synchronized electrical measurements and high-speed imaging. In air (140 V, 350 A), arc voltage increased with rise time, reaching 800 V, 840 V, and 1080 V at 0.5 ms, 1 ms, and 2 ms, respectively, while shorter rise times reduced arc duration but promoted reignition. In vacuum (140–200 V), arc voltage stabilized at 80–90 V, with longer rise times extending arc duration; round contacts exhibited faster voltage rise and higher peaks. In nitrogen (140–200 V), higher voltages were obtained at shorter rise times, reaching 2680 V, 2600 V, and 2320 V at 0.5 ms, 1 ms, and 2 ms, respectively, with reduced arc duration. Across all media, round contacts consistently produced higher arc voltages than flat contacts. These results demonstrate that drive dynamics and contact geometry critically influence arc voltage and duration, providing practical guidelines for the design of high-speed piezoelectric-based switching devices.

Plasma doi: 10.3390/plasma9020012

Authors: Rareș Iovănescu Radu P. Daia Anana C. Gîrlea Emil I. Slușanschi Cătălin M. Ticoș

We investigate laser wakefield electron acceleration in a periodic plasma density profile using 2D PIC simulations with the EPOCH code. The profile of the electron density has the form n(x)=n01+δsin2πx/x0, where n0 is the steady electron density, x0=100m is the spatial periodicity in the laser propagation direction and δ, taking the values 0, 0.1, 0.3, 0.5 and 0.7, is the modulation parameter. The bubble size varies with the modulated plasma density, thereby influencing the electron acceleration, which occurs within a continuously changing bubble structure. We propose an analytical model to estimate the energies of the accelerated electrons, and evaluate the maximum electron energies at 500 fs intervals for the five modulated density profiles. We then calculate the dephasing and depletion lengths for these modulated plasma profiles and examine their dependence on δ. The results show a growth in both lengths with δ, with depletion being the main limitation in these cases. Additionally, we compute and compare the transverse emittance of the self-injected electron bunches corresponding to the various density profiles at the same simulation time, and other characteristics, like the center energy and energy spread. Emittance is observed to experience a decrease with the increase in the modulation parameter.

Plasma doi: 10.3390/plasma9020011

Authors: Vladislav Kotov Marc Sackers Oleksandr Marchuk Michael Reinhart Gennady Sergienko Arkadi Kreter Mauricio Gago Bernhard Unterberg Sebastijan Brezinsek

Spatially resolved absolute intensities of the atomic lines Hα, Hβ, Hγ, and Hδ have been measured and analyzed in pure hydrogen plasma in the linear plasma device PSI-2. Two regimes have been investigated, with nominal (0.04 Pa) and elevated (0.5 Pa) gas pressure in the sample chamber. The measurements have been compared with local 0D calculations taking into account radiation from H(n=1), H2, and H2+ channels. A baseline plasma chemical mechanism developed in magnetic fusion research was applied to calculate the H2+ density. Both the plasma chemical mechanism and the population factors applied are based on Sawada–Fujimoto collision-radiative model of atomic and molecular hydrogen. The calculations were found to reproduce both the absolute radiation and the line radiation intensity ratios measured in the 0.04 Pa experiment with electron temperature Te = 2–10 eV and electron density ∼5 × 1017 m−3. An exception is the Hα/Hγ intensity ratio, which tends to be overestimated by the model. The calculations suggest that the majority of the observed Balmer radiation in this regime is due to the H2+ channel. At the same time, both the applied simplified approach without detailed transport modeling and the baseline mechanism were found to be inappropriate for the 0.5 Pa experiment with reduced Te = 1–5 eV. This experimental regime can serve as a benchmark of more sophisticated hydrogen plasma models.

Plasma doi: 10.3390/plasma9020010

Authors: Mahmood Nasser Layla Nasser Fatima Makhlooq Batool Abulwahab Elias Naser

A spatially resolved investigation of bacterial inactivation using a radiofrequency (13.56 MHz) capacitively coupled plasma (RF CCP) discharge operating in ambient air at 4.0 mbar is presented. The plasma was generated in a parallel-plate reactor without external gas precursors and characterized using Langmuir probe diagnostics and optical emission spectroscopy (OES). Electron densities on the order of 109 cm3 were measured near the powered electrode, exhibiting pronounced axial and radial gradients across the discharge volume. OES revealed strong excitation of oxygen- and nitrogen-containing emitters, including O I (777 nm), N2 s positive system (337–380 nm), and N2+ first negative system features, with emission intensities increasing monotonically with applied RF power. The bactericidal performance was evaluated using Escherichia coli American Type Culture Collection (ATCC) 11775 exposed at different axial and radial positions within the reactor. At a fixed exposure time of 60 s, the log10 reduction increased nonlinearly with RF power, rising from 0.29 at 20 W to 0.81 at 40 W, followed by a sharp transition to the assay reporting ceiling (≥2.95-log10 under the adopted half-count correction) at 50 W and above. Time-resolved measurements at 50 W demonstrated rapid inactivation kinetics, with measurable reductions occurring within 5–10 s and reaching the reporting ceiling within 60 s. In contrast, samples positioned at the chamber periphery or approximately 20 cm from the discharge center exhibited negligible inactivation, confirming strong spatial localization of the biocidal effect. These results identify a threshold-like operating regime in which increased discharge intensity produces rapid inactivation in the plasma core while remaining strongly position dependent. The findings establish medium pressure, air-based RF CCP as an efficient, gas-free, and spatially controllable platform for localized surface decontamination under non-thermal conditions.

Plasma doi: 10.3390/plasma9020009

Authors: Dae-Won Ok Dae-Yeol Pyo Hong-Sik Yun Yong-Seok Hwang Yong-Su Na

Internal reconnection events (IREs) are rapid magnetohydrodynamic phenomena that play an important role in the confinement and stability of spherical tokamak plasmas. Reliable identification of IREs in experimental data is challenging due to short discharge durations, ambiguous event boundaries, and the limited availability of labeled data. In this study, we propose an unsupervised, event-level IRE detection framework based on anomaly detection techniques and apply it to experimental data from the VEST spherical tokamak. The proposed framework combines a two-stage detection strategy using plasma current and Hα emission signals with sliding-window segmentation and event-level evaluation, enabling physically meaningful IRE identification without labeled training data. Three unsupervised models—K-Nearest Neighbors (KNN), One-Class Support Vector Machine (OCSVM), and an autoencoder (AE)—are evaluated within a unified framework. All models achieve stable detection performance, with precision exceeding 80% and recall above 70% under a precision-oriented operating point. To enhance detection robustness, a KNN-based cleaning procedure is introduced during training to remove noise-driven, locally isolated windows, significantly reducing spurious detections while preserving physically meaningful IRE signatures. Event-level analysis indicates that missed detections under this operating regime predominantly correspond to weak events with limited impact on global plasma behavior. The proposed framework is fully unsupervised, computationally efficient, and readily extensible to other spherical tokamak devices, providing a flexible foundation for incorporating additional diagnostics, such as Mirnov coil signals, toward precursor-aware detection and future predictive modeling of IRE activity.

Plasma doi: 10.3390/plasma9010008

Authors: Kenichi Yamazaki Hiroyuki Yasui Tsuyoshi Noguchi Yuuma Suenaga Akitoshi Okino

Atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) enables low-temperature coating in open air, yet the interplay between precursor activation and ambient-derived species remains unclear. Here, thin films from an amine precursor are deposited using a helium plasma and characterized by gas chromatography–mass spectrometry (GC-MS), a quartz crystal microbalance (QCM), and X-ray photoelectron spectroscopy (XPS). GC-MS indicates partial precursor conversion and formation of oxygen- and nitrogen-containing products, consistent with participation of ambient air and moisture. QCM identifies a limited precursor-concentration window in which mass increases monotonically during plasma exposure and remains constant after shutdown; outside this window, post-discharge mass loss occurs, indicating desorption of weakly bound species. XPS confirms carbon-rich films incorporating oxygen- and nitrogen-containing functionalities and complete substrate coverage at higher precursor concentrations.

Plasma doi: 10.3390/plasma9010007

Authors: Ron Grikshtas Sergey Efimov Nikita Asmedianov Yakov E. Krasik

Underwater electrical explosions of single metallic wires driven by microsecond current pulses are investigated and compared with previously reported sub-microsecond experiments. Current and voltage waveforms, streak camera shadow imaging, and one-dimensional hydrodynamic simulations are employed to characterize how the energy density, energy density deposition rate, and the generated shock waves in water depend on the wire parameters. It was found that, similar to the sub-microsecond timescale, the solid–liquid phase transition occurs later than thermodynamic calculations predicted, while the liquid–vapor phase transition happens sooner than expected, leading to a two-phase coexistence. Additionally, most materials show a notable resistance peak (Ti, Fe, Ni, Zn, Ag, Sn, Ta, Au) compared to a quasi-plateau for Cu and Mo or a continuous increase for Al and Pt. Moreover, the specific action integral values are significantly smaller than those observed in wire explosion experiments in vacuum. Finally, the plasma formed at peak resistive voltage is non-ideal but exhibits lower electron density, ionization degree, and temperature compared to the sub-microsecond case.

Plasma doi: 10.3390/plasma9010006

Authors: Ovidiu S. Stoican

A novel system aiming to electrically supply various cold plasma generators is proposed. It operates as a programmable linear current source which is able to maintain a dc constant discharge current at various discharge voltages required to sustain the plasma jet. Its design is based on a specific electronic device called a switchable current regulator, which considerably simplifies the circuit topology. Experimental results carried out in real operating conditions confirm the practical purpose of the proposed solution.

Plasma doi: 10.3390/plasma9010005

Authors: Myles Perry Sidmar Holoman Daniel Wozniak Shirshak Kumar Dhali

The generation of atmospheric pressure nonequilibrium plasma using electrical discharges is an active area of research due to its significance in a wide spectrum of applications including medicine, combustion, and manufacturing. In our attempt to create a helium plasma jet in a pin-plane discharge with a constant current source, we observed self-pulsating behavior. We present the results of the electrical, optical, and spectroscopic measurements carried out to characterize the discharge. The duration of the discharge is a few tens of nanoseconds, and the repetition rate is in the few tens of kHz. The effect of the gap distance and gas flow is discussed. The effective capacitance formed by the space charge in the discharge region plays an important role in determining the pulsing frequency. The results of voltage swing, current pulse, and light emission are also discussed. Such self-pulsating discharges can be used to produce helium plasmas under ambient conditions in applications such as plasma medicine.

Plasma doi: 10.3390/plasma9010004

Authors: Anbang Sun Yulin Guo Yanru Li Yifei Zhu

Filamentary mode, as a common phenomenon that appears in dielectric barrier discharge (DBD), is realized by rod-to-rod electrodes in N2-O2 mixtures at 80 mbar. The effects of the dielectric thickness on the characteristics of filamentary DBD are investigated through experiments and simulations. The discharges are driven by a positive unipolar nanosecond pulse voltage with 15.8 kV amplitude, 9 ns rise time (Tr10–90%), and 14 ns pulse width. The characteristics of filamentary DBD are recorded with an intensified charge-coupled device and a Pearson current probe in the experiment, and a 2D axisymmetric fluid mode is established to analyze the discharge. Surface discharges occur on the anode and cathode dielectric after the breakdown, and the discharge is gradually extinguished as the applied voltage decreases. A thinner total dielectric thickness (Da + Dc) leads to larger currents, stronger discharges, and wider discharge channels. These characteristics are consistent when the total dielectric thickness is the same but anode dielectric thickness and cathode dielectric thickness are different (Da ≠ Dc ≠ 0). If the anode is a metal electrode (Da = 0), the current will be substantially large, and two discharge modes are observed: stable mono-filament discharge mode and random multi-filament discharge mode. It is found in simulations that the dielectric thickness changes the electric field configuration. The electric field is stronger with the decrease in dielectric thickness and leads to a more intense ionization which is responsible for most of the observed effects.

Plasma doi: 10.3390/plasma9010003

Authors: Lian Farhadian Samira Amiri Khoshkar Vandani Hai-Feng Ji

Dielectric barrier discharge (DBD) plasma provides a solvent-free and energy-efficient approach for the in situ polymerization of styrene on cotton textiles. Traditional methods for polystyrene (PS) coating often require elevated temperatures, chemical initiators, or organic solvents, conditions that are incompatible with porous, heat-sensitive substrates such as cotton. In this work, we demonstrate that DBD plasma can initiate and sustain styrene polymerization directly on cotton fibers under ambient conditions. FT-IR spectroscopy confirms the consumption of the vinyl C=C bond and the formation of atactic, amorphous polystyrene. Thermogravimetric analysis indicates that the cotton coated with DBD polymerized PS exhibits enhanced thermal stability compared to cotton coated with commercial PS. Additionally, UV aging tests confirm that the plasma-deposited coating maintains its hydrophobicity after exposure to light. Together, these findings highlight DBD plasma as a sustainable and effective approach for producing hydrophobic, thermally robust, and UV-stable textile coatings without the need for solvents, initiators, or harsh processing conditions.

Plasma doi: 10.3390/plasma9010002

Authors: Stephan Fritzsche Houke Huang Aloka Kumar Sahoo

Recent advances in non-local thermodynamic equilibrium (non-LTE) plasma simulations, for example in modeling kilonova ejecta, have emphasized the need for consistent and reliable atomic data. Unlike LTE modeling, non-LTE calculations must include a consistent treatment of various photon-induced and collisional processes in order to describe realistic electron and photon distributions in the plasma. However, the available atomic data are often incomplete, inconsistently formatted, or even fail to indicate the main dependencies on the level structure and plasma parameters, thus limiting their practical use. To address these issues, we have extended Jac, the Jena Atomic Calculator (version v0.3.0), to provide direct access to relevant cross sections, plasma rates, and rate coefficients. Emphasis is placed on photoexcitation and ionization processes as well as their time-reversed counterparts—photo-de-excitation and photorecombination. Whereas most of these data are still based on empirical expressions, their dependence on the ionic level structure and plasma temperature is made explicit here. Moreover, the electron and photon distributions can be readily controlled and adjusted by the user. This transparent representation of atomic data for photon-mediated processes, together with a straightforward use, facilitates their integration into existing plasma codes and improves the interpretation of high-energy astrophysical phenomena. It may support also more accurate and flexible non-LTE plasma simulations.

Plasma doi: 10.3390/plasma9010001

Authors: J. E. Mastache-Mastache M. C. González H. Martínez B. Reyes-Ramírez

This computational study investigates the optical properties of a sixth-order Fibonacci quasi-periodic photonic crystal cavity designed for the infiltration of near-infrared colloidal quantum dots (QDs, e.g., InAs/ZnSe or PbS) and fully compatible with plasma-enhanced chemical vapor deposition (PECVD) using porous silicon layers. Using the transfer matrix method (TMM), we simulate transmission (T), reflection, absorption, electric field distributions and Purcell factors (F) for both TE and TM polarizations, incorporating the wavelength-dependent absorption of porous silicon. A multi-objective figure-of-merit is defined to simultaneously maximize transmission (T>95% at 800 nm) and the one-dimensional Purcell factor. The optimized structure (PH=0416) yields a quality factor Q≈4300, a 1D Purcell factor F1D≈3.6 and a realistic 3D Purcell enhancement estimated between 4 and 8 (under lateral confinement assumptions). This conservative estimate, derived via the effective index method to account for 3D effects, aligns with the detailed discussion within the article and is lower than the ideal upper bound of the 1D model. The integrated emission enhancement is approximately 3.0-fold. Monte Carlo simulations demonstrate remarkable robustness to fabrication tolerances (±10 nm thickness variations result in a <5% reduction in transmission), highlighting the structure’s scalability for PECVD-based processing. Comparison with periodic Bragg structures reveals superior angular stability and disorder tolerance in the Fibonacci design, positioning it as a promising platform for robust QD-based light sources and integrated refractive index sensors.

Plasma doi: 10.3390/plasma8040051

Authors: Weihua Jiang

This review article is the third of a three-article introductory series on virtual cathode oscillators. The first article has laid the theoretical ground for understanding the physical properties of the virtual cathode, and the second article has provided a numerical tool for studying virtual cathode oscillation. This third article focuses on the interaction between the electron beam and electromagnetic field. The virtual cathode oscillator has been studied for decades with the aim of developing it as high-power microwave source. The beam-field interaction has been one of the core issues that always perplexes both experimentalists and theorists. Using the physical model established in the first article and the numerical method described in the second article, this article is an attempt to answer some of the key questions based on a more comprehensive description of the device and its interaction process. This article is expected to serve as a reference for young researchers and students working on high-power microwaves and pulsed particle beams.

Plasma doi: 10.3390/plasma8040050

Authors: Veselin Vasilev Nikola Lazarov Svetlana Lazarova Tsvetelina Paunska Stanimir Kolev

The aim of this work is to provide an extensive experimental study of the performance of a novel magnetically and gas-flow-stabilized arc discharge for carbon dioxide (CO2) conversion and oxygen (O2) production on Mars. The proposed discharge provides an additional degree of freedom for easy scalability by adjusting its length. The discharge is examined at a pressure range of 200–612 mbar in order to optimize it for oxygen production on Mars, where low-pressure operation is preferable due to energy costs. Additionally, two quenching configurations with an actively cooled region are evaluated. They are compared to a benchmark configuration without additional cooling. Two high-voltage power supplies (PSs) are used, and the results are compared—a constant direct current (DC) and a pulsed unipolar current. The pulsed power supply offers better CO2 conversion performance at lower pressure due to stable operation in an arc regime. The energy cost for oxygen production on Mars is also presented, including a conservative estimation of the energy needed for compressing the Martian atmosphere at ambient pressure to the discharge operational pressure. It is discussed how this affects the energy cost of oxygen production.

Plasma doi: 10.3390/plasma8040049

Authors: Nikolai N. Bogachev Vyacheslav P. Stepin Vsevolod I. Zhukov Sergey E. Andreev Dmitry M. Karfidov Maksim S. Usachonak Evgeny M. Konchekov Namik G. Gusein-zade

This study investigates the axial electron density distribution in two plasma antenna configurations excited by a surface wave microwave discharge and its influence on the radiation pattern of antennas. The axial plasma electron density profiles were characterized using two non-invasive diagnostic techniques: the resonant cavity measurements in the TM110 mode and the waveguide transmission analysis. A linear decrease in the plasma electron density along the antenna was observed. The effective electrical length of the plasma antennas, accounting for this density distribution, is found to be approximately half the physical plasma column length. Numerical simulations employing COMSOL Multiphysics based on the Drude model revealed that a realistic nonuniform axial plasma electron density distribution markedly modifies the antenna radiation characteristics. For the wave-type plasma monopole antenna, this results in a shift in the emission maximum, a reduction in the main lobe amplitude, a nearly twofold broadening of the main lobe, and the disappearance of the side lobe. For the quarter-wave-type plasma asymmetric dipole antenna, there is a reduction in the main lobe amplitude without a shift in the maximum and a broadening of the main lobe due to an increase in the side-lobe level and its merging with the main lobe.

Plasma doi: 10.3390/plasma8040048

Authors: Shigeyuki Takagi Shih-Nan Hsiao Yusuke Imai Makoto Sekine Fumihiko Matsunaga

The fabrication of semiconductor devices with three-dimensional architectures imposes unprecedented demands on advanced plasma dry etching processes. These include the simultaneous requirements of high throughput, high material selectivity, and precise profile control. In conventional reactive ion etching (RIE), fluorocarbon plasma provides both accelerated ion species and reactive neutrals that etch the feature front, while the CFx radicals promote polymerization that protects sidewalls and enhance selectivity to the amorphous carbon layer (ACL) mask. In this work, we present computational results on the role of CF4 addition to hydrogen fluoride (HF) plasma for next-generation RIE, specifically cryogenic etching. Simulations were performed by varying the CF4 concentration in the HF plasma to evaluate its influence on ion densities, neutral species concentration, and electron density. The results show that the densities of CFx (x = 1–3) ions and radicals increase significantly with CF4 addition (up to 20%), while the overall plasma density and the excited HF species remain nearly unchanged. The results of plasma density and atomic fluorine density are consistent with the experimental observations of the HF/CF4 plasma using an absorption probe and the actimetry method. It was verified that the gas-phase reaction model proposed in this study can accurately reproduce the plasma characteristics of the HF/CF4 system. The coupling of HF-based etchants with CFx radicals enables polymerization that preserves SiO2 etching throughput while significantly enhancing etch selectivity against the ACL mask from 1.86 to 5.07, with only a small fraction (~10%) of fluorocarbon gas added. The plasma simulation provides new insights into enhancing the etching performance of HF-based cryogenic plasma etching by controlling the CF2 radicals and HF reactants through the addition of fluorocarbon gases.

Plasma doi: 10.3390/plasma8040047

Authors: Alexander S. Metel Marina A. Volosova Enver S. Mustafaev Yury A. Melnik Sergey N. Grigoriev

To increase the wear resistance of a hollow ceramic product, it is necessary to apply wear-resistant coatings to all its surfaces, including the internal surfaces. Before the coating deposition, the surface must be processed with a beam of energetic particles to ensure its adhesion. In this study, a scheme for processing internal surfaces of hollow cylinders with fast argon atoms is proposed and tested. Simultaneous treatment of all surfaces of the rotating ceramic cylinder allowed for deposition of a uniform TiB2 coating on both sides of the cylinder and a decrease in the abrasion wear by several times.

Plasma doi: 10.3390/plasma8040046

Authors: Mingyan Chen Chenhong Wang Tian Xie Zheng Chen Guimin Xu

This study investigated the inactivation effect and influencing factors of cold atmospheric plasma (CAP) treatment with Salmonella typhimurium and Staphylococcus aureus populations on three food contact materials (FCMs)—kraft paper, 304 stainless steel, and glass. The CAP was generated as an atmospheric helium plasma jet (15 kV, 10.24 kHz, He 4 L/m), and the experimental results indicated that its inactivation effects on two bacterial species gradually increased as the plasma treatment duration increased (0, 1, 2, 3, 4, and 5 min). Three classical sterilization kinetic models (Log-linear, Weibull, and Log-linear + Shoulder + Tail) were employed to evaluate the inactivation efficiency of plasma against bacteria FCMs. Combined with the coefficient of determination (R2), accuracy factor (Af), and bias factor (Bf), together with the root mean square error (RMSE), it can be concluded that the Log-linear + Shoulder + Tail model had the highest fitting degree among the three sterilization kinetics models. Salmonella typhimurium exhibited weaker resistance than Staphylococcus aureus to the same CAP treatment. Under the same conditions, CAP had the strongest bactericidal effect on the bacteria on the glass surface, followed by those on the 304 stainless steel, and had the weakest bactericidal effect on the bacteria on the kraft paper surface, which might be related to the surface hydrophilicity and roughness of the FCMs. The above results indicated that CAP’s inactivation effect may be influenced by the microbial species as well as the surface characteristics of FCMs. This study provides useful information for future applications of CAP in enhancing food safety.

Plasma doi: 10.3390/plasma8040045

Authors: Viktoriia V. Gudkova Valentin D. Borzosekov Maria A. Zimina Igor V. Moryakov Dmitry V. Malakhov Namik Gusein-zade Evgeny M. Konchekov

This study investigates the physicochemical processes in aqueous solutions treated with a high-current (up to 300 A) pulsed multispark discharge. Pulse length was 2 μs at a 50 Hz repetition rate. The discharge occurred within bubbles of argon injected between the stainless-steel electrodes at the constant flow rate. The erosion of electrode material during the discharge led to iron and other alloy components entering the liquid. Optical emission spectra confirmed the erosion of electrode material (Fe, Cr, Ni atoms and ions). EDTA and its disodium salt were used in order to study their effect on the metal particle formation process. Treatment with deionized water led to an increase in conductivity and the generation of hydrogen peroxide (up to 1200 µM). In contrast, the presence of EDTA and its disodium salt drastically altered the reaction pathways: the H2O2 yield decreased, and the solution conductivity dropped substantially for the acidic form of EDTA, while the decrease was minor for EDTA-Na2. This effect is attributed to the buffered chelation of eroded metal ions, forming stable Fe-EDTA complexes, as confirmed by a characteristic absorption band at 260 nm. The results demonstrate the critical role of complex-forming agents in modulating plasma–liquid interactions, shifting the process from direct erosion products to the formation of stable coordination compounds.

Plasma doi: 10.3390/plasma8040044

Authors: Dariusz Korzec Florian Freund Isabelle Doelfs Florian Zacherl Lucas Kudala Hans-Peter Rabl

The atmospheric pressure plasma jet (APPJ) is a popular type of cold atmospheric plasma (CAP). APPJs based on a pulsed atmospheric arc (PAA) are widely spread in industrial processing. A plasma jet of this type, PlasmaBrush PB3 (PB3), is a subject of diverse research activities. The characteristic feature of PB3 is the generation of a low-current (300 mA), high-voltage (1500 V) pulsed (54 kHz) atmospheric arc. A gas flow vortex is used to stabilize the arc and to sustain the circular motion of the cathodic arc foot. During long periods of operation, nozzles acting as arc discharge cathodes erode. Part of the eroded material is emitted as nanoparticles (NPs). These NPs are not wanted in many processing applications. Knowledge of the number, type, and size distribution of emitted NPs is essential to minimize their emissions. In this study, NPs in the size range of 6 to 220 nm, emitted from four different nozzles operated with PB3, are investigated. The differences between the nozzles are in the eroded surface material (copper, tungsten, and nickel), the diameter of the nozzle orifice, the length of the discharge channel, and the position of the cathodic arc foot. Significant differences in the particle size distribution (PSD) and particle mass distribution (PMD) of emitted NPs are observed depending on the type and condition of the nozzle and their operating time. Monomodal and bimodal PMD models are used to approximate emissions from the nozzles with tungsten and copper cores, respectively. The skew-normal distribution function is deemed suitable. The results of this study can be used to control NP emissions, both to avoid them and to utilize them intentionally.

Plasma doi: 10.3390/plasma8040043

Authors: Jiawen Xie Fubao Jin Shangang Ma Jinqiang Shi Yanming Qi

To explore the potential of plasma technology in regulating seed germination, this study compared the effects of direct treatment with needle-plate electrodes using DC and pulse power supplies, and indirect treatment with plasma-activated water on the growth characteristics of Bromus inermis seeds. By comparing different pulse power parameters, including voltage, pulse width, frequency, and duration, it was found that treatments at 15 kV, 2500 ns, 6 kHz, and 10 min significantly increased the surface hydrophilicity and germination performance of the seeds. The best conditions for DC power supply were 15 kV and 10 min. Indirect treatment with plasma-activated water (15 kV, 10 min) effectively broke the seed dormancy by regulating active nitrogen oxygen particle components, increasing the germination percentage by 50%. Analysis of antioxidant enzyme activity showed that in seedlings the activities of superoxide dismutase (SOD) and peroxidase (POD) increased by 75% and 21%, respectively, after treatment, revealing the mechanism of oxidative stress response induced by plasma. This study provides theoretical and technical references for the application of plasma technology in enhancing seed vitality and agricultural practices.

Plasma doi: 10.3390/plasma8040042

Authors: Shirshak Kumar Dhali Stuart Reyes

When a gas is overvolted at or near atmospheric pressure, it results in a streamer discharge formation. Electrode geometries exert significant impact on the electrical breakdown of gases by altering the spatial profile of the electric field. In many applications the efficient generation of radicals is critical and is determined by the characteristics of the streamer discharge. We examine the effect of electrode geometry on the streamer characteristics and the production of radicals. This is performed for three different electrode geometries: plane–plane, pin–plane, and pin–pin. A two-dimensional rotationally symmetric fluid model is used for the streamer discharge simulation in the hydrogen/air gas mixture. The spatial profile of electron density and the electric field for point electrodes show significant differences when compared to plane electrodes. However, the efficiency of radical generation shows similar trends for the electrode configurations studied. We also present the results of spatial electrical energy density distribution which in turn determines spatial excited species distribution. These results can inform the design of specific applications.

Plasma doi: 10.3390/plasma8040041

Authors: Sung-Young Yoon Eun Jeong Hong Junghyun Lim Seungil Park Sangheum Eom Seong Bong Kim Seungmin Ryu

We present an experimentally validated, engineering-oriented framework for the design and operation of pin-to-water (PTW) atmospheric discharges to produce hydrogen peroxide (H2O2) on demand. Motivated by industrial needs for safe, point-of-use oxidant supply, we combine time-resolved diagnostics (FTIR, OES), liquid-phase analysis (ion chromatography, pH, conductivity), and coupled plasma-chemistry/fluid simulations to link plasma state to aqueous H2O2 yield. Under the tested conditions (14.3 kHz, 0.2 kW; electrode to quartz wall distance 12–14 mm; coolant setpoints 0–40 °C), H2O2 concentration follows a reproducible non-monotonic trajectory: rapid accumulation during the early treatment (typical peak at ~15–25 min), followed by decline with continued operation. The decline coincides with a robust vibrational-temperature (Tvib) threshold near ~4900 K measured from N2 emission, and with concurrent NOX accumulation and bulk acidification. Global chemistry modeling and Fluent flow fields reproduce the observed trend and show that both vibrational excitation (kinetics) and convective transport (mass/heat transfer) determine the productive time window. Based on these results, we formulate practical design rules—electrode gap (power density), discharge current control, thermal/flow management, water quality, and OES-based Tvib monitoring with an automated stop rule—that maximize H2O2 yield while avoiding NOX-dominated suppression. The study provides a clear path for transforming mechanistic plasma insights into deployable, industrial H2O2 generator designs.

Plasma doi: 10.3390/plasma8040040

Authors: Subash Mohandoss Harshini Mohan Natarajan Balasubramaniyan Sivachandiran Loganathan

The win–win situation of dye degradation and nitrogen fixation in wastewater using non-thermal plasma (NTP) were investigated in this study. Specifically, the feasibility of utilizing plasma-treated dye-contaminated wastewater for seed germination and plant growth was explored. Crystal Violet (CV) and Rhodamine B (RhB) dyes were used as model pollutants, while Sorghum bicolor (great millet) seeds were used to assess germination rates and plant growth responses. In untreated wastewater containing CV and RhB, approximately 45% of seeds germinated after three days, but no significant stem or root growth was observed after 11 days. Plasma treatment significantly enhanced dye degradation, with efficiency improving as treatment time and input power increased. After 16 min of plasma treatment at 1.3 ± 0.2 W input power, about 99% degradation efficiency was achieved for both CV (0.0122 mM) and RhB (0.0104 mM). This degradation was primarily driven by reactive oxygen and nitrogen species (RONS) generated by plasma discharge. When sorghum seeds were germinated using plasma-treated wastewater, the germination rate increased to 65% after three days—20% higher than with untreated wastewater. Furthermore, after 11 days, the average stem length reached 9 cm, while the average root length extended to 7 cm. These findings highlight NTP as a promising and sustainable method for degrading textile industry pollutants while simultaneously enhancing crop productivity through the reuse of treated wastewater.

Plasma doi: 10.3390/plasma8040039

Authors: Nadia Imtiaz Saeed Ur Rehman Liu Chao Rui Yan Richard Marchand

We present three dimensional particle-in-cell simulations of current-voltage characteristics of the hemispherical Langmuir probe (LAP), onboard the China Seismo-Electromagnetic Satellite (CSES). Using realistic plasma parameters and background magnetic fields obtained from the International Reference Ionosphere (IRI) and International Geomagnetic Reference Field (IGRF) models, we simulate probe–plasma interactions at three locations: the equatorial region and two magnetically conjugate mid-latitude sites: Millstone Hill (Northern Hemisphere) and Rothera (Southern Hemisphere). The simulations, performed using the PTetra PIC code, incorporate realistic LAP geometry and spacecraft motion in the ionospheric plasma. Simulated current voltage characteristics or I–V curves are compared against in-situ LAP measurements from CSES Orbit-026610, with Pearson’s correlation coefficients used to assess agreement. Our findings indicate how plasma temperature, density, and magnetization affect sheath structure and probe floating potential. The study highlights the significance of kinetic modeling in enhancing diagnostic accuracy, particularly in variable sheath regimes where classic analytical models such as the Orbital-Motion-Limited (OML) theory may be inadequate.

Plasma doi: 10.3390/plasma8040038

Authors: Alexander S. Metel Marina A. Volosova Enver S. Mustafaev Yury A. Melnik Sergey N. Grigoriev

The removal of defective surface layers can substantially improve the quality of various products. It can be carried out using beams of accelerated ions or fast argon atoms. However, it is difficult to process the inner surface of narrow channels. In the present work, a narrow beam of fast argon atoms is used to sputter and polish the inner surface of drawing dies with 5.7 mm wide working channels. Due to the high angle of incidence to the channel walls, sputtering with fast argon atoms decreased their roughness to Ra ~ 0.004 µm.

Plasma doi: 10.3390/plasma8030037

Authors: Ping Huang Zhipeng Wu Li Tian Biao Zhang Yuang Long Zhenyu Liu Yiyi Zhang

Plasma arcs generated by lightning strikes are prone to tripping distribution lines, especially 10 kV lines. To reduce the lightning-induced tripping rate of 10 kV distribution lines and ensure the safe operation of power systems, this paper proposes a same-level double-fracture lightning protection device containing the electronegative gas trifluoroiodomethane (CF3I). A mathematical model of the gas arc-extinguishing process is established based on magnetohydrodynamics. Meanwhile, the mechanism of CF3I in the arc-extinguishing process is analyzed according to its physical and chemical properties, and the arc-extinguishing process is simulated using COMSOL Multiphysics 6.0. The results show that (1) the arc-extinguishing effect is optimal when the horizontal distance of the compression pipeline of the device is 9 mm; (2) under the action of power frequency currents with different initial phases of π/2 and 0, the arc-extinguishing device can extinguish the arc within 800 μs without re-ignition; and (3) in the arc-extinguishing process involving CF3I, the arc can be extinguished within 710 μs, which is 11.2% quicker than that without CF3I. Meanwhile, CF3I can effectively reduce the arc temperature at the initial stage of arc extinguishing, avoiding damage caused by excessive internal compression of the device.

Plasma doi: 10.3390/plasma8030036

Authors: Nicolas Bente Alfredo Cuellar Valencia Hubert Piquet

A system-level study of a NOx abatement process by means of non-thermal plasma (NTP) generated with dielectric barrier discharges (DBDs) is the framework of this article. With the goal of system improvement, the kinetic reaction simulation software ZdPlaskin is considered to select the most favorable operating conditions in order to optimize NOx abatement (deNOx). A parametric exploration of the performance, through variations in operating conditions (temperature, power injection pattern, and input gas mixture composition), requires highly numerous simulations; thus, the shortest possible computation times with robust results are of significant interest. As such, an analysis and filtering method is proposed and detailed to build a minimized chemical kinetic reaction set, allowing us to reliably analyze the impact of the selected operating conditions for the DBD reactor on treatment performance.

Plasma doi: 10.3390/plasma8030035

Authors: Vladislav Maksimov Adi Haim Ron Grikshtas Alexander Kostinskiy Elhanan Magid John G. Leopold Yakov E. Krasik

Time- and space-resolved radiation emitted by the plasma produced by a 0.8 ns duration at full width half maximum, ~600 MW maximum power microwave (~9.6 GHz) pulse traversing a hydrogen-, helium-, or air-filled circular waveguide, is studied. Gas ionization by microwaves is an old subject but the regime investigated in the present experimental research, of very high-power microwaves and very short pulses using modern diagnostic tools, is new and follows a series of new studies performed so far only in our laboratory, revealing non-linear phenomena never observed before. In the present research, plasma radiation is observed along a slit made in a circular waveguide wall by either an intensified fast frame camera or a streak camera. Using calibrated input and output couplers, the transmission and reflection coefficients of the high-power microwaves were determined over a broad range of gas pressures, 0.1 kPa < P < 90 kPa. It was found that the intensity of the plasma light emission increases significantly after the high-power microwave pulse has left the waveguide. Depending on pressure, the radiation is either uniform along the slit, while the front of the emitted light follows the microwave pulse at a velocity close to its group velocity, or it remains in the vicinity of the input window, indicating that the plasma density is above critical density. It was also found that the radial distribution of radiation depends on pressure. At pressures <10 kPa, when the electron oscillatory energy reaches 20 keV close to the waveguide axis, light emission forms faster near the waveguide walls, where the ionization rate is maximal. Otherwise, when pressure is >80 kPa, light emission is most intense on the axis where the electron oscillatory energy is ~100 eV and the ionization rate is maximal. We also studied the UV radiation from the plasma, the duration of which was found to be longer than the duration of visible light emission. This indicates the existence of energetic electrons for tens of ns after the high-power microwave pulse has left the observation region. Considering that the emitted light intensity depends on the plasma density and temperature, the observed data may be used for a comparison with the results of collisional radiative models if the electron time and spatial energy distribution is known.

Plasma doi: 10.3390/plasma8030034

Authors: Michael Okebiorun Dalton Miller Kenneth A. Cornell Jim Browning

The study evaluates the efficacy of an image-guided CAP treatment method with a plasma device capable of rapid biofilm removal from chicken tissue. The plasma treatment operating configuration includes a gas mixture of Argon and H2O at a flowrate of 1.5 lpm. An X-Y stage was used to move the chicken sample below the stationary plasma scalpel at a speed of 0.1 mm/s. The discharge voltage and current were maintained between 3.2 and 3.7 kV (AC 20 kHz), and at 3 mA, respectively. The electrode gap and sample distance were set to 0.6 mm and 4 mm. This configuration facilitated effective biofilm removal, as confirmed by CFU analysis and 3D microscopic analysis showing a >99% reduction in biofilm post treatment with an etch rate of 2.2–5.8 µm/s and an impact width of up to 300 µm. The plasma scalpel electrode temperature reached 94.7 °C, while the targeted biofilm area was heated to 36.3 °C, suggesting non-thermal biofilm disruption. Three-dimensional microscopic analysis revealed biofilm thickness on chicken tissues ranging from 20 to 180 µm, comparable to biofilm loads on mammalian tissues. In conclusion, the study highlights the potential of CAP devices as a promising solution for biofilm debridement.

Plasma doi: 10.3390/plasma8030033

Authors: Juan Casanova-Chafer Pedro Atienzar Carla Bittencourt

Theoretical studies have indicated that borophene is a promising two-dimensional material characterized by remarkable chemical, mechanical, and electrical properties. Nonetheless, its practical applications in areas such as catalysis and gas sensing are hindered by the limited density of reactive sites in its pristine form. To address this limitation, the present study explores the controlled fluorination of borophene nanoflakes as a strategy to modify their surface chemistry and enhance the availability of active sites. Furthermore, it is anticipated that surface fluorination will improve hydrophobicity, which is crucial for reducing humidity-related interference in sensing applications. In this study, we report the successful functionalization of borophene nanoflakes with fluorine using a plasma arc discharge technique for the first time. Borophene nanolayers were synthesized via a sonochemical-assisted exfoliation method, yielding nanosheets with an average lateral dimension of approximately 100 nm. The fluorinated samples were characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). A systematic investigation of plasma exposure durations demonstrated that fluorine was effectively introduced as a dopant while maintaining the crystallinity of the borophene lattice.

Plasma doi: 10.3390/plasma8030032

Authors: Khadija Shabbir Brian Leard Zibo Wang Sai Tej Paruchuri Tariq Rafiq Eugenio Schuster

The Multi-Mode Model (MMM) is a physics-based anomalous transport model integrated into TRANSP for predicting electron and ion thermal transport, electron and impurity particle transport, and toroidal and poloidal momentum transport. While MMM provides valuable predictive capabilities, its computational cost, although manageable for standard simulations, is too high for real-time control applications. MMMnet, a neural network-based surrogate model, is developed to address this challenge by significantly reducing computation time while maintaining high accuracy. Trained on TRANSP simulations of DIII-D discharges, MMMnet incorporates an updated version of MMM (9.0.10) with enhanced physics, including isotopic effects, plasma shaping via effective magnetic shear, unified correlation lengths for ion-scale modes, and a new physics-based model for the electromagnetic electron temperature gradient mode. A key advancement is MMMnet’s ability to predict all six transport coefficients, providing a comprehensive representation of plasma transport dynamics. MMMnet achieves a two-order-of-magnitude speed improvement while maintaining strong correlation with MMM diffusivities, making it well-suited for real-time tokamak control and scenario optimization.

Plasma doi: 10.3390/plasma8030031

Authors: Yangyang Ma Wenle Song Junlei Zhao Lei Wang Shenghui Mu Kun Wang

The Thomas–Fermi model and its quantum and exchange corrections with mathematic manipulations and numerical approaches are primarily investigated. The reduced ideal electron chemical potential is adopted as the initial value for the iterative solution of the Thomas–Fermi model. A new transformation for the quantum and exchange equations is proposed to apply the boundary conditions easily. Both the Thomas–Fermi equation and correction equations are solved with the Runge–Kutta algorithm. The mathematical difficulties in controlling the computing accuracy of the equations containing the Fermi–Dirac integral are settled. The equation of state, based on the Thomas–Fermi model with its quantum and exchange corrections, is constructed and compared with relevant data.

Plasma doi: 10.3390/plasma8030030

Authors: Florin Enescu Codrina Ionita Dan Gheorghe Dimitriu Roman Schrittwieser

In this article, we present an overview of our investigations on the formation and behavior of space charge structures in an argon discharge plasma on gridded and smooth spherical hollow electrodes with and without orifices. Four experiments are described, in which we have used the following: (1) one spherical gridded sphere with one orifice, (2) one hollow smooth stainless steel sphere with two opposing orifices, (3) two smooth polished stainless steel spherical electrodes without orifices, (4) two smooth polished stainless steel spherical electrodes with opposing orifices. The experiments were conducted at the University of Innsbruck in a stainless steel cylindrical chamber (the former Innsbruck DP machine—IDP), and at the Alexandru Ioan Cuza University of Iaşi (Romania) in a Pyrex Vacuum Chamber (PCH). As diagnostics, we have used mainly optical emission spectroscopy to determine electron temperature and density.

Plasma doi: 10.3390/plasma8030029

Authors: Xinliang Xu Yihang Chen Yulin Zhou Zhanhui Wang Xueke Wu Bo Li Jiang Sun Junzhao Zhang Da Li

The mechanisms by which rotation influences zonal flows (ZFs) in plasma are incompletely understood, presenting a significant challenge in the study of plasma dynamics. This research addresses this gap by investigating the role of non-inertial effects—specifically centrifugal and Coriolis forces—on Geodesic Acoustic Modes (GAMs) and ZFs in rotating tokamak plasmas. While previous studies have linked centrifugal convection to plasma toroidal rotation, they often overlook the Coriolis effects or inconsistently incorporate non-inertial terms into magneto-hydrodynamic (MHD) equations. In this work, we derive self-consistent drift-ordered two-fluid equations from the collisional Vlasov equation in a non-inertial frame, and we modify the Hermes cold ion code to simulate the impact of rotation on GAMs and ZFs. Our simulations reveal that toroidal rotation enhances ZF amplitude and GAM frequency, with Coriolis convection playing a critical role in GAM propagation and the global structure of ZFs. Analysis of simulation outcomes indicates that centrifugal drift drives parallel velocity growth, while Coriolis drift facilitates radial propagation of GAMs. This work may provide valuable insights into momentum transport and flow shear dynamics in tokamaks, with implications for turbulence suppression and confinement optimization.

Plasma doi: 10.3390/plasma8030028

Authors: Stuart Reyes Shirshak Kumar Dhali

Some of the popular and successful atmospheric pressure fuel/air plasma-assisted combustion methods use repetitive ns pulsed discharges and dielectric-barrier discharges. The transient phase in such discharges is dominated by transport under strong space charge from ionization fronts, which is best characterized by the streamer model. The role of the nonthermal plasma in such discharges is to produce radicals, which accelerates the chemical conversion reaction leading to temperature rise and ignition. Therefore, the characterization of the streamer and its energy partitioning is essential to develop a predictive model. We examine the important characteristics of streamers that influence combustion and develop some macroscopic parameters. Our results show that the radicals’ production efficiency at an applied field is nearly independent of time and the radical density generated depends only on the electrical energy density coupled to the plasma. We compare the results of the streamer model to the zero-dimensional uniform field Townsend-like discharge, and our results show a significant difference. The results concerning the influence of energy density and repetition rate on the ignition of a hydrogen/air fuel mixture are presented.

Plasma doi: 10.3390/plasma8030027

Authors: Dariusz Korzec Florian Freund Christian Bäuml Patrik Penzkofer Oliver Beier Andreas Pfuch Klaus Vogelsang Frank Froehlich Stefan Nettesheim

Reactive oxygen–nitrogen species (RONS) production in a Peltier-cooled hybrid dielectric barrier discharge (HDBD) reactor operated with humid air is characterized. Fourier-transform infrared spectroscopy (FTIR) is used to determine the RONS in the HDBD-produced gases. The presence of molecules O3, NO2, N2O, N2O5, and HNO3 is evaluated. The influence of HDBD reactor operation parameters on the FTIR result is discussed. The strongest influence of Peltier cooling on RONS chemistry is reached at conditions related to a high specific energy input (SEI): high voltage and duty cycle of plasma width modulation (PWM), and low gas flow. Both PWM and Peltier cooling can achieve a change in the chemistry from oxygen-based to nitrogen-based. N2O5 and HNO3 are detected at a low humidity of 7% in the reactor input air but not at humidity exceeding 90%. In addition to the FTIR analysis, the plasma-activated water (PAW) is investigated. PAW is produced by bubbling the HDBD plasma gas through 12.5 mL of distilled water in a closed-loop circulation at a high SEI. Despite the absence of N2O5 and HNO3 in the gas phase, the acidity of the PAW is increased. The pH value decreases on average by 0.12 per minute.

Plasma doi: 10.3390/plasma8030026

Authors: Joel G. Rogers Andrew A. Egly Yoon S. Roh Robert E. Terry Frank J. Wessel

In this study, particle-in-cell (PIC) simulation was used to validate a conceptual design for a quasi-spherical, net power, hydrogen-plus-boron-11-fueled fusion reactor incorporating high-temperature superconducting (HTS) magnets. By burning a fully thermalized plasma, our proposed MET6 reactor uses the principles of the 1980 magneto-electrostatic trap design of Yushmanov to improve the classic Polywell design. Because the input power consumed by the reactor will barely balance the waste bremsstrahlung radiation, future research must focus on reducing the bremsstrahlung losses to reach practical net power levels. The first step to reducing bremsstrahlung, explored in this paper, is to tune the reactor parameters to reduce the energies of trapped electrons. We assume the quality factor Q can be approximated as the ratio of fusion power output divided by bremsstrahlung power loss. Thus, assuming the particles’ power loss is negligible compared to bremsstrahlung power loss, the resulting quality factor is estimated to be Q ≈ 1.3.

Plasma doi: 10.3390/plasma8020025

Authors: Harshini Mohan Subash Mohandoss Natarajan Balasubramaniyan Sivachandiran Loganathan

Non-thermal plasma (NTP)-assisted material synthesis and surface modification provide a promising approach in various applications, particularly in wastewater treatment. In this study, we reported the synthesis of photocatalytic zinc oxide (ZnO) from zinc hydroxide (Zn(OH)2) utilizing NTP discharge generated by dielectric barrier discharge (DBD). The results demonstrated that the 40 min plasma treatment at 200 °C (ZnO-P) with a voltage of 20 kV significantly improved the material’s physicochemical properties compared to conventional calcination at 600 °C (ZnO-600). ZnO-P exhibited better crystallinity, a significantly reduced particle size of 41 nm, and a narrower band gap of 3.1 eV compared to ZnO-600. Photocatalytic performance was evaluated through crystal violet degradation, where ZnO-P achieved an 60% degradation rate after 90 min of UV exposure, whereas ZnO-600 exhibited only a 50% degradation rate under identical conditions. These findings underscore the effectiveness of NTP synthesis in enhancing the surface properties of ZnO, leading to superior photocatalytic performance.

Plasma doi: 10.3390/plasma8020024

Authors: Jean de Dieu Nibigira Richard Marchand

Predicting the behaviour of the Earth’s ionosphere is crucial for the ground-based and spaceborne technologies that rely on it. This paper presents a novel way of inferring ionospheric electron density profiles and electron temperature profiles using machine learning. The analysis is based on the Nearest Neighbour (NNB) and Radial Basis Function (RBF) regression models. Synthetic data sets used to train and validate these two inference models are constructed using the International Reference Ionosphere (IRI 2020) model with randomly chosen years (1987–2022), months (1–12), days (1–31), latitudes (−60 to 60°), longitudes (0, 360°), and times (0–23 h), at altitudes ranging from 95 to 600 km. The NNB and RBF models use the constructed ionosonde-like profiles to infer complete ISR-like profiles. The results show that the inference of ionospheric electron density profiles is better with the NNB model than with the RBF model, while the RBF model is better at inferring the electron temperature profiles. A major and unexpected finding of this research is the ability of the two models to infer full electron temperature profiles that are not provided by ionosondes using the same truncated electron density data set used to infer electron density profiles. NNB and RBF models generally over- or underestimate the inferred electron density and electron temperature values, especially at higher altitudes, but they tend to produce good matches at lower altitudes. Additionally, maximum absolute relative errors for electron density and temperature inferences are found at higher altitudes for both NNB and RBF models.

Plasma doi: 10.3390/plasma8020023

Authors: Roman S. Zemskov Maxim V. Barkov Evgeniy S. Blinov Konstantin F. Burdonov Vladislav N. Ginzburg Anton A. Kochetkov Aleksandr V. Kotov Alexey A. Kuzmin Sergey E. Perevalov Il’ya A. Shaikin Sergey E. Stukachev Ivan V. Yakovlev Alexander A. Soloviev Andrey A. Shaykin Efim A. Khazanov Julien Fuchs Mikhail V. Starodubtsev

A pioneering detailed comparative study of the dynamics of plasma flows generated by high-power nanosecond and high-intensity femtosecond laser pulses with similar fluences of up to 3×104 J/cm2 is presented. The experiments were conducted on the petawatt laser facility PEARL using two types of high-power laser radiation: femtosecond pulses with energy exceeding 10 J and a duration less than 60 fs, and nanosecond pulses with energy exceeding 10 J and a duration on the order of 1 ns. In the experiments, high-velocity (>100 km/s) flows of «femtosecond» (created by femtosecond laser pulses) and «nanosecond» plasmas propagated in a vacuum across a uniform magnetic field with a strength over 14 T. A significant difference in the dynamics of «femtosecond» and «nanosecond» plasma flows was observed: (i) The «femtosecond» plasma initially propagated in a vacuum (no B-field) as a collimated flow, while the «nanosecond» flow diverged. (ii) The «nanosecond» plasma interacting with external magnetic field formed a quasi-spherical cavity with Rayleigh–Taylor instability flutes. In the case of «femtosecond» plasma, such flutes were not observed, and the flow was immediately redirected into a narrow plasma sheet (or «tongue») propagating across the magnetic field at an approximately constant velocity. (iii) Elongated «nanosecond» and «femtosecond» plasma slabs interacting with a transverse magnetic field broke up into Rayleigh–Taylor «tongues». (iv) The ends of these «tongues» in the femtosecond case twisted into vortex structures aligned with the ion motion in the external magnetic field, whereas the «tongues» in the nanosecond case were randomly oriented. It was suggested that the twisting of femtosecond «tongues» is related to Hall effects. The experimental results are complemented by and consistent with numerical 3D magnetohydrodynamic simulations. The potential applications of these findings for astrophysical objects, such as short bursts in active galactic nuclei, are discussed.

Plasma doi: 10.3390/plasma8020022

Authors: Justinas Masionis Darius Čiužas Edvinas Krugly Martynas Tichonovas Tadas Prasauskas Dainius Martuzevičius

Bioaerosol particles contribute to the reduced indoor air quality and cause various health issues, thus their concentration must be managed. Air cleaning is one of the most viable technological options for reducing quantities of indoor air contaminants. This study assesses the effectiveness of a prototype multi-stage air cleaner in reducing bioaerosol particle viability and concentrations. The single-pass type unit consisted of non-thermal plasma (NTP), ultraviolet-C (UV-C) irradiation, bipolar ionization (BI), and electrostatic precipitation (ESP) stages. The device was tested under controlled laboratory conditions using Escherichia coli (Gram-negative) and Lactobacillus casei (Gram-positive) bacteria aerosol at varying airflow rates (50–600 m3/h). The device achieved over 99% inactivation efficiency for both bacterial strains at the lowest airflow rate (50 m3/h). Efficiency declined with increasing airflow rates but remained above 94% at the highest flow rate (600 m3/h). Among the individual stages, NTP demonstrated the highest standalone inactivation efficiency, followed by UV-C and BI. The ESP stage effectively captured inactivated bioaerosol particles, preventing re-emission, while an integrated ozone decomposition unit maintained ozone concentrations below safety thresholds. These findings show the potential of multi-stage air cleaning technology for reducing bioaerosol contamination in indoor environments, with applications in healthcare, public spaces, and residential settings.

Plasma doi: 10.3390/plasma8020021

Authors: Hasupama Jayasinghe Liliana Arevalo Richard Morrow Vernon Cooray

Implementing a computationally efficient numerical model for a single streamer discharge is essential to understand the complex processes such as lightning initiation and electrical discharges in high voltage systems. In this paper, we present a streamer discharge simulation in air, by solving one-dimensional (1D) drift diffusion reaction (DDR) equations for charged species with the disc approximation for electric field. A recently developed fourth-order space and time-centered implicit finite difference method (FDM) with a flux-corrected transport (FCT) method is applied to solve the DDR equations, followed by a comparative simulation using the well-established explicit FDM with FCT. The results demonstrate good agreement between implicit and explicit FDMs, verifying their reliability for streamer modeling. The total electrons, total charge, streamer position, and hence the streamer bridging time obtained using the FDMs with FCT agree with the same streamer computed in the literature using different numerical methods and dimensions. The electric field is obtained with good accuracy due to the inclusion of image charges representing the electrodes in the disc method. This accuracy can be further improved by introducing more image charges. Both implicit and explicit FDMs effectively capture the key streamer behavior, including the variations in charged particle densities and electric field. However, the implicit FDM is computationally more efficient.

Plasma doi: 10.3390/plasma8020020

Authors: Adam D. Light Hariharan Srinivasulu Christopher J. Hansen Michael R. Brown

The force-free magnetic field solution formed in a high-aspect ratio cylinder is a non-axisymmetric (m=1), closed magnetic structure that can be produced in laboratory experiments. Force-free equilibria can have strong field gradients that break the usual adiabatic invariants associated with particle motion, and gyroradii at measured conditions can be large relative to the gradient scale lengths of the magnetic field. Individual particle motion is largely unexplored in force-free systems without axisymmetry, and it is unclear how the large gradients influence confinement. To understand more about how particles remain confined in these configurations, we simulate a thermal distribution of protons moving in a high-aspect-ratio force-free magnetic field using a Boris stepper. The particle loss is logarithmic in time, which suggests trapping and/or periodic orbits. Many particles do remain confined in particular regions of the field, analogous to trapped particles in other magnetic configurations. Some closed flux surfaces can be identified, but particle orbits are not necessarily described by these surfaces. We show examples of orbits that remain on well-defined surfaces and discuss the statistical properties of confined and escaping particles.

Plasma doi: 10.3390/plasma8020019

Authors: Md Jahangir Alam Abubakar Hamza Sadiq Jaroslav Kristof Mahedi Hasan Farhana Begum Yamano Tomoki Kazuo Shimizu

The blood–brain barrier (BBB) limits drug delivery to the brain, particularly for large or hydrophilic molecules. Brain microvascular endothelial cells (bEND.3), which form part of the BBB, play a critical role in regulating drug uptake. This study investigates the use of cold atmospheric microplasma (CAM) to enhance membrane permeability and facilitate drug delivery in bEND.3 cells. CAM generates reactive oxygen species (ROS) that modulate membrane properties. We exposed bEND.3 cells to CAM at varying voltages (3, 3.5, 4, and 4.5 kV) and measured drug uptake using the fluorescent drug FD-150, fluorescence intensity, ROS levels, membrane lipid order, and membrane potential. The results showed a significant increase in fluorescence intensity and drug concentration in the plasma-treated cells compared to controls. ROS production, measured by DCFH-DA staining, was higher in the plasma-treated cells, supporting the hypothesis that CAM enhances membrane permeability through ROS-induced changes. Membrane lipid order, assessed using the LipiORDER probe, shifted from the liquid-ordered (Lo) to liquid-disordered (Ld) phase, indicating increased membrane fluidity. Membrane depolarization was detected with DisBAC2(3) dye, showing increased fluorescence in the plasma-treated cells. Cell viability, assessed by trypan blue and LIVE/DEAD™ assays, revealed transient damage at higher voltages (≥4 kV), with recovery after 24 h. These results suggest that CAM enhances drug delivery in bEND.3 cells by modulating membrane properties via ROS production and changes in membrane potential. CAM offers a promising strategy for improving drug delivery to the brain, with potential applications in brain-targeted therapies.

Plasma doi: 10.3390/plasma8020018

Authors: Muhammad Waqar Ahmed Kainat Gul Sohail Mumtaz

Cold atmospheric plasma (CAP) acts as a powerful antibacterial tool in the food industry, effectively eliminating E. coli and a wide range of pathogens, including bacteria, viruses, fungi, spores, and biofilms in meat and vegetables. Unlike traditional bactericidal methods, CAP leverages an arsenal of reactive species, including reactive oxygen species (ROS) such as ozone (O3) and hydroxyl radicals (OH•), and reactive nitrogen species (RNS) like nitric oxide (NO•), alongside UV radiation and charged particles. These agents synergistically dismantle E. coli’s cell membranes, proteins, and DNA, achieving high degradation rates without thermal or chemical damage to processed food. This non-thermal, eco-friendly technology preserves food’s nutritional and sensory integrity, offering a transformative edge over conventional approaches. It emphasizes the critical need to optimize treatment parameters (exposure time, gas composition, power) to unlock CAP’s full potential. This review explores CAP’s effectiveness in degrading E. coli, emphasizing the optimization of treatment parameters for practical food industry applications and its potential as a scalable food safety solution. It is crucial to conduct further studies to enhance its implementation, establishing CAP as a fundamental element of advanced food processing technologies and a key measure for protecting public health.

Plasma doi: 10.3390/plasma8020017

Authors: Jean-Christophe Pain

We present analytical estimates for the maximum line strength in a transition array, as well as for the numbers of strong and weak lines. For that purpose, two main assumptions are used as concerns the line strength distribution. The first one, due to Porter and Thomas, is more suitable for J−J′ sets, where J is the total atomic angular momentum, and the second one, based on a decreasing-exponential modeling of the line-amplitude distribution, is more relevant for an entire transition array. We also review the different approximations of overlapping and blanketing (band model), insisting on the computation and properties of the Elsasser function. We compare different approximations of the Ladenburg–Reiche function giving the equivalent width of an ensemble of lines in a frequency bin and discuss the possibility of using statistical indicators, such as the Chernoff bound or the Gini coefficient (initially introduced in economics for the measurement of income inequality), in the statistical characterization of transition arrays.

Plasma doi: 10.3390/plasma8020016

Authors: Shi Chen Qishuo Zhang Qianyi Feng Ziyue Yu Jingyi Mai Hongping Zhang Lili Huang Chengjin Huang Mu Li

Tantalum is extensively used in inertial confinement fusion research for targets in radiation transport experiments, hohlraums in magnetized fusion experiments, and lining foams for hohlraums to suppress wall motions. To comprehend the physical processes associated with these applications, detailed information regarding the ionization composition and electrical conductivity of tantalum plasma across a wide range of densities and temperatures is essential. In this study, we calculate the densities of ionization species and the electrical conductivity of partially ionized, nonideal tantalum plasma based on a simplified theoretical model that accounts for high ionization states up to the atomic number of the element and the lowering of ionization energies. A comparison of the ionization compositions between tantalum and copper plasmas highlights the significant role of ionization energies in determining species populations. Additionally, the average electron–neutral momentum transfer cross-section significantly influences the electrical conductivity calculations, and calibration with experimental measurements offers a method for estimating this atomic parameter. The impact of electrical conductivity in the intermediate-density range on the laser absorption coefficient is discussed using the Drude model.

Plasma doi: 10.3390/plasma8020015

Authors: Abubakar Hamza Sadiq Md Jahangir Alam Mahedi Hasan Farhana Begum Tomoki Yamano Jaroslav Kristof Kazuo Shimizu

Low temperature plasmas (LTPs) generated at atmospheric pressure and room temperature have gained increasing attention in biomedical research due to their ability to control cellular behavior through the production of reactive oxygen and nitrogen species (RONS), electric fields, and UV radiation. Among several LTP configurations, dielectric barrier discharge (DBD) plasma has been extensively studied for its ability to stimulate controlled biological effects while maintaining low gas temperature, making it suitable for cell-based applications. This study designed a novel spiral-wound DBD plasma device to investigate the voltage-dependent effects of plasma discharge on DBC1.2 epithelial cells. Plasma was applied at 2 kVp-p, 3 kVp-p, and 4 kVp-p to evaluate its effect on cellular permeability, mitochondrial activity, viability, and apoptosis. FITC-dextran-70 (FD-70, MW: 70 kDa) was used as a model permeation marker to assess cellular uptake. The results showed a voltage-dependent increase in FD-70 uptake, suggesting improved plasma-assisted drug delivery. The cell mitochondrial activity, evaluated with a MT-1 MitoMP detection kit, revealed that plasma exposure at 2 kVp-p and 3 kVp-p slightly enhanced mitochondrial membrane potential (MMP), signifying increased metabolic and mitochondrial activity, whereas exposure at 4 kVp-p led to a reduction in MMP, suggesting oxidative stress and early apoptosis. Early and late apoptosis was further assessed using FITC Annexin-V and propidium iodide (PI). The results showed enhanced cell viability and a reduced apoptotic cell at 2 kVp-p and 3 kVp-p plasma exposure when compared to the control. However, at 4 kV, there was a decline in cell viability and an increase in apoptosis, suggesting a shift towards plasma-induced cytotoxicity. This study established a safe plasma exposure threshold for DBC1.2 cells and explored the potential use of a spiral-wound DBD plasma device for biomedical applications, particularly in drug delivery and cell modulation.

Plasma doi: 10.3390/plasma8020014

Authors: Takaki Miyamoto Jeanielle Amurao Eiji Minami Haruo Kawamoto

Lignin is a natural aromatic macromolecule present in wood and an abundant resource on Earth, yet it is hardly used. In this study, an aqueous solution plasma treatment was investigated for the catalyst-free production of valuable chemicals from lignin. To elucidate the decomposition mechanism, the aqueous solution plasma treatment was applied to the fundamental lignin aromatic model compounds—phenol, guaiacol, and syringol. The results showed that the decomposition rate followed the order syringol > guaiacol > phenol, indicating that electron-donating methoxy groups enhance reactivity. These aromatic model compounds underwent hydroxylation at the ortho and para positions, oxidative ring cleavage, and fragmentation, leading to the formation of various dicarboxylic acids, primarily oxalic acid. All these reactions were promoted by hydroxyl radicals generated from water. Ultimately, decarbonylation and decarboxylation of carboxyl groups resulted in gasification, mainly producing H2, CO, and CO2. These results provide fundamental insights into lignin decomposition and demonstrate that aqueous solution plasma is a promising method for producing dicarboxylic acids from lignin under mild conditions without catalysts.

Plasma doi: 10.3390/plasma8020013

Authors: Weihua Jiang

This review article is the continuation of a previous publication, by the same author, on one dimensional theory of space charge effect and virtual cathode. The virtual cathode is known to be unstable. However, the process of virtual cathode oscillation is very complicated both physically and mathematically. No satisfactory theoretical model exists that can fully describe the oscillatory behavior of the virtual cathode. On the other hand, computer simulations allow us to numerically observe this phenomenon and establish certain relations between the electron beam parameters and the virtual cathode characteristics. This article explains the detailed procedure of numerical modeling by dealing with the one-dimensional case as an example. A sample code written in the C language is attached at the end following the main text. This article is expected to serve as a reference for young researchers and students who are interested in computer simulations of intense particle beams and high-power microwave generation.

Plasma doi: 10.3390/plasma8010012

Authors: Dmitry Levko

This paper reviews the state of the art of our understanding of the mechanisms of runaway electron generation in pressurized gases from the numerical modeling perspective. Since the energy relaxation length of these electrons is comparable to the interelectrode spacing, these electrons can be captured only using the kinetic approach. Therefore, only the results from kinetic models are discussed here. Special attention is given to pulsed discharges, which play an important role in modern industry. It is concluded that the mechanisms of runaway electron generation are defined by the gap overvoltage and the discharge gap geometry. For small and moderate overvoltages, runaway electrons are primarily generated at the heads of fast ionization waves or streamers. Due to their long energy relaxation length, these electrons can pre-ionize the discharge gap far from their origin, accelerating ionization and starting new avalanches. At high overvoltages, cathode surface irregularities enhance the local electric field, leading to electron emission into the interelectrode space. These electrons, injected into the strong electric field, gain high energy and reach discharge walls with extremely high energies measuring tens and hundreds of electron volts. These electrons not only pre-ionize the gas but also stimulate the emission of high-energy photons, which can further contribute to the pre-ionization of the discharge gap.

Plasma doi: 10.3390/plasma8010011

Authors: Jiří Fujera Petr Hoffer Václav Prukner Milan Šimek

Plasma-assisted treatment is a potentially interesting technology for advanced seed processing. In this work, we address the issue of defining and quantifying the plasma dose during the exposure of seeds to microdischarges formed in a barrier discharge configuration fed with synthetic air at atmospheric pressure. Using advanced imaging and other optoelectrical diagnostics, we identify suitable conditions for the formation of microdischarges developing exclusively between the powered electrode and the seed coat, which allows for the relatively accurate quantification of the plasma dose for an individual barley seed. In addition to determining the microdischarge energy/power consumed to treat a single seed during controlled exposure, we also provide an estimate of the electric field and gas temperature, which are key parameters that can affect seed viability. In this way, each individually exposed seed can be linked to the exact exposure time, total number, energy, and temperature of the microdischarges that came into contact with it. This is fundamentally different from conventional “averaging” approaches based on the simultaneous exposure of many seeds, which makes it virtually impossible to correlate the responses of individual seeds with the corresponding individual plasma dose. Finally, we propose a minimal treatment protocol that could allow for the more direct interpretation of the results of subsequent biological tests to reveal seed responses to specific plasma–chemical stimuli during germination and seedling growth.

Plasma doi: 10.3390/plasma8010010

Authors: Blake Haist Richard E. Wirz

Network analysis is a convenient method for analyzing cold atmospheric plasma (CAP) devices across a wide range of operating conditions. By using frequency and voltage as nodes in the network, edges are formed between nodes when the combination of voltage and frequency results in an ignited plasma jet. Singular value decomposition is used to identify modalities in the network that are representative of operational modes in the plasma jet. An analysis of the spectra produced by the jet provides validation of the operational modes and shows that voltage and frequency predominately affect the operation of the jet with remarkable independence.

Plasma doi: 10.3390/plasma8010009

Authors: Aleksey Chaplygin Mikhail Yakimov Sergey Vasil’evskii Mikhail Kotov Ilya Lukomskii Semen Galkin Andrey Shemyakin Nikolay Solovyov Anatoly Kolesnikov

This paper investigates a novel combined laser and plasma heating test technique. Integrating the 1.5 kW Raycus RFL-C1500 laser source into the VGU-4 Inductively Coupled Plasma Facility (IPMech RAS) allowed the study of fine-grain MPG-7 graphite ablation in the high-temperature range from 2920 to 3865 K under exposure to subsonic nitrogen plasma flow and combined exposure to nitrogen plasma flow and laser irradiation. Graphite nitridation and sublimation were observed. The subsonic nitrogen plasma flow was characterized by numerical modeling, probes, and spectral measurements. The proposed experimental approach is promising for simulating the entry conditions of planetary mission vehicles into different atmospheres.

Plasma doi: 10.3390/plasma8010008

Authors: Espedito Vassallo Miriam Saleh Matteo Pedroni Anna Cremona Dario Ripamonti

A capacitively coupled radio-frequency argon plasma, used for tungsten sputtering deposition, is characterized using Langmuir probe measurements. Druyvesteyn’s method is used to evaluate plasma parameters through the integral of the Electron Energy Distribution Function (EEDF). In the pressure range analyzed (0.6–10 Pa), the obtained distributions are not Maxwellian, which suggests some depletion of electrons with higher energies. The obtained plasma parameters are compared with those derived from the graphical method. The electron temperature obtained via the graphical method is always lower than the effective temperatures derived from EEDFs, and vice versa, the electron density is overestimated by the graphical method. Optical Emission Spectroscopy is used to monitor the atoms sputtered in the plasma process. The behavior of excited species correlates with the plasma parameters.

Plasma doi: 10.3390/plasma8010007

Authors: Richard Morrow

An implicit flux-corrected transport (FCT) and diffusion algorithm was developed and used in many gas discharge calculations. Such calculations require the use of a fine mesh where the electric field changes rapidly; that is, near electrodes or in a streamer front. If diffusion is included using an explicit method, then the von Neumann stability condition severely limits the time-step that can be used; however, this limitation does not apply to implicit methods. Further, for gas discharge calculations including space-charge effects, it is necessary to solve the continuity equations with no negative number densities nor point-by-point oscillation in the number density. This is because the electron number densities are finely balanced with the ion number densities to determine the space-charge distribution and hence the electric field which drives the motion of the particles. An efficient way to solve the particle transport equation, with the required properties, is to use FCT. The most accurate form of FCT developed by the author is implicit fourth-order FCT; hence, the method presented incorporates implicit diffusion into the implicit fourth-order FCT scheme to produce a robust algorithm that has been successfully used in many calculations.

Plasma doi: 10.3390/plasma8010006

Authors: Mobish A. Shaji Francis Eboh Alexander Rabinovich Liran Dor Alexander Fridman

Municipal waste gasification presents a promising avenue to extract useful energy from waste through syngas. This technology’s application is limited by tar formation (long-chain hydrocarbons), which can decrease energy conversion efficiency and applications of raw syngas. Non-thermal plasma-based tar degradation is a simple and cost-effective alternative to existing thermal and catalytic tar mitigation methods. While plasma stimulates tar reformation reactions like steam reformation, there are thermodynamic energy requirements associated with these endothermic processes. Determining thermodynamic energy requirements and the equilibrium composition of products during tar reformation can aid with the proper optimization of the treatment process. In the present study, thermodynamic modeling and experimental validation are conducted to study energy requirements and product formation during the plasma-assisted steam reformation of tar present in raw syngas with an inlet temperature of 300 °C and 30% moisture content. The thermodynamic study evaluated the effect of adding air into the system (to increase the temperature by oxidizing a portion of raw syngas). Results show that up to 75% of energy requirement can be brought down by adding up to 30% air; experimental validation using gliding arc discharge with 30% air addition agrees with the thermodynamic model finding. The thermodynamic model predicted an increase in H2 and CO concentration with the degradation of tar, but experimental validation reported a reduction in H2 and CO concentration with the degradation of tar, as syngas was consumed to increase the temperature to support oxidation, owing to the low temperature (300 °C) and significant moisture presence (~30%) of raw syngas analyzed in this study.

Plasma doi: 10.3390/plasma8010005

Authors: Mahrukh Mahrukh Sen-Hui Liu Li Zhang Sohail Husnain Cheng-Chung Yang Xiao-Tao Luo Chang-Jiu Li

This study employs computational fluid dynamics (CFD) to analyze the in-flight dynamics of particles in an Ar–H2 atmospheric plasma spray (APS) torch with a modified diverging-type nozzle. The focus is on optimizing injection parameters—plasma gas flow rates, input power, and carrier gas flow rates—to enhance coating microstructure and deposition efficiency by achieving superheated molten metal droplets. Using a discrete phase model, the heat and momentum transfer of Ni/Al/C (2 wt.% diamond) composite powders (30–110 µm) within the plasma jet were simulated. Results show that particle characteristics, such as temperature and oxidation, can be controlled by adjusting plasma jet temperature (T∞) and velocity (U∞). Smaller particles heat faster, reaching higher temperatures with increased evaporation and oxidation rates. The modified nozzle enables Ni-based alloy particles to reach ~2500 °C, reducing oxygen inclusion in the plasma jet core. This setup allows for the control of the onset of carbon and oxygen reactions, wherein carbon serves as a sacrificial element, protecting the base alloy elements (such as aluminum) from excessive oxidation.

Plasma doi: 10.3390/plasma8010004

Authors: Alfred YiuFai Wong Chun-Ching Shih

An approach to achieve nuclear fusion utilizing the formation of high densities of electrons and neutrals is described. The abundance of low energy free electrons produces intense electric fields that reduce the Coulomb barrier in nuclear fusion. Meanwhile, high-density rotating neutrals provide high centrifugal forces to achieve the extreme pressure gradients of electrons and consequent negative electric fields to reduce the ion repulsive Coulombic fields. These high-density neutrals also provide better stability and higher reaction rates. Ion–neutral coupling is responsible for the control of neutral dynamics. Since high-frequency excitations favor the generation of free electrons, pulsed operations are recommended to achieve fusion with higher gains.

Plasma doi: 10.3390/plasma8010003

Authors: Manoj K. Deka Balaram Pradhan Apul N. Dev Deepsikha Mahanta Jalil Manafian Khaled H. Mahmoud

In this study, the effects of pressure anisotropy and viscosity on the propagation of shock waves in spin-polarized degenerate quantum magnetoplasma are studied under the influence of the streaming energy of ion beams. The effects of different suitable plasma parameters on the shock wave’s potential profile are studied using the steady state solution of the Zakharov–Kuznetsov–Burgers (Z–K–B) equation, as well as the numerical simulation of the governing non-linear Z–K–B equation. First-order analysis of the non-linear wave propagation depicted a new beam-induced stable mode whose Mach number may be subsonic or supersonic depending on the anisotropic pressure combination in the presence of different spin density polarization ratios. This is the first observation of this new beam-induced stable mode in ion beam plasma, apart from the other existing modes of ion beam plasma systems, namely, the fast beam mode, the slow beam mode, the inherent ion acoustic mode, and the coupled mode, which also has unique propagation characteristics compared to the other modes. The spin density polarization ratio of spin-up and spin-down electrons have an unprecedented effect on the polarity and the direction of propagation of different shock wave modes in such plasma systems. Apart from the spin effect, anisotropic pressure combinations, as well as the viscosity of ions and ion beams, also play an outstanding role in controlling the nature of propagation of shock waves, especially in the newly detected beam-induced stable mode, and depending on the viscosity parameters of ions and ion beams, both oscillatory and monotonic shock waves can propagate in such plasma.

Plasma doi: 10.3390/plasma8010002

Authors: Spiros Alexiou

The purpose of this paper is to address conflicting results regarding a simple criterion that has been proposed as decisive in determining whether accounting for spiralling electron trajectories increases or decreases the widths of hydrogen lines in a parameter range relevant to the spectral lines of white dwarfs. We analyse the claims in detail and also provide explicit calculations. It is shown that the recent attempts to justify a simple theory are erroneous and miss important physics.

Plasma doi: 10.3390/plasma8010001

Authors: Juan Long Tiejun Wang Fukang Yin Yaoxiang Liu Yingxia Wei Chengpu Liu Yuxin Leng

The evolution of the THz waveform generated from the two-color air filament was experimentally investigated by moving an iris along the plasma channel. By taking the differentiation of the measured THz waveforms, the local longitudinally resolved THz waves along a 54 mm-long filament were obtained. The local THz pulse underwent periodic phase shifts. A theoretical deduction indicates that the phase shifts are mainly caused by the dispersion in the plasma channel which plays a dominant role in the evolution of the local THz waveforms.

Plasma doi: 10.3390/plasma7040053

Authors: Sagi Turiel Alexander Gribov Daniel Maler Yakov E. Krasik

Theta Pinch is one of the promising methods for the generation of hot and dense plasma. In this paper, we describe the results of experimental research on a small-scale Theta Pinch created with Helium or Hydrogen plasmas. Different plasma diagnostics, namely, optical, microwave cut-off, laser interferometry, visible spectroscopy, Thomson scattering, and Laser-Induced Fluorescence were used to characterize the time- and space-resolved evolution of the plasma parameters, and the specific features of these diagnostic results obtained are discussed. The measured plasma density and the electron and ion temperature evolution, obtained by these various diagnostic tools, agree to a satisfactory level. These methods will be applied for studies of the parameters of the plasma in the device that is being developed by the nT-Tao company towards fusion energy.

Plasma doi: 10.3390/plasma7040052

Authors: S. K. H. Auluck R. Verma R. S. Rawat

This paper explores a recently proposed scalable z-pinch-based space propulsion engine in greater detail. This concept involves a “modified plasma focus with a tapered anode that transports current from a pulsed power source to a consumable portion of the anode in the form of a hypodermic needle tube continuously extruded along the axis of the device”. This tube is filled with a gas at a high pressure and also optionally with an axial magnetic field. The current enters the metal tube through its contact with the anode and returns to the cathode via the plasma sliding over its outer wall. The resulting rapid electrical explosion of the metal tube partially transfers current to a snowplough shock in the fill gas. Both the metal plasma and the fill gas form axisymmetric converging shells. Their interaction forms a hot and dense plasma of the fill gas surrounded by the metal plasma. Its ejection along the axis provides the impulse needed for propulsion. In a nonnuclear version, the fill gas could be xenon or hydrogen. Its unique energy density scaling could potentially lead to a neutron-deficient nuclear fusion drive based on the proton-boron avalanche fusion reaction by lining the tube with solid decaborane. In order to explore the inherent potential of this idea as a scalable space propulsion engine, this paper discusses its theoretical foundations and outlines the first iteration of a conceptual engineering design study for a proof-of-concept experiment based on the UNU-ICTP Plasma Focus facility at the Nanyang Technological University, Singapore.

Plasma doi: 10.3390/plasma7040051

Authors: Valentin Boutrouche Juan Pablo Trelles

The Atmospheric Pressure Glow Discharge (APGD) is a relatively simple and versatile plasma source used in a wide range of applications. Active cooling of the cathode can effectively mitigate instabilities, leading to glow-to-arc transitions. This study investigates the effect of varying the degree of cathode cooling in APGD with a planar cathode in helium. The plasma flow model incorporates mass conservation, chemical species transport, momentum conservation, conservation of thermal energy of heavy species and of electrons, and electrostatics. The model is applied to time-dependent simulations through a three-dimensional computational domain describing the whole discharge, without geometric symmetry or steady-state assumptions. Simulations of an experimentally characterized APGD explore the effects of electric current and cathode cooling—ranging from thermally insulated to extreme convective cooling. Results show the formation of an annular region with high electric field over the cathode surface under conditions of high current and low cooling.

Plasma doi: 10.3390/plasma7040050

Authors: Gurbax Singh Lakhina Satyavir Singh

An analysis of the Magnetospheric Multiscale (MMS) spacecraft data shows the presence of slow electrostatic solitary waves (SESWs) in the Earth’s plasma sheet, which have been interpreted as slow electron holes (SEHs). An alternative mechanism based on slow ion-acoustic solitons is proposed for these SESWs. The SESWs are observed in the region where double humped ion distributions and hot electrons co-exist. Our theoretical model considers the plasma in the SESW region to consist of hot electrons with a vortex distribution, core Maxwellian protons drifting parallel to the magnetic field, B and beam protons drifting anti-parallel to B. Parallel propagating nonlinear ion-acoustic waves are studied using the Sagdeev pseudopotential technique. The analysis yields four types of modes, namely, two slow ion-acoustic (SIA1 and SIA2) solitons and two fast ion-acoustic (FIA1 and FIA2) solitons. All solitons have positive potentials. Except the FIA1 solitons which propagate parallel to B; the other three types propagate anti-parallel to B. Good agreement is found between the amplitudes of electrostatic potential, the electric field, the widths and speed of SIA1 and SIA2 solitons, and the observed properties of SESWs by the MMS spacecraft.

Plasma doi: 10.3390/plasma7040049

Authors: Jamiah Thomas Alexander G. Volkov

A cold atmospheric-pressure He-plasma jet (CAPPJ) interacts with air and water, producing reactive oxygen and nitrogen species (RONS), including biologically active ions, radicals, and molecules such as NOx, H2O2, HNO3, HNO2, and O3. These compounds can activate interfacial redox processes in biological tissues. The CAPPJ can oxidize N2 to HNO3 and water to H2O2 at the interface between plasma and water. It can also induce the oxidation of water-soluble redox compounds in various organisms and in vitro. This includes salicylic acid, hydroquinone, and mixtures of antioxidants such as L (+)-ascorbic acid sodium salt with NADPH. It can react with redox indicators, such as ferroin, in a three-phase system consisting of air, CAPPJ, and water. Without reducing agents in the water, the CAPPJ will oxidize the water and decrease the pH of the solution. When antioxidants such as ascorbate, 1,4-hydroquinone, or NADPH are present in the aqueous phase, the CAPPJ oxidizes these substances first and then oxidizes water to H2O2. The multielectron mechanisms of the redox reactions in the plasma-air/water interfacial area are discussed and analyzed.

Plasma doi: 10.3390/plasma7040048

Authors: Svetlana Lazarova Tsvetelina Paunska Veselin Vasilev Khristo Tarnev Snejana Iordanova Stanimir Kolev

This work investigates CO2 conversion using atmospheric pressure low-current gliding discharges (GD). The following three modifications are studied: classic GD; magnetically accelerated GD (MAGD); and magnetically retarded GD (MRGD). In the latter two, permanent magnets produce a magnetic field that either accelerates or retards the discharge downstream. The gas flow is confined between quartz plates and the electrodes, with varying channel thicknesses. The magnetic configurations improve the performance compared to the classic GD, with up to 30% higher energy efficiency and up to a 50% higher conversion rate. The highest conversion rate is 11–12% with 10% energy efficiency, while the highest efficiency is 40% with 5% conversion, achieved with MRGD and MAGD at channel thicknesses of 2 mm and 3 mm.

Plasma doi: 10.3390/plasma7040047

Authors: Samira Amiri Khoshkar Vandani Lian Farhadian Alex Pennycuick Hai-Feng Ji

This work explores the polymerization of sodium 4-styrenesulfonate (NaSS) inside filter paper using dielectric barrier discharge (DBD) plasma and its application in the environmental field. The plasma-based technique, performed under mild conditions, solves common problems associated with conventional polymerization inside porous materials. The polymerization process was monitored using Fourier-transform infrared (FTIR) spectroscopy, which confirmed the consumption of double bonds, particularly in NaSS samples containing the optimal concentration of crosslinker divinyl benzene (DVB) (0.25% wt). Our work demonstrates the effectiveness and promise of DBD plasma as a substitute polymerization approach, especially for those in porous materials.

Plasma doi: 10.3390/plasma7040046

Authors: Tomiris Ismagambetova Mukhit Muratov Maratbek Gabdullin

In this paper, a new ion–ion screened potential was numerically calculated, which takes into account the ion core effect, i.e., the influence of strongly bound electrons. The pseudopotential model describing the shielding of ion cores and the screening using the density response function in the long wavelength approximation were used. To study the influence of this ion core effect on dense plasma’s structural and thermodynamic properties, the integral Ornstein–Zernike equation was solved in the hypernetted chain approximation. Our results show that the ion core has a significant impact on ionic radial distribution functions and thermodynamic properties when compared to the results obtained for the Yukawa potential, which does not take the ion core into account. Increasing the steepness of the core edge or decreasing the depth of the minimum leads to more pronounced screening due to bound electrons.

Plasma doi: 10.3390/plasma7040045

Authors: Xiao Song Brian Leard Zibo Wang Sai Tej Paruchuri Tariq Rafiq Eugenio Schuster

A free-boundary equilibrium solver for an axisymmetric tokamak geometry was developed based on the finite difference method and Picard iteration in a rectangular computational area. The solver can run either in forward mode, where external coil currents are prescribed until the converged magnetic flux function ψ(R,Z) map is achieved, or in inverse mode, where the desired plasma boundary, with or without an X-point, is prescribed to determine the required coil currents. The equilibrium solutions are made consistent with prescribed plasma parameters, such as the total plasma current, poloidal beta, or safety factor at a specified flux surface. To verify the mathematical correctness and accuracy of the solver, the solution obtained using this numerical solver was compared with that from an analytic fixed-boundary equilibrium solver based on the EAST geometry. Additionally, the proposed solver was benchmarked against another numerical solver based on the finite-element and Newton-iteration methods in a triangular-based mesh. Finally, the proposed solver was compared with equilibrium reconstruction results in DIII-D experiments.

Plasma doi: 10.3390/plasma7040044

Authors: Achraf Hani Karim Saber Alyen Abahazem Nofel Merbahi

This paper focuses on the determination of and improvement in the energy efficiency of plasma jets. To achieve this goal, an equivalent electrical model of a discharge reactor was developed, incorporating variable electrical parameters. The evolution of these parameters was determined by a mathematical identification method based on the recursive least squares algorithm (RLSA). The good agreement between the measured currents and those calculated using our electrical circuit, as well as the significant shapes of the estimated parameters, confirmed the accuracy of the parameter estimation method. This allowed us to use these parameters to determine the energy delivered to the reactor and that used during the discharge. This made our reactor controllable at the energy level. Thus, the ratio between these two energies allowed us to calculate the energy efficiency of plasma jets at each discharge instant. We also studied the effect of the applied voltage on efficiency. We found that efficiency was increased from 75% to 90% by increasing the voltage from 6 kV to 8 kV. All the results found in this work were interpreted and compared with the discharge behavior. This proposed model will help us to choose the right operating conditions to reach the maximum efficiency.

Plasma doi: 10.3390/plasma7040043

Authors: Sergey N. Grigoriev Alexander S. Metel Marina A. Volosova Enver S. Mustafaev Yury A. Melnik

Mechanical polishing of a product makes it possible to decrease the roughness of its surface to Ra = 0.001 µm by rubbing it with a fine abrasive contained in a fabric or other soft material. This method takes too much time and is associated with abrasive particles and microscopic scratches remaining after the processing. As such, a non-contact treatment with plasma and accelerated particles has been chosen in the present work to study polishing of ceramic samples. The small angular divergence of fast argon atoms made it possible to obtain the dependence of the sample roughness on the angle α of the atom’s incidence on its surface. It was found that the roughness weakly depends on the angle α, if not exceeding the threshold value αo ~ 50°, and rapidly decreases with increasing α > αo. Polishing with fast argon atoms leads to a noticeable decrease in friction of ceramic samples.

Plasma doi: 10.3390/plasma7040042

Authors: Hideaki Miura

Hall magnetohydrodynamic simulations are often carried out to study the subjects of instabilities and turbulence of space and nuclear fusion plasmas in which sub-ion-scale effects are important. Hall effects on a structure formation at a small scale in homogeneous and isotropic turbulence are reviewed together with a simple comparison to a (non-Hall) MHD turbulence simulation. A comparison between MHD and Hall MHD simulations highlights a fine structure in Hall MHD turbulence. This enhancement of the fine structures by the Hall term can be understood in relation to the whistler waves at the sub-ion scale. The generation and enhancement of fine-scale sheet, filamentary, or tubular structures do not necessarily contradict one another.