Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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18 pages, 5201 KiB  
Article
Pulsed Dielectric Barrier Discharges for Gas-Phase Composition Control: A Simulation Model
by Ruggero Barni, Prince Alex and Claudia Riccardi
Plasma 2023, 6(4), 735-752; https://doi.org/10.3390/plasma6040050 - 12 Dec 2023
Cited by 2 | Viewed by 1999
Abstract
We present results obtained from the numerical simulation of the gas-phase chemical kinetics in atmospheric pressure air non-equilibrium plasmas. In particular, we addressed the effect of the pulsed operation mode of a planar dielectric barrier discharge. As conjectured, the large difference in the [...] Read more.
We present results obtained from the numerical simulation of the gas-phase chemical kinetics in atmospheric pressure air non-equilibrium plasmas. In particular, we addressed the effect of the pulsed operation mode of a planar dielectric barrier discharge. As conjectured, the large difference in the time scales involved in the fast dissociation of molecules in plasmas and their subsequent reactions to produce stable chemical species makes the presence of a continuously repeated plasma production stage unnecessary and a waste of electrical power and efficiency. The results on NOx remediation, ozone production, water vapor and ammonia dissociation are discussed. A few comparisons with experimental findings in a dielectric barrier discharge reactor already used for applications are also briefly addressed. Our results clearly indicate a pattern for the optimization of the discharge using a carefully designed repetition rate and duty cycle. Full article
(This article belongs to the Special Issue Dielectric Barrier Discharges 2023)
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21 pages, 13047 KiB  
Article
Experimental Progress in the Development of a Metal Foil Pump for DEMO
by Yannick Kathage, Alejandro Vazquez Cortes, Stefan Merli, Christian Day, Thomas Giegerich, Stefan Hanke, Juri Igitkhanov, Andreas Schulz and Matthias Walker
Plasma 2023, 6(4), 714-734; https://doi.org/10.3390/plasma6040049 - 28 Nov 2023
Cited by 3 | Viewed by 2155
Abstract
Experimental findings to contribute to the preliminary design of a metal foil pump for fuel separation in the Direct Internal Recycling loop of the DEMO fusion device are presented. In parametric studies on a small-scale superpermeation experiment with a microwave plasma source and [...] Read more.
Experimental findings to contribute to the preliminary design of a metal foil pump for fuel separation in the Direct Internal Recycling loop of the DEMO fusion device are presented. In parametric studies on a small-scale superpermeation experiment with a microwave plasma source and two different metal foil materials, niobium Nb and vanadium V, a substantial increase in permeation with plasma power and with a decrease in pressure was observed. To ease operation in the typical fusion environment, in-situ heating procedures were developed to recover from impurity contamination. The temperature independence of plasma-driven permeation from 600 to 900 °C metal foil temperature was demonstrated. No proof of an isotopic effect for plasma-driven permeation of protium and deuterium could be found. The highest repeatable permeation flux achieved was 6.7 Pa∙m3/(m2∙s) or ~5.5 × 10−3 mol H/(m2∙s). The found compression ratios do safely allow the operation of the metal foil pump using ejector pumps as backing stages for the permeate. In a dedicated experimental setup, the operation of the plasma source in a strong magnetic field was tested. Parametric studies of pressure, power input, magnetic flux density, field gradient and field angle are presented. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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15 pages, 2801 KiB  
Article
Generation of Plasma-Activated Fluids for Successful Disinfection of Pseudomonas aeruginosa in Liquid Environments and Determination of Microbial Damage
by Mareike Hummert, Paul Leenders, Alexander Mellmann, Karsten Becker and Thorsten Kuczius
Plasma 2023, 6(4), 699-713; https://doi.org/10.3390/plasma6040048 - 21 Nov 2023
Cited by 4 | Viewed by 2322
Abstract
The application of the non-thermal atmospheric pressure plasma technology is a promising tool for microbial inactivation. During the activation process, many reactive substances and radicals arise associated with physicochemical changes in the fluid and massive pH drop. In this study, we analyzed and [...] Read more.
The application of the non-thermal atmospheric pressure plasma technology is a promising tool for microbial inactivation. During the activation process, many reactive substances and radicals arise associated with physicochemical changes in the fluid and massive pH drop. In this study, we analyzed and optimized plasma activation settings and conditions of water and liquids to obtain inactivation of the waterborne microorganism Pseudomonas aeruginosa in a liquid environment. The minimal electrical output was 60 Watt with 20 min activation time followed by 30 min contact time with 108 cells/mL. Using higher electrical power (>90 W) with a Lab Unit generating plasma-activated water, a shorter activation time (<10 min) was sufficient for bacterial inactivation. The organic and inorganic composition of the activated liquid with different mineral salt concentrations is of utmost importance for the yield of reactive species during the plasma activation process and consequently for the antimicrobial effect. Plasma-activated fluids with high organic and inorganic contents demonstrated lower inactivation efficiencies than low loaded fluids; yet antimicrobial efficacy could be achieved by increasing the electrical power and activation time. For sufficient inactivation of bacterial suspensions, at least half a volume unit of plasma-activated water had to be added after appropriately optimized activation. Further dilutions reduced the antimicrobial effect. PAW lost activity after being left standing for a prolonged time after activation, so for maximizing the antimicrobial effect a direct use after activation is recommendable. Bacterial inactivation was shown by the absence of colony forming units on culture media and, at the molecular level, damage to the membrane and inactivation of enzymes were observed. Plasma-activated fluids demonstrated a high potential in applications as microbiological disinfectant in liquids. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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12 pages, 1213 KiB  
Article
Comparison of Saturation Rules Used for Gyrokinetic Quasilinear Transport Modeling
by Scott E. Parker, Calder S. Haubrich, Stefan Tirkas, Qiheng Cai and Yang Chen
Plasma 2023, 6(4), 611-622; https://doi.org/10.3390/plasma6040042 - 12 Oct 2023
Cited by 2 | Viewed by 2107
Abstract
Theory-based transport modeling has been widely successful and is built on the foundations of quasilinear theory. Specifically, the quasilinear expression of the flux can be used in combination with a saturation rule for the toroidal mode amplitude. Most transport models follow this approach. [...] Read more.
Theory-based transport modeling has been widely successful and is built on the foundations of quasilinear theory. Specifically, the quasilinear expression of the flux can be used in combination with a saturation rule for the toroidal mode amplitude. Most transport models follow this approach. Saturation rules are heuristic and difficult to rigorously derive. We compare three common saturation rules using a fairly accurate quasilinear expression for the fluxes computed using local linear gyrokinetic simulation. We take plasma parameters from experimental H-mode profiles and magnetic equilibrium and include electrons, deuterium, and carbon species. We find that the various saturation rules provide qualitatively similar behavior. This may help to explain why the different theory-based transport models can all predict core tokamak profiles reasonably well. Comparisons with nonlinear local and global gyrokinetic simulations are discussed. Full article
(This article belongs to the Special Issue New Insights into Plasma Theory, Modeling and Predictive Simulations)
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15 pages, 3273 KiB  
Article
Inclusion of Biological Targets in the Analysis of Electrical Characteristics of Non-Thermal Plasma Discharge
by Julia Sutter, Jascha Brettschneider, Sara Mamchur, Fred Krebs, Sophia Gershman and Vandana Miller
Plasma 2023, 6(3), 577-591; https://doi.org/10.3390/plasma6030040 - 15 Sep 2023
Cited by 4 | Viewed by 1875
Abstract
In Plasma Medicine studies, the effect of non-thermal plasma (NTP) on biological targets is typically correlated with the amount of stable reactive oxygen and nitrogen species produced in a liquid medium. The effect of NTP and the response of the biological target on [...] Read more.
In Plasma Medicine studies, the effect of non-thermal plasma (NTP) on biological targets is typically correlated with the amount of stable reactive oxygen and nitrogen species produced in a liquid medium. The effect of NTP and the response of the biological target on cellular redox mechanisms is overlooked in these investigations. Additionally, the influence of electrical properties of cells on the physical properties of NTP is neglected. Therefore, we used a floating electrode dielectric barrier discharge plasma to explore the impact of cell structure, size, and viability of the biological target on the physical properties of NTP. Lissajous figures were used to determine circuit capacitance and energy per cycle during NTP exposure of different cell suspensions. We show that both, structural integrity and active enzymic processes of cells change the electrical properties of NTP. Correlations were also drawn between NTP-produced hydrogen peroxide and nitrite with measured capacitance. Our studies indicate that the observed changes between different cell suspensions may be due to a feedback loop between the biological target and the NTP source. In future studies, a more detailed analysis is needed to improve the control of clinical NTP devices. Full article
(This article belongs to the Special Issue Dielectric Barrier Discharges 2023)
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11 pages, 1448 KiB  
Article
Plasma Energy Loss by Cathode Heat Conduction in a Vacuum Arc: Cathode Effective Voltage
by Isak I. Beilis
Plasma 2023, 6(3), 492-502; https://doi.org/10.3390/plasma6030034 - 11 Aug 2023
Cited by 2 | Viewed by 1335
Abstract
The importance of understanding the energy loss specifics by the cathode for vacuum arc metallic plasma generation and its applications were emphasized. To this end, the heat conduction losses per unit current were characterized by the cathode effective voltage uef, which [...] Read more.
The importance of understanding the energy loss specifics by the cathode for vacuum arc metallic plasma generation and its applications were emphasized. To this end, the heat conduction losses per unit current were characterized by the cathode effective voltage uef, which is weakly dependent on the current. In this paper, a physical model and a mathematical approach were developed to describe the energy dissipation due to heat conduction in the cathode body, which is heated by energy outflowed from the adjacent plasma. The arc plasma generation was considered by taking into account the kinetics of the heavy particle fluxes in the non-equilibrium layer near the vaporizing surface. The phenomena of electric sheath, heat and mass transfer at the cathode were taken into account. The self-consistent numerical analysis was performed with a system of equations for a copper cathode spot. The transient analysis starts from the spot initiation, modeled by the plasma arising at the initial time determined by the kind of arc triggering, up to spot development. The results of the calculations show that the cathode effective voltage uef is determined by the cathode temperature as a function of spot time. The calculated evolution of the voltage uef shows that the steady state of uef is approximately 7 V, and it is reached when the cathode temperature reaches a steady state at approximately one microsecond. This essential result provides an explanation for the good agreement with the experimental cathode effective voltage (6–8 V) measured for the arc duration from one millisecond up to a few seconds. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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12 pages, 2343 KiB  
Article
Isotope Detection in Microwave-Assisted Laser-Induced Plasma
by Ali M. Alamri, Jan Viljanen, Philip Kwong and Zeyad T. Alwahabi
Plasma 2023, 6(3), 466-477; https://doi.org/10.3390/plasma6030032 - 1 Aug 2023
Cited by 3 | Viewed by 1957
Abstract
Isotope detection and identification is paramount in many fields of science and industry, such as in the fusion and fission energy sector, in medicine and material science, and in archeology. Isotopic information provides fundamental insight into the research questions related to these fields, [...] Read more.
Isotope detection and identification is paramount in many fields of science and industry, such as in the fusion and fission energy sector, in medicine and material science, and in archeology. Isotopic information provides fundamental insight into the research questions related to these fields, as well as insight into product quality and operational safety. However, isotope identification with established mass-spectrometric methods is laborious and requires laboratory conditions. In this work, microwave-assisted laser-induced breakdown spectroscopy (MW-LIBS) is introduced for isotope detection and identification utilizing radical and molecular emission. The approach is demonstrated with stable B and Cl isotopes in solids and H isotopes in liquid using emissions from BO and BO2, CaCl, and OH molecules, respectively. MW-LIBS utilizes the extended emissive plasma lifetime and molecular-emission signal-integration times up to 900 μs to enable the use of low (~4 mJ) ablation energy without compromising signal intensity and, consequently, sensitivity. On the other hand, long plasma lifetime gives time for molecular formation. Increase in signal intensity towards the late microwave-assisted plasma was prominent in BO2 and OH emission intensities. As MW-LIBS is online-capable and requires minimal sample preparation, it is an interesting option for isotope detection in various applications. Full article
(This article belongs to the Special Issue Advances in Laser Plasma Spectroscopy Applications)
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24 pages, 2059 KiB  
Article
Validating the Multi-Mode Model’s Ability to Reproduce Diverse Tokamak Scenarios
by Tariq Rafiq, Zibo Wang, Shira Morosohk, Eugenio Schuster, Jan Weiland, Wilkie Choi and Hyun-Tae Kim
Plasma 2023, 6(3), 435-458; https://doi.org/10.3390/plasma6030030 - 24 Jul 2023
Cited by 4 | Viewed by 1636
Abstract
A large-scale validation exercise was conducted to assess the multi-mode model (MMM) anomalous transport model in the integrated modeling code TRANSP. The validation included 6 EAST discharges, 17 KSTAR discharges, 72 JET ITER-like wall D-D discharges, and 4 DIII-D fusion plasma discharges. Using [...] Read more.
A large-scale validation exercise was conducted to assess the multi-mode model (MMM) anomalous transport model in the integrated modeling code TRANSP. The validation included 6 EAST discharges, 17 KSTAR discharges, 72 JET ITER-like wall D-D discharges, and 4 DIII-D fusion plasma discharges. Using the MMM, the study computed anomalous thermal, particle, impurity, and momentum transport within TRANSP. Simulations for EAST, KSTAR, and JET focused on electron and ion temperatures and safety factor profiles, while DIII-D simulations also considered electron density, toroidal rotation frequency, and flow shear. The predicted profiles were compared to experimental data at the diagnostic time, quantifying the comparison using root-mean-square (RMS) deviation and relative offsets. The study found an average RMS deviation of 9.3% for predicted electron temperature and 10.5% for ion temperature, falling within the experimental measurement error range 20%. The MMM model demonstrated computational efficiency and the ability to accurately reproduce a wide range of discharges, including various scenarios and plasma parameters, such as plasma density, gyroradius, collisionality, beta, safety factor and heating method variations. Full article
(This article belongs to the Special Issue New Insights into Plasma Theory, Modeling and Predictive Simulations)
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16 pages, 1222 KiB  
Article
Plasma-Assisted Abatement of Per- and Polyfluoroalkyl Substances (PFAS): Thermodynamic Analysis and Validation in Gliding Arc Discharge
by Mikaela J. Surace, Jimmy Murillo-Gelvez, Mobish A. Shaji, Alexander A. Fridman, Alexander Rabinovich, Erica R. McKenzie, Gregory Fridman and Christopher M. Sales
Plasma 2023, 6(3), 419-434; https://doi.org/10.3390/plasma6030029 - 17 Jul 2023
Cited by 5 | Viewed by 2632
Abstract
Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic organofluorine surfactants that are resistant to typical methods of degradation. Thermal techniques along with other novel, less energy-intensive techniques are currently being investigated for the treatment of PFAS-contaminated matrices. Non-equilibrium plasma is one [...] Read more.
Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic organofluorine surfactants that are resistant to typical methods of degradation. Thermal techniques along with other novel, less energy-intensive techniques are currently being investigated for the treatment of PFAS-contaminated matrices. Non-equilibrium plasma is one technique that has shown promise for the treatment of PFAS-contaminated water. To better tailor non-equilibrium plasma systems for this application, knowledge of the energy required for mineralization, and in turn the roles that plasma reactive species and heat can play in this process, would be useful. In this study, fundamental thermodynamic equations were used to estimate the enthalpies of reaction (480 kJ/mol) and formation (−4640 kJ/mol) of perfluorooctanoic acid (PFOA, a long-chain legacy PFAS) in water. This enthalpy of reaction estimate indicates that plasma reactive species alone cannot catalyze the reaction; because the reaction is endothermic, energy input (e.g., heat) is required. The estimated enthalpies were used with HSC Chemistry software to produce a model of PFOA defluorination in a 100 mg/L aqueous solution as a function of enthalpy. The model indicated that as enthalpy of the reaction system increased, higher PFOA defluorination, and thus a higher extent of mineralization, was achieved. The model results were validated using experimental results from the gliding arc plasmatron (GAP) treatment of PFOA or PFOS-contaminated water using argon and air, separately, as the plasma gas. It was demonstrated that PFOA and PFOS mineralization in both types of plasma required more energy than predicted by thermodynamics, which was anticipated as the model did not take kinetics into account. However, the observed trends were similar to that of the model, especially when argon was used as the plasma gas. Overall, it was demonstrated that while energy input (e.g., heat) was required for the non-equilibrium plasma degradation of PFOA in water, a lower energy barrier was present with plasma treatment compared to conventional thermal treatments, and therefore mineralization was improved. Plasma reactive species, such as hydroxyl radicals (OH) and/or hydrated electrons (e(aq)), though unable to accelerate an endothermic reaction alone, likely served as catalysts for PFOA mineralization, helping to lower the energy barrier. In this study, the activation energies (Ea) for these species to react with the alpha C–F bond in PFOA were estimated to be roughly 1 eV for hydroxyl radicals and 2 eV for hydrated electrons. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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15 pages, 1345 KiB  
Article
A Boltzmann Electron Drift Diffusion Model for Atmospheric Pressure Non-Thermal Plasma Simulations
by Arturo Popoli, Fabio Ragazzi, Giacomo Pierotti, Gabriele Neretti and Andrea Cristofolini
Plasma 2023, 6(3), 393-407; https://doi.org/10.3390/plasma6030027 - 7 Jul 2023
Cited by 4 | Viewed by 1900
Abstract
We introduce a fluid computational model for the numerical simulation of atmospheric pressure dielectric barrier discharge plasmas. Ion and neutral species are treated with an explicit drift diffusion approach. The Boltzmann relation is used to compute the spatial distribution of electrons as a [...] Read more.
We introduce a fluid computational model for the numerical simulation of atmospheric pressure dielectric barrier discharge plasmas. Ion and neutral species are treated with an explicit drift diffusion approach. The Boltzmann relation is used to compute the spatial distribution of electrons as a function of the electrostatic potential and the ionic charge density. This technique, widely used to speed up particle and fluid models for low-pressure conditions, poses several numerical challenges for high-pressure conditions and large electric field values typical of applications involving atmospheric-pressure plasmas. We develop a robust algorithm to solve the non-linear electrostatic Poisson problem arising from the Boltzmann electron approach under AC electric fields based on a charge-conserving iterative computation of the reference electric potential and electron density. We simulate a volumetric reactor in dry air, comparing the results yielded by the proposed method with those obtained when the drift diffusion approach is used for all charged species, including electrons. We show that the proposed methodology retains most of the physical information provided by the reference modeling approach while granting a substantial advantage in terms of computation time. Full article
(This article belongs to the Special Issue Dielectric Barrier Discharges 2023)
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11 pages, 520 KiB  
Article
What Machine Learning Can and Cannot Do for Inertial Confinement Fusion
by Baolian Cheng and Paul A. Bradley
Plasma 2023, 6(2), 334-344; https://doi.org/10.3390/plasma6020023 - 1 Jun 2023
Cited by 2 | Viewed by 2539
Abstract
Machine learning methodologies have played remarkable roles in solving complex systems with large data, well-defined input–output pairs, and clearly definable goals and metrics. The methodologies are effective in image analysis, classification, and systems without long chains of logic. Recently, machine-learning methodologies have been [...] Read more.
Machine learning methodologies have played remarkable roles in solving complex systems with large data, well-defined input–output pairs, and clearly definable goals and metrics. The methodologies are effective in image analysis, classification, and systems without long chains of logic. Recently, machine-learning methodologies have been widely applied to inertial confinement fusion (ICF) capsules and the design optimization of OMEGA (Omega Laser Facility) capsule implosion and NIF (National Ignition Facility) ignition capsules, leading to significant progress. As machine learning is being increasingly applied, concerns arise regarding its capabilities and limitations in the context of ICF. ICF is a complicated physical system that relies on physics knowledge and human judgment to guide machine learning. Additionally, the experimental database for ICF ignition is not large enough to provide credible training data. Most researchers in the field of ICF use simulations, or a mix of simulations and experimental results, instead of real data to train machine learning models and related tools. They then use the trained learning model to predict future events. This methodology can be successful, subject to a careful choice of data and simulations. However, because of the extreme sensitivity of the neutron yield to the input implosion parameters, physics-guided machine learning for ICF is extremely important and necessary, especially when the database is small, the uncertain-domain knowledge is large, and the physical capabilities of the learning models are still being developed. In this work, we identify problems in ICF that are suitable for machine learning and circumstances where machine learning is less likely to be successful. This study investigates the applications of machine learning and highlights fundamental research challenges and directions associated with machine learning in ICF. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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14 pages, 12225 KiB  
Article
EUV/VUV Spectroscopy for the Study of Carbon Impurity Transport in Hydrogen and Deuterium Plasmas in the Edge Stochastic Magnetic Field Layer of Large Helical Device
by Tetsutarou Oishi, Shigeru Morita, Masahiro Kobayashi, Gakushi Kawamura, Yasuko Kawamoto, Tomoko Kawate, Suguru Masuzaki, Chihiro Suzuki and Motoshi Goto
Plasma 2023, 6(2), 308-321; https://doi.org/10.3390/plasma6020021 - 12 May 2023
Cited by 3 | Viewed by 1845
Abstract
The ergodic layer in the Large Helical Device (LHD) consists of stochastic magnetic fields exhibiting a three-dimensional structure that is intrinsically formed by helical coils. Spectroscopic diagnostics was employed in the extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) wavelength ranges to investigate emission [...] Read more.
The ergodic layer in the Large Helical Device (LHD) consists of stochastic magnetic fields exhibiting a three-dimensional structure that is intrinsically formed by helical coils. Spectroscopic diagnostics was employed in the extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) wavelength ranges to investigate emission lines of carbon impurities in both hydrogen (H) and deuterium (D) plasmas, aiming to elucidate the impact of distinct bulk ions on impurity generation and transport in the edge plasmas of the LHD. The emission intensity of carbon CIII, CIV, CV, and CVI lines is significantly higher in the D plasma compared to the H plasma, indicating a greater sputtering rate of carbon materials in the D plasma, resulting in a higher quantity of carbon impurities originating from the divertor plates. A Doppler profile measurement of the second order of CIV line emission (1548.20 × 2 Å) was attempted using a 3 m normal-incidence VUV spectrometer in the edge plasma at a horizontally elongated plasma position. The flow velocity reaches its maximum value close to the outermost region of the ergodic layer, and the observed flow direction aligns with the friction force in the parallel momentum balance. The flow velocity increases with the electron density in H plasmas, suggesting that the friction force becomes more dominant in the force balance at higher density regimes. This leads to an increase in the impurity flow, which can contribute to the impurity screening. In contrast, the flow velocity in the D plasma is smaller than that in the H plasma. The difference in flow values between D and H plasmas, when the friction force term dominates in the momentum balance, could be attributed to the mass dependence of the thermal velocity of the bulk ions. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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27 pages, 6355 KiB  
Article
From Repeatability to Self-Organization of Guided Streamers Propagating in a Jet of Cold Plasma
by Henri Decauchy and Thierry Dufour
Plasma 2023, 6(2), 250-276; https://doi.org/10.3390/plasma6020019 - 5 May 2023
Cited by 1 | Viewed by 2580
Abstract
In this work, a jet of cold plasma is generated in a device supplied in helium and powered with a high-voltage nanopulse power supply, hence generating guided streamers. We focus on the interaction between these guided streamers and two targets placed in a [...] Read more.
In this work, a jet of cold plasma is generated in a device supplied in helium and powered with a high-voltage nanopulse power supply, hence generating guided streamers. We focus on the interaction between these guided streamers and two targets placed in a series: a metal mesh target (MM) at floating potential followed by a metal plate target (MP) grounded by a 1500 Ω resistor. We demonstrate that such an experimental setup allows to shift from a physics of streamer repeatability to a physics of streamer self-organization, i.e., from the repetition of guided streamers that exhibit fixed spatiotemporal constants to the emergence of self-organized guided streamers, each of which is generated on the rising edge of a high-voltage pulse. Up to five positive guided streamers can be self-organized one after the other, all distinct in space and time. While self-organization occurs in the capillary and up to the MM target, we also demonstrate the existence of transient emissive phenomena in the inter-target region, especially a filamentary discharge whose generation is directly correlated with complexity order Ω. The mechanisms of the self-organized guided streamers are deciphered by correlating their optical and electrical properties measured by fast ICCD camera and current-voltage probes, respectively. For the sake of clarity, special attention is paid to the case where three self-organized guided streamers (α, β and γ) propagate at vα = 75.7 km·s–1, vβ = 66.5 km·s–1 and vγ = 58.2 km·s–1), before being accelerated in the vicinity of the MM target. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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13 pages, 23963 KiB  
Article
Characteristics of Double-Layer, Large-Flow Dielectric Barrier Discharge Plasma Source for Toluene Decomposition
by Mao Xu, Yohei Fukuyama, Kazuki Nakai, Zhizhi Liu, Yuki Sumiya and Akitoshi Okino
Plasma 2023, 6(2), 212-224; https://doi.org/10.3390/plasma6020016 - 3 Apr 2023
Cited by 4 | Viewed by 2798
Abstract
The direct decomposition of toluene-containing humidified air at large flow rates was studied in two types of reactors with dielectric barrier discharge (DBD) features in ambient conditions. A scalable large-flow DBD reactor (single-layer reactor) was designed to verify the feasibility of large-flow plasma [...] Read more.
The direct decomposition of toluene-containing humidified air at large flow rates was studied in two types of reactors with dielectric barrier discharge (DBD) features in ambient conditions. A scalable large-flow DBD reactor (single-layer reactor) was designed to verify the feasibility of large-flow plasma generation and evaluate its decomposition characteristics with toluene-containing humidified air, which have not been investigated. In addition, another large-flow DBD reactor with a multilayer structure (two-layer reactor) was developed as an upscale version of the single-layer reactor, and the scalability and superiority of the features of the multilayer structure were validated by comparing the decomposition characteristics of the two reactors. Consequently, the large-flow DBD reactor showed similar decomposition characteristics to those of the small-flow DBD reactor regarding applied voltage, flow velocity, flow rate, and discharge length, thus justifying the feasibility of large-flow plasma generation. Additionally, the two-layer reactor is more effective than the single-layer reactor, suggesting multilayer configuration is a viable scheme for further upscaled DBD systems. A high decomposition rate of 59.5% was achieved at the considerably large flow rate of 110 L/min. The results provide fundamental data and present guidelines for the implementation of the DBD plasma-based system as a solution for volatile organic compound abatement. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences 2023)
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19 pages, 2899 KiB  
Article
Plane Parallel Barrier Discharges for Carbon Dioxide Splitting: Influence of Discharge Arrangement on Carbon Monoxide Formation
by Ronny Brandenburg, Milko Schiorlin, Michael Schmidt, Hans Höft, Andrei V. Pipa and Volker Brüser
Plasma 2023, 6(1), 162-180; https://doi.org/10.3390/plasma6010013 - 6 Mar 2023
Cited by 8 | Viewed by 2522
Abstract
A planar volume dielectric barrier discharge (DBD) in pure carbon dioxide (CO2) for the formation of carbon monoxide (CO) is examined by combined electrical and CO density measurements. The influence of the type of electrode, the barrier material, the barrier thickness, [...] Read more.
A planar volume dielectric barrier discharge (DBD) in pure carbon dioxide (CO2) for the formation of carbon monoxide (CO) is examined by combined electrical and CO density measurements. The influence of the type of electrode, the barrier material, the barrier thickness, and the discharge gap on the plasma power and the CO formation is analyzed systematically. The electrical characterization by means of charge-voltage plots is based on the simplest equivalent circuit model of DBDs, extended by the so-called partial surface discharge effect and the presence of parallel parasitic capacitances. The stackable discharge arrangement in this study enables one to elucidate the influence of parasitic capacitances, which can be overlooked in the application of such plasma sources. The determination of the discharge voltage from charge-voltage plots and the validity of the so-called Manley power equation are revised by taking into account non-uniform coverage as well as parasitic capacitances. The energy yield (EY) of CO is analyzed and compared with the literature. No correlations of EY with the mean reduced electric field strength or the geometric parameters of the DBD arrangement are observed. Full article
(This article belongs to the Special Issue Dielectric Barrier Discharges 2023)
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23 pages, 7129 KiB  
Article
Application of Plasma Bridge for Grounding of Conductive Substrates Treated by Transferred Pulsed Atmospheric Arc
by Dariusz Korzec, Markus Hoffmann and Stefan Nettesheim
Plasma 2023, 6(1), 139-161; https://doi.org/10.3390/plasma6010012 - 5 Mar 2023
Cited by 1 | Viewed by 2326
Abstract
An atmospheric pressure plasma jet (APPJ) sustained by a pulsed atmospheric arc (PAA) transferred on an electrically conducting surface was operated with a mean power of 700 W, a pulse frequency of 60 kHz, and a gas mixture of N2 and H [...] Read more.
An atmospheric pressure plasma jet (APPJ) sustained by a pulsed atmospheric arc (PAA) transferred on an electrically conducting surface was operated with a mean power of 700 W, a pulse frequency of 60 kHz, and a gas mixture of N2 and H2 with up to 10% H2, flowing at 30 to 70 SLM. It was shown that the plasma bridge ignited between the grounded injector and electrically conducting and floating substrates can be used for electrical grounding. This allowed for arc transfer on such substrates. The plasma bridge was stable for Argon flow through the injector from 3 to 10 SLM. Its length was between 5 and 15 mm. The plasma bridge current was 350 mA. The copper contact pads on an alumina electronic board were treated using the plasma bridge sustained by Ar injection for grounding. First, an oxide film of about 65 nm was grown by a compressed dry air (CDA) plasma jet. Then, this film was reduced at a speed of 4 cm2/s by forming gas 95/5 (95% of N2 and 5% of H2) plasma jet. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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12 pages, 3294 KiB  
Article
Understanding the Role of Plasma Bullet Currents in Heating Skin to Mitigate Risks of Thermal Damage Caused by Low-Temperature Atmospheric-Pressure Plasma Jets
by Shunya Hashimoto, Hideo Fukuhara, Endre J. Szili, Chiaki Kawada, Sung-Ha Hong, Yuta Matsumoto, Tatsuru Shirafuji, Masayuki Tsuda, Atsushi Kurabayashi, Mutsuo Furihata, Hiroshi Furuta, Akimitsu Hatta, Keiji Inoue and Jun-Seok Oh
Plasma 2023, 6(1), 103-114; https://doi.org/10.3390/plasma6010009 - 27 Feb 2023
Cited by 4 | Viewed by 2729
Abstract
Low-temperature atmospheric-pressure plasma jets are generally considered a safe medical technology with no significant long-term side effects in clinical studies reported to date. However, there are studies emerging that show plasma jets can cause significant side effects in the form of skin burns [...] Read more.
Low-temperature atmospheric-pressure plasma jets are generally considered a safe medical technology with no significant long-term side effects in clinical studies reported to date. However, there are studies emerging that show plasma jets can cause significant side effects in the form of skin burns under certain conditions. Therefore, with a view of developing safer plasma treatment approaches, in this study we have set out to provide new insights into the cause of these skin burns and how to tailor plasma treatments to mitigate these effects. We discovered that joule heating by the plasma bullet currents is responsible for creating skin burns during helium plasma jet treatment of live mice. These burns can be mitigated by treating the mice at a further distance so that the visible plasma plume does not contact the skin. Under these treatment conditions we also show that the plasma jet treatment still retains its medically beneficial property of producing reactive oxygen species in vivo. Therefore, treatment distance is an important parameter for consideration when assessing the safety of medical plasma treatments. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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17 pages, 7958 KiB  
Review
Flexible Cold Atmospheric Plasma Jet Sources
by Carles Corbella, Sabine Portal and Michael Keidar
Plasma 2023, 6(1), 72-88; https://doi.org/10.3390/plasma6010007 - 16 Feb 2023
Cited by 6 | Viewed by 6513
Abstract
The properties of non-thermal atmospheric pressure plasma jets (APPJs) make them suitable for industrial and biomedical applications. They show many advantages when it comes to local and precise surface treatments, and there is interest in upgrading their performance for irradiation on large areas [...] Read more.
The properties of non-thermal atmospheric pressure plasma jets (APPJs) make them suitable for industrial and biomedical applications. They show many advantages when it comes to local and precise surface treatments, and there is interest in upgrading their performance for irradiation on large areas and uneven surfaces. The generation of charged species (electrons and ions) and reactive species (radicals), together with emitted UV photons, enables a rich plasma chemistry that should be uniform on arbitrary sample profiles. Lateral gradients in plasma parameters from multi-jets should, therefore, be minimized and addressed by means of plasma monitoring techniques, such as electrical diagnostics and optical emission spectroscopy analysis (OES). This article briefly reviews the main strategies adopted to build morphing APPJ arrays and ultra-flexible and long tubes to project cold plasma jets. Basic aspects, such as inter-jet interactions and nozzle shape, have also been discussed, as well as potential applications in the fields of polymer processing and plasma medicine. Full article
(This article belongs to the Special Issue Latest Review Papers in Plasma Science 2023)
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14 pages, 3890 KiB  
Article
Analysis and Mitigation of Pulse-Pile-Up Artifacts in Plasma Pulse-Height X-ray Spectra
by Taosif. Ahsan, Charles P. S. Swanson, Chris Galea, Sangeeta P. Vinoth, Tony Qian, Tal Rubin and Samuel A. Cohen
Plasma 2023, 6(1), 58-71; https://doi.org/10.3390/plasma6010006 - 2 Feb 2023
Cited by 1 | Viewed by 2619
Abstract
Pulse pile-up in pulse-height energy analyzers increases when the incident rate of pulses increases relative to the inverse of the dead time per pulse of the detection system. Changes in the observed energy distributions with incident rate and detector-electronics-formed pulse shape then occur. [...] Read more.
Pulse pile-up in pulse-height energy analyzers increases when the incident rate of pulses increases relative to the inverse of the dead time per pulse of the detection system. Changes in the observed energy distributions with incident rate and detector-electronics-formed pulse shape then occur. We focus on weak high energy tails in X-ray spectra, important for measurements on partially ionized, warm (50–500 eV average electron energy), pure hydrogen plasma. A first-principles two-photon pulse-pile-up model is derived specific to trapezoidal-shaped pulses; quantitative agreement is found between the measurements and the model’s predictions. The model is then used to diagnose pulse-pile-up tail artifacts and mitigate them in relatively low count-rate spectra. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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13 pages, 2975 KiB  
Article
Physical Properties of Plasma-Activated Water
by Mobish Shaji, Alexander Rabinovich, Mikaela Surace, Christopher Sales and Alexander Fridman
Plasma 2023, 6(1), 45-57; https://doi.org/10.3390/plasma6010005 - 30 Jan 2023
Cited by 8 | Viewed by 4549
Abstract
Recent observations of plasma-activated water (PAW)’s surfactant behavior suggest that the activation of water with non-equilibrium plasma can decrease the surface tension of the water. This suggested change to the surface tension also indicates that the addition of plasma can lead to changes [...] Read more.
Recent observations of plasma-activated water (PAW)’s surfactant behavior suggest that the activation of water with non-equilibrium plasma can decrease the surface tension of the water. This suggested change to the surface tension also indicates that the addition of plasma can lead to changes in the physical properties of the water, knowledge of which can expand existing PAW applications and open new ones. While the chemical behavior of PAW has been extensively analyzed, to the best of our knowledge the physical properties of PAW have not been investigated. This study focuses on the need for experimental determination of PAW’s physical properties—namely, surface tension, viscosity, and contact angle. The experimental results of this study show that the addition of plasma lowers the surface tension of water at room temperature, increases the viscosity of water at high temperatures, and lowers the contact angle of droplets on glass surfaces at room temperatures. Potential factors influencing these changes include plasma alteration of the mesoscopic structure of water at low temperatures and plasma additives acting as foreign particles in water at higher temperatures. Ultimately, this investigation demonstrates that the physical properties of water change due to plasma activation, which could lead to potential industrial applications of PAW as a surfactant or as a washing-out and cleaning agent. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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26 pages, 690 KiB  
Article
Hydrogen-, Helium-, and Lithium-like Bound States in Classical and Quantum Plasmas
by Werner Ebeling and Gerd Röpke
Plasma 2023, 6(1), 1-26; https://doi.org/10.3390/plasma6010001 - 28 Dec 2022
Cited by 3 | Viewed by 2251
Abstract
We study the effective interactions and the mass action constants for pair and triple associations in classical and quantum plasmas. Avoiding double counting, we derive new expressions for the mass action constants. The calculations resulted in values that were substantially smaller than the [...] Read more.
We study the effective interactions and the mass action constants for pair and triple associations in classical and quantum plasmas. Avoiding double counting, we derive new expressions for the mass action constants. The calculations resulted in values that were substantially smaller than the standard ones in relevant temperature ranges by up to 50 percent. On this basis, we determine the pressure of H, He and Li plasmas and the osmotic coefficient of electrolytes with higher charges such as, e.g., seawater. Classical and quantum Coulomb systems show strong similarities. The contributions in low orders with respect to the interaction e2 are suppressed by thermal and screening effects. The contributions of weakly bound states, near the continuum edge, to the mass action constants are reduced, replacing the exponential functions with cropped exponentials. The new mass action constants are consistent with well-known extended limiting cases of screening effects. We analyze classical examples including the salts CaCl2 and LaCl3, and a model of seawater including multiple associations. In the case of quantum systems, we follow the work of Planck–Brillouin–Larkin for H plasmas and study He and Li plasmas. The equation of state (EoS) for wide-density regions is obtained through the concatenation of the EoS for the low-density region of partial ionization with the EoS of degenerate plasmas, where all bound states are dissolved and Fermi, Hartree–Fock and Wigner contributions dominate. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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41 pages, 4900 KiB  
Review
A Comprehensive Review on Amplification of Laser Pulses via Stimulated Raman Scattering and Stimulated Brillouin Scattering in Plasmas
by Renju Miriam Cheriyan, Nikhil Varghese, R. S. Sooraj, Kavya H. Rao and N. Smijesh
Plasma 2022, 5(4), 499-539; https://doi.org/10.3390/plasma5040037 - 24 Nov 2022
Cited by 1 | Viewed by 2775
Abstract
The demand for high-intensity lasers has grown ever since the invention of lasers in 1960, owing to their applications in the fields of inertial confinement fusion, plasma-based relativistic particle accelerators, complex X-ray and gamma-ray sources, and laboratory astrophysics. To create such high-intensity lasers, [...] Read more.
The demand for high-intensity lasers has grown ever since the invention of lasers in 1960, owing to their applications in the fields of inertial confinement fusion, plasma-based relativistic particle accelerators, complex X-ray and gamma-ray sources, and laboratory astrophysics. To create such high-intensity lasers, free-running lasers were either Q-switched or mode-locked to increase the peak power to the gigawatt range. Later, chirped pulse amplification was developed, allowing the generation of peak power up to 1012 W. However, the next generation of high-intensity lasers might not be able to be driven by the solid-state technology alone as they are already operating close to their damage thresholds. In this scenario, concepts of amplification based on plasmas has the potential to revolutionize the laser industry, as plasma is already a broken-down medium, and hence does not pose any problems related to the damage thresholds. On the other hand, there are many other aspects that need to be addressed before developing technologies based on plasma-based amplification, and they are being investigated via theoretical and numerical methods and supported by several experiments. In this report, we review the prospects of employing plasma as the medium of amplification by utilising stimulated scattering techniques, such as the stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) techniques, to modulate high-power laser pulses, which would possibly be the key to the next generation of high-power lasers. The 1980s saw the commencement of research in this field, and possibilities of obtaining high peak powers were verified theoretically with the help of numerical calculations and simulations. The extent of amplification by these stimulated scattering schemes are limited by a number of instabilities such as forward Raman scattering (FRS), filamentation, etc., and here, magnetised plasma played an important role in counteracting these parasitic effects. The current research combines all these factors to experimentally realise a large-scale plasma-based amplifier, which can impact the high-energy laser industry in the near future. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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8 pages, 411 KiB  
Article
Effects of Plasma on Physical Properties of Water: Nanocrystalline-to-Amorphous Phase Transition and Improving Produce Washing
by Jinjie He, Alexander Rabinovich, Dmitri Vainchtein, Alexander Fridman, Christopher Sales and Mikhail N. Shneider
Plasma 2022, 5(4), 462-469; https://doi.org/10.3390/plasma5040034 - 3 Nov 2022
Cited by 2 | Viewed by 1981
Abstract
Washing fresh produce using Plasma-activated water recently became a promising eco-friendly alternative to using chemical additives such as Chlorine. We discuss the produce-washing experiments that illustrate that addition of plasma to washing water is a multi-faced phenomena. Not only it increases the sterilization [...] Read more.
Washing fresh produce using Plasma-activated water recently became a promising eco-friendly alternative to using chemical additives such as Chlorine. We discuss the produce-washing experiments that illustrate that addition of plasma to washing water is a multi-faced phenomena. Not only it increases the sterilization ability of water by killing pathogens, but it also has improved washibility: the ability to remove pathogens from the cleaning surface. We propose an explanation of these features based on the recently discoveries that many physical and chemical properties of water change their temperature dependence between about 35 and 60 degrees Celsius. In particular, heat conductance, light absorption, and surface tension all change their temperature dependence. These drastic changes were associated with water gradually changing its mesoscopic structure: while at the higher temperatures water is a uniform media (amorphous state), at the temperatures below transition it consists of many nano-to-micro-scale clusters (crystalline state). This transition is similar to the second order phase transition. In the present paper we propose that treating water with non-thermal plasma (adding plasma-created active compounds) can lower the temperature of the transition and thus cause a significant change in such physical quantities as surface tension, viscosity, freezing rate, and wettability and washability. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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11 pages, 9040 KiB  
Article
Plasma Deposition to Improve Barrier Performance of Biodegradable and Recyclable Substrates Intended for Food Packaging
by Espedito Vassallo, Matteo Pedroni, Marco Aloisio, Hao Chen, Giuseppe Firpo, Silvia Maria Pietralunga and Dario Ripamonti
Plasma 2022, 5(4), 451-461; https://doi.org/10.3390/plasma5040033 - 28 Oct 2022
Cited by 4 | Viewed by 2198
Abstract
The extensive application of biodegradable polymers in the food packaging industries was partially limited due to poor barrier performances. In the present work, we investigated the improvement of oxygen barrier performances by means of the deposition of a few nanometres of SiOx coatings [...] Read more.
The extensive application of biodegradable polymers in the food packaging industries was partially limited due to poor barrier performances. In the present work, we investigated the improvement of oxygen barrier performances by means of the deposition of a few nanometres of SiOx coatings on Poly(butylene succinate) (PBS) films. The coated samples produced by the plasma-enhanced chemical vapor deposition technique were tested in terms of morphology and composition of the surface and barrier properties. Barrier performances studied as a function of SiOx thickness were greatly improved and a reduction of at least 99% was achieved for oxygen transmission rate. In order to reduce the formation of residual stress between PBS substrate and SiOx coatings, a proper buffer layer (silicon organic SiOxCyHz) was used. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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15 pages, 7339 KiB  
Article
A Plasma-Based Decontamination Process Reveals Potential for an in-Process Surface-Sanitation Method
by Thomas Weihe, Uta Schnabel, Mathias Andrasch, Jörg Stachowiak, Frank Tübbecke and Jörg Ehlbeck
Plasma 2022, 5(3), 351-365; https://doi.org/10.3390/plasma5030027 - 6 Sep 2022
Cited by 4 | Viewed by 1918
Abstract
Methods, which use an indirect plasma treatment for the inactivation of microorganisms in foods, claim a vastly growing field of research. This paper presents a method that uses plasma-processed air (PPA) as a sanitizer. In addition to a sanitation concept for the decontamination [...] Read more.
Methods, which use an indirect plasma treatment for the inactivation of microorganisms in foods, claim a vastly growing field of research. This paper presents a method that uses plasma-processed air (PPA) as a sanitizer. In addition to a sanitation concept for the decontamination of produce in the value chain, the presented method offers a possible application as an “in-process” surface sanitation. PPA provides antimicrobial-potent species, which are predominantly reactive nitrogen species (RNS); this has an outstanding groove penetration property. In an experimental approach, surfaces, made from materials, which are frequently used for the construction of food-processing plants, were inoculated with different microorganisms. Listeria monocytogenes (ATCC 15313), Staphylococcus aureus (ATCC 6538), Escherichia coli (ATCC 10538), Salmonella enterica subsp. enterica serovar Typhimurium (ATCC 43971), and Salmonella enterica subsp. enterica serovar Enteritidis (ATCC 13076) are all microorganisms that frequently appear in foods and possess the risk for cross-contamination from the plant to the produce or vice versa. The contaminated samples were treated for various treatment times (1–5 min) with PPA of different antimicrobial potencies. Subsequently, the microbial load on the specimens was determined and compared with the load of untreated samples. As a result, reduction factors (RF) up to several log10-steps were obtained. Although surface and the bacterial strain showed an influence on the RF, the major influence was seen by a prolongation of the treatment time and an increase in the potency of the PPA. Full article
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18 pages, 5327 KiB  
Article
Simple Parametric Model for Calculation of Lateral Electromagnetic Loads in Tokamaks at Asymmetric Vertical Displacement Events (AVDE)
by Sergey Sadakov, Fabio Villone, Guglielmo Rubinacci and Salvatore Ventre
Plasma 2022, 5(3), 306-323; https://doi.org/10.3390/plasma5030024 - 25 Jul 2022
Cited by 1 | Viewed by 1868
Abstract
This paper describes a family of relatively simple numerical models for calculation of asymmetric electromagnetic (EM) loads at all tokamak structures and coils at asymmetric vertical plasma displacement events (AVDE). Unlike currently known AVDE studies concentrated on plasma physics, these models have a [...] Read more.
This paper describes a family of relatively simple numerical models for calculation of asymmetric electromagnetic (EM) loads at all tokamak structures and coils at asymmetric vertical plasma displacement events (AVDE). Unlike currently known AVDE studies concentrated on plasma physics, these models have a practical purpose to calculate detailed time-dependent patterns of AVDE-induced EM loads everywhere in the tokamak. They are built to intrinsically assure good-enough EM load balance (opposite net forces and torques for the Vacuum Vessel and the Magnets with zero total for the entire tokamak), as needed for consequent simulation of the tokamak’s dynamic response to AVDE, as well as for the development of tokamak monitoring algorithms and tokamak simulators. To achieve these practical goals, the models work in a manner of parametric study. They do not intervene in details of plasma physics, but run at widely varied input assumptions on AVDE evolution and severity. Their outputs will fill a library of ready-for-use lateral EM loads for multiple variants of AVDE evolution and severity. The tokamak physics community can select any variant from the library, and engineers can pick ready-for-use AVDE loads. Investigated here, EM models represent one already known approach and one newly suggested. The latter attempts to reflect the helical pattern of halo currents in plasma and delivers richer outcomes and, thus, can be preferred as the single practical model for parametric calculations. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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11 pages, 3472 KiB  
Article
Chamber with Inverted Electrode Geometry for Measuring and Control of Ion Flux-Energy Distribution Functions
by Christian Schulze, He Li, Leonie Mohn, Martin Müller and Jan Benedikt
Plasma 2022, 5(3), 295-305; https://doi.org/10.3390/plasma5030023 - 23 Jun 2022
Cited by 2 | Viewed by 4253
Abstract
Measurements of ion flux-energy distribution functions at the high sheath potential of the driven electrode in a classical low-pressure asymmetric capacitively coupled plasma are technically difficult as the diagnostic device needs to float with the applied radio frequency voltage. Otherwise, the ion sampling [...] Read more.
Measurements of ion flux-energy distribution functions at the high sheath potential of the driven electrode in a classical low-pressure asymmetric capacitively coupled plasma are technically difficult as the diagnostic device needs to float with the applied radio frequency voltage. Otherwise, the ion sampling is disturbed by the varying electric field between the grounded device and the driven electrode. To circumvent such distortions, a low-pressure plasma chamber with inverted electrode geometry, where the larger electrode is driven and the smaller electrode is grounded, has been constructed and characterized. Measurements of the ion flux-energy distribution functions with an energy-selective mass spectrometer at the high sheath potential of the grounded electrode are presented for a variety of conditions and ions. The potential for suppressing low-energy ions from resonant charge transfer collisions in the sheath by the dilution of the working gas is demonstrated. Additionally, the setup is supplemented by an inductively coupled plasma that controls the plasma density and consequently the ion flux to the substrate while the radio frequency bias controls the ion energy. At high ion energies, metal ions are detected as a consequence of the ionization of sputtered electrode material. The proposed setup opens a way to study precisely the effects of ion treatment for a variety of substrates such as catalysts, polymers, or thin films. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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15 pages, 1241 KiB  
Article
Impact of Internal Faraday Shields on RF Driven Hydrogen Discharges
by David Rauner, Dominikus Zielke, Stefan Briefi and Ursel Fantz
Plasma 2022, 5(3), 280-294; https://doi.org/10.3390/plasma5030022 - 21 Jun 2022
Cited by 3 | Viewed by 4051
Abstract
At RF plasma reactors operated at high power, internal Faraday shields are required to shield dielectric vessel or windows from erosion due to isotropic heat and particle fluxes. By utilizing a flexible and diagnostically well-equipped laboratory setup, crucial effects that accompany the application [...] Read more.
At RF plasma reactors operated at high power, internal Faraday shields are required to shield dielectric vessel or windows from erosion due to isotropic heat and particle fluxes. By utilizing a flexible and diagnostically well-equipped laboratory setup, crucial effects that accompany the application of internal Faraday shields at low-pressure hydrogen (and deuterium) RF discharges are identified and quantified in this contribution. Both an inductively coupled plasma (ICP) utilizing a helical coil and a low-field helicon discharge applying a Nagoya-type III antenna at magnetic fields of up to 12 mT are investigated. Discharges are driven at 4 MHz and in the pressure range between 0.3 and 10 Pa while the impact of the Faraday shields on both the RF power transfer efficiency and spectroscopically determined bulk plasma parameters (electron density and temperature, atomic density) is investigated. Three main effects are identified and discussed: (i) due to the Faraday shield, the measured RF power transfer efficiency is globally reduced. This is mainly caused by increased power losses due to induced eddy currents within the electrostatic shield, as accompanying numerical simulations by a self-consistent fluid model demonstrate. (ii) The Faraday shield reduces the atomic hydrogen density in the plasma by one order of magnitude, as the recombination rate of atoms on the metallic (copper) surfaces of the shield is considerably higher compared to the dielectric quartz walls. (iii) The Faraday shield suppresses the transition of the low-field helicon setup to a wave heated regime at the present conditions. This is attributed to a change of boundary conditions for wave propagation, as the plasma is in direct contact with the conductive surfaces of the Faraday shield rather than being operated in a laterally fully dielectric vessel. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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11 pages, 1601 KiB  
Article
Pulsed Spherical Tokamak—A New Approach to Fusion Reactors
by Mikhail Gryaznevich, Valery A. Chuyanov and Yuichi Takase
Plasma 2022, 5(2), 247-257; https://doi.org/10.3390/plasma5020019 - 18 May 2022
Cited by 5 | Viewed by 3638
Abstract
Traditionally, spherical tokamak (ST) reactors are considered to operate in a steady state. This paper analyses the advantages of a pulsed ST reactor. The methodology developed for conventional tokamak (CT) reactors is used and it is shown that advantages of a pulsed operation [...] Read more.
Traditionally, spherical tokamak (ST) reactors are considered to operate in a steady state. This paper analyses the advantages of a pulsed ST reactor. The methodology developed for conventional tokamak (CT) reactors is used and it is shown that advantages of a pulsed operation are even more pronounced in an ST reactor because of its ability to operate at a higher beta, therefore achieving a higher bootstrap current fraction, which, together with a lower inductance, reduces requirements for magnetic flux from the central solenoid for the plasma current ramp-up and sustainment. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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10 pages, 2621 KiB  
Article
Poly (O-Aminophenol) Produced by Plasma Polymerization Has IR Spectrum Consistent with a Mixture of Quinoid & Keto Structures
by Natalie M. Stuart and Karl Sohlberg
Plasma 2022, 5(2), 196-205; https://doi.org/10.3390/plasma5020015 - 14 Apr 2022
Viewed by 2350
Abstract
A vibrational analysis of various poly(o-aminophenol) structures has been undertaken using first principles methods. It is shown that a mixture of quinoid and keto forms of poly(o-aminophenol) gives rise to a simulated spectrum that replicates the experimental infrared spectra of plasma-produced poly(o-aminophenol) better [...] Read more.
A vibrational analysis of various poly(o-aminophenol) structures has been undertaken using first principles methods. It is shown that a mixture of quinoid and keto forms of poly(o-aminophenol) gives rise to a simulated spectrum that replicates the experimental infrared spectra of plasma-produced poly(o-aminophenol) better than either the quinoid or keto poly(o-aminophenol) spectra alone. An unassigned peak in the spectrum is attributed to hydrogen bonding to the silica substrate. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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8 pages, 791 KiB  
Article
Enhancement of Nuclear Fusion in Plasma Oscillation Systems
by Alfred YiuFai Wong and Chun-Ching Shih
Plasma 2022, 5(1), 176-183; https://doi.org/10.3390/plasma5010013 - 17 Mar 2022
Cited by 2 | Viewed by 3808
Abstract
Concepts of dynamic oscillations of positive and negative ions to enhance fusion reactions are examined in this paper. Collective oscillations of positive and negative ions produce large oscillating electrostatic fields and could provide a significant reduction of the Coulomb potential barrier between the [...] Read more.
Concepts of dynamic oscillations of positive and negative ions to enhance fusion reactions are examined in this paper. Collective oscillations of positive and negative ions produce large oscillating electrostatic fields and could provide a significant reduction of the Coulomb potential barrier between the two interacting species (such as hydrogen anion H− and B+ in the hydrogen-boron fusion reaction). The negative hydrogen ions can be produced by populating low-temperature electrons around the neutral hydrogen atoms in a rotation chamber. The existence of H− ensures the stability of the plasma and the effectiveness of fusion interactions between H− and B+. In this paper, theoretical analyses of such oscillations systems will be presented and the conditions for fusion enhancement are discussed. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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22 pages, 544 KiB  
Article
Adaptive Algorithm for the Generation of Superconfigurations in Hot-Plasma Opacity Calculations
by Jean-Christophe Pain
Plasma 2022, 5(1), 154-175; https://doi.org/10.3390/plasma5010012 - 4 Mar 2022
Cited by 5 | Viewed by 2899
Abstract
In hot plasmas, such as the ones encountered in astrophysics or laser-fusion studies, the number of ionic excited states may become huge, and the relevant electron configurations cannot always be handled individually. The Super Transition Array approach enables one to calculate the massic [...] Read more.
In hot plasmas, such as the ones encountered in astrophysics or laser-fusion studies, the number of ionic excited states may become huge, and the relevant electron configurations cannot always be handled individually. The Super Transition Array approach enables one to calculate the massic photo-absorption cross-section (or radiative opacity) in a statistical manner consisting of grouping configurations close in energy into superconfigurations. One of the main issues of the method, beyond its spectral resolution, is the determination of the most relevant configurations that contribute to opacity. In this work, we discuss different aspects of the generation of superconfigurations in a hot plasma and propose a new adaptive algorithm. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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19 pages, 8889 KiB  
Article
Application of Nitrogen Piezoelectric Direct Discharge for Increase in Surface Free Energy of Polymers
by Dariusz Korzec, Florian Hoppenthaler, Thomas Andres, Sophia Guentner and Simona Lerach
Plasma 2022, 5(1), 111-129; https://doi.org/10.3390/plasma5010009 - 9 Feb 2022
Cited by 6 | Viewed by 3226
Abstract
The subject of this study is the application of the piezoelectric direct discharge (PDD) operated with nitrogen to control the surface free energy (SFE) of polymers. The activation area, defined as the area of the zone reaching the SFE of 58 mN/m for [...] Read more.
The subject of this study is the application of the piezoelectric direct discharge (PDD) operated with nitrogen to control the surface free energy (SFE) of polymers. The activation area, defined as the area of the zone reaching the SFE of 58 mN/m for high-density polyethylene (HDPE) and poly (methyl methacrylate) (PMMA), is characterized. For HDPE, the activation area was characterized as a function of the distance from 1 to 16 mm, the nitrogen flow from 5 to 20 SLM, and the treatment time from 1 to 32 s. For larger distances, where SFE does not exceed 58 mN/m, the water contact angle is evaluated. The activation area for nitrogen PDD is typically a factor of 3 higher than for air with all other conditions the same. A maximum static activation area of 15 cm2 is reached. The plasma treatment of lens panels made of PMMA is presented as application example. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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16 pages, 2998 KiB  
Article
Plasma Co-Polymerization of HMDSO and Limonene with an Atmospheric Pressure Plasma Jet
by Gerrit Wulf, Bernd Mayer and Uwe Lommatzsch
Plasma 2022, 5(1), 44-59; https://doi.org/10.3390/plasma5010004 - 4 Jan 2022
Cited by 7 | Viewed by 3761
Abstract
Plasma co-polymers (co-p) were deposited with an atmospheric pressure plasma jet (APPJ) using a precursor mixture containing hexamethyldisiloxane (HMDSO) and limonene. A coating with fragments from both precursors and with siloxane, carbonyl and nitrogen functional groups was deposited. The flow rate of limonene [...] Read more.
Plasma co-polymers (co-p) were deposited with an atmospheric pressure plasma jet (APPJ) using a precursor mixture containing hexamethyldisiloxane (HMDSO) and limonene. A coating with fragments from both precursors and with siloxane, carbonyl and nitrogen functional groups was deposited. The flow rate of limonene was found to be an important parameter for plasma co-polymerization to tune the formation and structure of the functional groups. The FTIR and XPS analysis indicates that with increasing flow rate of limonene a higher proportion of carbon is bound to silicon. This is related to a stronger incorporation of fragments from limonene into the siloxane network and a weaker fragmentation of HMDSO. The formation mechanism of the nitroxide and carboxyl groups can be mainly differentiated into in-plasma and post-plasma reactions, respectively. Full article
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14 pages, 2015 KiB  
Article
The Effect of Excited Species on the Collisional Energy of Argon Inductively Coupled Plasmas: A Global Model Study
by Júlia Karnopp, Bernardo Magaldi, Julio Sagás and Rodrigo Pessoa
Plasma 2022, 5(1), 30-43; https://doi.org/10.3390/plasma5010003 - 4 Jan 2022
Cited by 6 | Viewed by 5008
Abstract
Global modeling of inductively coupled plasma (ICP) reactors is a powerful tool to investigate plasma parameters. In this article, the argon ICP global model is revisited to explore the effect of excited species on collisional energy through the study of different approaches to [...] Read more.
Global modeling of inductively coupled plasma (ICP) reactors is a powerful tool to investigate plasma parameters. In this article, the argon ICP global model is revisited to explore the effect of excited species on collisional energy through the study of different approaches to particle and energy balance equations. The collisional energy loss is much more sensitive to modifications in the balance equations than the electron temperature. According to the simulations, the multistep ionization reduces the collisional energy loss in all investigated reaction sets and the inclusion of heavy species reactions has negligible influence. The plasma parameters obtained, such as total energy loss and electron temperature, were compared with experimental results from the literature. The simulated cases that have more excited species and reactions in the energy balance are in better agreement with the experimental measurements. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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18 pages, 3883 KiB  
Article
The Nanosecond Impulsive Breakdown Characteristics of Air, N2 and CO2 in a Sub-mm Gap
by Ting Liu, Igor Timoshkin, Mark P. Wilson, Martin J. Given and Scott J. MacGregor
Plasma 2022, 5(1), 12-29; https://doi.org/10.3390/plasma5010002 - 30 Dec 2021
Cited by 6 | Viewed by 3130
Abstract
The present paper investigates the breakdown characteristics—breakdown voltage, with breakdown occurring on the rising edge of the applied HV impulses, and time to breakdown—for gases of significance that are present in the atmosphere: air, N2 and CO2. These breakdown characteristics [...] Read more.
The present paper investigates the breakdown characteristics—breakdown voltage, with breakdown occurring on the rising edge of the applied HV impulses, and time to breakdown—for gases of significance that are present in the atmosphere: air, N2 and CO2. These breakdown characteristics have been obtained in a 100 µm gap between an HV needle and plane ground electrode, when stressed with sub-µs impulses of both polarities, with a rise time up to ~50 ns. The scaling relationships between the reduced breakdown field Etip/N and the product of the gas number density and inter-electrode gap, Nd, were obtained for all tested gases over a wide range of Nd values, from ~1020 m−2 to ~1025 m−2. The breakdown field-time to breakdown characteristics obtained at different gas pressures are presented as scaling relationships of Etip/N, Nd, and Ntbr for each gas, and compared with data from the literature. Full article
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51 pages, 3297 KiB  
Review
Electrostatic Solitary Structures in Space Plasmas: Soliton Perspective
by Gurbax Singh Lakhina, Satyavir Singh, Rajith Rubia and Selvaraj Devanandhan
Plasma 2021, 4(4), 681-731; https://doi.org/10.3390/plasma4040035 - 21 Oct 2021
Cited by 26 | Viewed by 4369
Abstract
Occurrence of electrostatic solitary waves (ESWs) is ubiquitous in space plasmas, e.g., solar wind, Lunar wake and the planetary magnetospheres. Several theoretical models have been proposed to interpret the observed characteristics of the ESWs. These models can broadly be put into two main [...] Read more.
Occurrence of electrostatic solitary waves (ESWs) is ubiquitous in space plasmas, e.g., solar wind, Lunar wake and the planetary magnetospheres. Several theoretical models have been proposed to interpret the observed characteristics of the ESWs. These models can broadly be put into two main categories, namely, Bernstein–Green–Kruskal (BGK) modes/phase space holes models, and ion- and electron- acoustic solitons models. There has been a tendency in the space community to favor the models based on BGK modes/phase space holes. Only recently, the potential of soliton models to explain the characteristics of ESWs is being realized. The idea of this review is to present current understanding of the ion- and electron-acoustic solitons and double layers models in multi-component space plasmas. In these models, all the plasma species are considered fluids except the energetic electron component, which is governed by either a kappa distribution or a Maxwellian distribution. Further, these models consider the nonlinear electrostatic waves propagating parallel to the ambient magnetic field. The relationship between the space observations of ESWs and theoretical models is highlighted. Some specific applications of ion- and electron-acoustic solitons/double layers will be discussed by comparing the theoretical predictions with the observations of ESWs in space plasmas. It is shown that the ion- and electron-acoustic solitons/double layers models provide a plausible interpretation for the ESWs observed in space plasmas. Full article
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221 pages, 61334 KiB  
Review
Update on the Scientific Status of the Plasma Focus
by Sunil Auluck, Pavel Kubes, Marian Paduch, Marek J. Sadowski, Vyacheslav I. Krauz, Sing Lee, Leopoldo Soto, Marek Scholz, Ryszard Miklaszewski, Hellmut Schmidt, Alexander Blagoev, Maurizio Samuelli, Yeow Sing Seng, Stuart Victor Springham, Alireza Talebitaher, Cristian Pavez, Mohammad Akel, Seong Ling Yap, Rishi Verma, Karel Kolacek, Paul Lee Choon Keat, Rajdeep S. Rawat, Ali Abdou, Guixin Zhang and Tõnu Laasadd Show full author list remove Hide full author list
Plasma 2021, 4(3), 450-669; https://doi.org/10.3390/plasma4030033 - 7 Sep 2021
Cited by 38 | Viewed by 7485
Abstract
This paper is a sequel to the 1998 review paper “Scientific status of the Dense Plasma Focus” with 16 authors belonging to 16 nations, whose initiative led to the establishment of the International Center for Dense Magnetized Plasmas (ICDMP) in the year 2000. [...] Read more.
This paper is a sequel to the 1998 review paper “Scientific status of the Dense Plasma Focus” with 16 authors belonging to 16 nations, whose initiative led to the establishment of the International Center for Dense Magnetized Plasmas (ICDMP) in the year 2000. Its focus is on understanding the principal defining characteristic features of the plasma focus in the light of the developments that have taken place in the last 20 years, in terms of new facilities, diagnostics, models, and insights. Although it is too soon to proclaim with certainty what the plasma focus phenomenon is, the results available to date conclusively indicate what it is demonstrably not. The review looks at the experimental data, cross-correlated across multiple diagnostics and multiple devices, to delineate the contours of an emerging narrative that is fascinatingly different from the standard narrative, which has guided the consensus in the plasma focus community for several decades, without invalidating it. It raises a question mark over the Fundamental Premise of Controlled Fusion Research, namely, that any fusion reaction having the character of a beam-target process must necessarily be more inefficient than a thermonuclear process with a confined thermal plasma at a suitably high temperature. Open questions that need attention of researchers are highlighted. A future course of action is suggested that individual plasma focus laboratories could adopt in order to positively influence the future growth of research in this field, to the general benefit of not only the controlled fusion research community but also the world at large. Full article
(This article belongs to the Special Issue Feature Papers in Plasma Sciences)
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12 pages, 774 KiB  
Article
Aerosol Charging with a Piezoelectric Plasma Generator
by Helmut Krasa, Mario A. Schriefl, Martin Kupper, Alexander Melischnig and Alexander Bergmann
Plasma 2021, 4(3), 377-388; https://doi.org/10.3390/plasma4030027 - 16 Jul 2021
Cited by 3 | Viewed by 3705
Abstract
A novel piezoelectric plasma generator developed by TDK Electronics GmbH & Co. OG, the CeraPlas®, was investigated for its feasibility as a charger for aerosol particles. The CeraPlas® charger was directly compared to a commercially available bipolar X-ray charger regarding [...] Read more.
A novel piezoelectric plasma generator developed by TDK Electronics GmbH & Co. OG, the CeraPlas®, was investigated for its feasibility as a charger for aerosol particles. The CeraPlas® charger was directly compared to a commercially available bipolar X-ray charger regarding its efficiency of charging atomized NaCl particles in a size range from 30 nm to 100 nm. First results show the ability of the CeraPlas® to perform bipolar aerosol charging with high reproducibility, and measurements of the charge distribution in the Nit product yielded about 1012 m−3 s for our experimental charging configuration. Unwanted generation of ozone was suppressed by a dedicated charging chamber and operation in N2 atmosphere. Full article
(This article belongs to the Special Issue Piezoelectric Direct Discharge)
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13 pages, 3810 KiB  
Article
RF and Microwave Ion Sources Study at Institute of Modern Physics
by Qian Y. Jin, Yu G. Liu, Yang Zhou, Qi Wu, Yao J. Zhai and Liang T. Sun
Plasma 2021, 4(2), 332-344; https://doi.org/10.3390/plasma4020022 - 6 Jun 2021
Cited by 11 | Viewed by 8958
Abstract
Intense ion beam production is of high importance for various versatile applications from accelerator injectors to secondary ion mass spectrometry (SIMS). For these purposes, different types of ion beams are needed and, accordingly, the optimum plasma to produce the desired ion beams. RF-type [...] Read more.
Intense ion beam production is of high importance for various versatile applications from accelerator injectors to secondary ion mass spectrometry (SIMS). For these purposes, different types of ion beams are needed and, accordingly, the optimum plasma to produce the desired ion beams. RF-type plasma features a simple structure, high plasma density and low plasma temperature, which is essential for negative ion beam production. A very compact RF-type ion source using a planar coil antenna has been developed at IMP for negative molecular oxygen ion beam production. In terms of high-intensity positive ion beam production, 2.45 GHz microwave power-excited plasma has been widely used. At IMP, we developed a 2.45 GHz plasma source with both ridged waveguide and coaxial antenna coupling schemes, tested successfully with intense beam production. Thanks to the plasma built with an external planar coil antenna, high O2 production efficiency has been achieved, i.e., up to 43%. With 2.45 GHz microwave plasma, the ridged waveguide can support a higher power coupling of high efficiency that leads to the production of intense hydrogen beams up to 90 emA, whereas the coaxial antenna is less efficient in power coupling to plasma but can lead to attractive ion source compactness, with a reasonable beam extraction of several emA. Full article
(This article belongs to the Special Issue Low Temperature Plasmas for Ion Beam Generation)
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