Search Results (15)

To clarify the background of multiple burst (MB) specifications in the aviation lightning test standards, a broadband lightning electromagnetic field measurement system was employed to collect 91 sets of VLF/LF band nature flash data. A total of 719 typical repetitive pulse (RP) groups were identified, and 163,589 single pulse samples were analyzed statistically. The variational mode decomposition (VMD) method and a trend-free correlation on index (TFCI) were used to extract RPs from the slowly varying trends and high-frequency noises from the measured data. The time-domain characteristics of four kinds of RPs corresponding to the lightning discharge events—initial breakdown pulse (IBP), regular pulse bursts (RPB), chaotic pulse train (CPT), and dart-stepped leader (DSL)—were investigated. By comparing previous statistics and the definition in current international aviation standards, the intrinsic correlation between RPs and the defined parameters of MBs was explored. New recommendations for the MB test standard were subsequently proposed. Full article

We propose a pulsed modulated digital holographic microscopy (PM-DHM) technique for the dynamic measurement of flowing microparticles in microfluidic systems. By digitally tuning the pulse width and the repetition rate of a laser source within a single-frame exposure, this method enables the recording of multiple images of flowing microparticles at different time points within a single hologram, allowing the quantification of velocity and acceleration. We demonstrate the feasibility of PM-DHM by measuring the velocity, acceleration, and forces exerted on PMMA microspheres and red blood cells flowing in microfluidic chips. Compared to traditional frame-sampling-based imaging methods, this technique has a much higher time resolution (in a range of microseconds) that is limited only by the pulse duration. This method demonstrates significant potential for high-throughput label-free flow cytometry detection and offers promising applications in drug development and cell analysis. Full article

We present a 520 μJ microsecond burst-mode pulse fiber amplifier with a GHz-tunable intra-burst repetition rate and a nearly flat-top pulse envelope. The amplifier architecture comprises a microsecond pulse seed, a high-bandwidth electro-optic modulator (EOM), two pre-amplifier stages, a waveform-compensated acoustic-optic modulator (AOM), and two main amplifier stages. To address amplified spontaneous emission (ASE) and nonlinear effects, a multistage synchronous pumping scheme that achieved a maximum energy output of 520 μJ and has a peak power of 160 W was used. To produce a flat-topped burst pulse envelope, the AOM generates an editable waveform with a leading edge and a high trailing edge to compensate for waveform distortion, resulting in a 5 μs nearly flat-top pulse envelope at maximum energy. The laser provides an adjustable intra-burst pulse repetition rate range of 1–5 GHz through the high-bandwidth EOM modulation. The intra-burst pulse jitter time of the laser remains below 4.31 ps at different frequencies. Moreover, the beam quality of the amplifier is M²x = 1.04 and M²y = 1.1. This amplifier exhibits promising potential and can be further amplified as an optical drive source for high-power, large-bandwidth microwave photon (MWP) radar applications. Meanwhile, it is also potentially applicable as a pulse source for high-speed optical communications, the high-precision processing of special materials, and LIDAR ranging. Full article

The phenomenon known as bipolar cancellation is observed when biphasic nanosecond electric field pulses are used, which results in reduced electroporation efficiency when compared to unipolar pulses of the same parameters. Basically, the negative phase of the bipolar pulse diminishes the effect of the positive phase. Our study aimed to investigate how bipolar cancellation affects Ca²⁺ electrochemotherapy and cellular response under varying electric field intensities and pulse durations (3–7 kV/cm, 100, 300, and 500 ns bipolar 1 MHz repetition frequency pulse bursts, n = 100). As a reference, standard microsecond range parametric protocols were used (100 µs × 8 pulses). We have shown that the cancellation effect is extremely strong when the pulses are closely spaced (1 MHz frequency), which results in a lack of cell membrane permeabilization and consequent failure of electrochemotherapy in vitro. To validate the observations, we have performed a pilot in vivo study where we compared the efficacy of monophasic (5 kV/cm × ↑500 ns × 100) and biphasic sequences (5 kV/cm × ↑500 ns + ↓500 ns × 100) delivered at 1 MHz frequency in the context of Ca²⁺ electrochemotherapy (B16-F10 cell line, C57BL/6 mice, n = 24). Mice treated with bipolar pulses did not exhibit prolonged survival when compared to the untreated control (tumor-bearing mice); therefore, the bipolar cancellation phenomenon was also occurrent in vivo, significantly impairing electrochemotherapy. At the same time, the efficacy of monophasic nanosecond pulses was comparable to 1.4 kV/cm × 100 µs × 8 pulses sequence, resulting in tumor reduction following the treatment and prolonged survival of the animals. Full article

The impact induced by cavitation bubble collapse can be utilized for mechanical surface treatment to improve fatigue properties of metals including additive manufactured metallic materials. A peening method using cavitation impact induced by a pulsed laser is called “laser cavitation peening (LCP)”. Normally, a Q-switched Nd:YAG laser, whose pulse width is a few nanoseconds, is used for LCP, which improves the fatigue strength. The problem with LCP is that the processing time is too slow. If a laser pulse whose pulse width is a few hundred microseconds can be utilized for LCP, the repetition frequency can be increased drastically using other types of laser systems such as a fiber laser. In the present paper, in order to reveal the possibility of LCP using a pulsed laser width of a few hundred microseconds, the use of LCP with a normal-oscillation Nd:YAG laser (pulse width ≈ 200 μs) was investigated. It is demonstrated that LCP with the normal-oscillation Nd:YAG laser produced curvature in an aluminum alloy plate. The shock pressure wave and impulsive vibration of the target surface at the first collapse of laser cavitation (LC), which was induced by the normal-oscillation Nd:YAG laser, was 3–4 times larger than those of laser ablation (LA). Full article

A Pockels cell driver (PCD) can be viewed as a high-voltage pulse width generator for controlling the bi-refringence effect of electro-optical crystals. The main features of a PCD include a high repetition rate, fast on and off switching, variable pulse duration, and a true square pulse shape. The most commonly used PCD has a narrow pulse width tuning range, typically within a few microseconds. In this paper, we propose a PCD based on a novel pulse width modulation (PWM) signal-generation strategy that can continuously adjust its pulse width with a minimum step size of 10 ns and no restriction on the maximum width. Therefore, it is easily compatible with both “On-type” and “Off-type” applications of the electro-optic crystal quarter-wave voltage. The experimental results show that the rising and falling times of the proposed PCD are approximately 7.3 ns and 7.8 ns, respectively, with a maximum repetition rate of 1 MHz and a maximum voltage of approximately 2.0 kV. Finally, the functionality of the PCD is demonstrated in a home-built slab laser. Full article

The most likely intentional high-power electromagnetic (EM) interference, threatening the operation of technologically advanced electronic infrastructure, will have the forms of sub- and nanosecond ultra-wideband (UWB) pulses, several hundred nanosecond pulses of attenuated sinusoids, and sub- and microsecond sinusoidal pulses. The protection of electronic objects against high-power EM pulses is provided by different types of metal enclosure shields with technological apertures, inside which sensitive electronic objects can be placed. These technological apertures allow external EM interference to penetrate into the enclosure, making the EM shielding imperfect. The EM protection against the EM pulses has been mainly assessed based on the so-called shielding effectiveness (SE) parameters. The SE parameters (SE^e and SE^m for the electric and magnetic fields, respectively) are useful for designing and comparing EM shields. In relatively small shielding enclosures, which have recently become the subject of interest, the SE parameters have been studied for relatively long transient EM interference, longer than 150 ns, i.e., for the EM pulses whose duration is much longer than the time that the pulse takes to pass the small enclosure. In this work, we dealt with an ultrashort transient interference pulse, the duration of which was much shorter than the pulse transit time through the enclosure. The intentional high-power EM subnanosecond UWB pulse is an example of such a pulse. For such an ultrashort pulse, we studied the EM shielding performance of a small size enclosure numerically (W:H:D = 455 mm:50 mm:463 mm) with aperture (W:H = 80 mm:30 mm). The ultrashort EM interference pulse of a Gaussian distribution of the electric and magnetic fields with amplitudes of 10⁶ V/m and 2.68·10³ A/m, respectively, applied in this study, had a duration of 0.0804 ns (FWHM). This means that the high-power EM interference pulse was about 18 times shorter than the time that it takes to pass the enclosure (equal to about 1.5 ns). Our numerical simulations of the subnanosecond high-power EM interference of the interior of the enclosure with aperture were performed in the time domain using the commercial code CST Microwave Studio. First of all, the time-domain simulations resulted in 2D and 3D images and 2D vector maps of the electric and magnetic fields, which visualized the temporal and spatial development of the EM field in the enclosure with aperture caused by the incident subnanosecond high-power EM interference. The development of the associated electric and magnetic fields proceeded in two phases: first in the form of EM waves and later as an interference pattern, traveling forth and back between the front and rear enclosure walls. Due to the energy loss through the aperture, suffered by the traveling EM field and the tendency of the EM field to be evenly distributed over time throughout the entire enclosure, the amplitudes of the EM field decreased about 30 times within 90 ns. Despite the energy loss, the EM field developed in the enclosure existed at least 1000 times longer than the subnanosecond duration of the incident EM pulse (i.e., at least 90 ns as demonstrated by the numerical calculation). Apart from the EM field development visualization, the time-domain simulation enabled easy tracking of the temporal behavior of the EM field in selected points in the enclosure. Such tracking showed that each point in the enclosure was passed by a series of subnanosecond EM pulses, called internal EM pulses, over a relatively long time (at least over the simulation duration of 90 ns). This means that over 90 ns, the points in the enclosure were repeatedly influenced by the series of about 500 subnanosecond internal EM pulses. The amplitudes of many of these pulses were only (3–5) times lower than that of the incident EM pulse. Despite the lower amplitudes, these internal pulses may cause severe EM interference inside the enclosure. This shows a substantial change in the nature of the EM interference caused by a subnanosecond high-power EM plane wave when a given point is not shielded (a single interference of the subnanosecond strong EM pulse) and when a given point is shielded by the enclosure with aperture (a repetitive interference of subnanosecond weaker EM pulses). With the time dependence of the EM field amplitudes obtained from the time-domain calculation at a selected point in the enclosure, it is easy to determine the SE^e and SE^m at that point as a function of time. In this way, evaluation of the local SE^e and SE^m (for selected points in the enclosure) can be performed. Moreover, the 2D and 3D images and 2D vector maps calculated in the time domain for a given time enabled easy calculation of the SE^e and SE^m maps for various times. Such maps, shown for the first time in this paper, give a more global view of the shielding properties of the enclosure with an aperture. This all shows the advantages of the use of the time-domain approach for studying EM shielding in the case of ultrashort EM interferences. Full article

Gene electrotransfer (GET) is a widely used method for nucleic acids’ delivery into cells. We explored, evaluated, and demonstrated the potential use of different pulse durations for introducing plasmid DNA (pDNA) into cells in vitro and compared the efficiency and dynamics of transgene expression after GET. We performed experiments on cell suspensions of 1306 fibroblasts and C2C12 myoblasts with four ranges of pulse durations (nanosecond, high frequency bipolar (HF-BP), and micro- and millisecond). Six different concentrations of pDNA encoding green fluorescent protein were used. We show that GET can be achieved with nanosecond pulses with a low pulse repetition rate (10 Hz). The GET’s efficiency depends on the pDNA concentration and cell line. Time dynamics of transgene expression are comparable between millisecond, microsecond, HF-BP, and nanosecond pulses but depend greatly on cell line. Lastly, based on the data obtained in the experiments of pDNA concentration effect on GET the model of the probability of pDNA and cell membrane contact during GET was developed. The model shows that pDNA migration is dominated by diffusion for nanosecond and HF-BP pulses and by electrophoresis for micro- and millisecond pulses. Modeling results can provide valuable guidance for further experiments and interpretations of the results obtained by various pulse protocols. Full article

High repetition-rate X-ray free-electron lasers (XFELs) enable the study of fast dynamics on microsecond time scales. Split-delay lines (SDLs) further bring the time scale down to femtoseconds by splitting and delaying the XFEL pulses. Crystals and multilayers are two common types of optical elements in SDLs, offering either long delay ranges or high temporal accuracy. In this work, we introduce the design of a hybrid SDL for the coherent diffraction endstation of Shanghai High Repetition Rate XFEL and Extreme Light Facility (SHINE). It uses crystals for the first branch and multilayers for the second one, thus simultaneously offering a relatively long delay range and high temporal accuracy. Moreover, a third branch can be installed to switch the SDL to the all-crystal configuration for longer delay ranges. Full article

A non-thermal plasma-water system using a microsecond pulsed high-voltage power supply was investigated with air, nitrogen, oxygen, and argon gas feedings individually. Optical emission spectroscopy (OES) was utilized to characterize the primary active species inside the plasmas generated by different gas feedings. The OES method was also employed to estimate the neutral gas and electron temperatures. The pH and the oxidation-reduction potential (ORP) of plasma-activated water (PAW) were measured in the liquid phase. An ion chromatography system (ICS) was employed to present the PAW activity, such as nitrite and nitrate species. Moreover, hydrogen peroxide as a secondary active species inside the activated water, generated by the gases mentioned above, was measured by potassium permanganate titration. It was found that the gas species have a noticeable effect on the pH level as well as the ORP of PAW. In the cases of argon and oxygen plasmas, the pH level of PAW does not change significantly. In contrast, the pH values of PAW generated by air and nitrogen plasmas decline sharply during the treatment time. Moreover, the gas species have a significant impact on the concentrations of nitrite, nitrate, and hydrogen peroxide generated in PAW. The activated water generated by oxygen plasma provides the highest level of hydrogen peroxide. Although the consumed power of argon plasmas was half of the other plasma sources, it provides relatively high hydrogen peroxide contents compared to the nitrogen and air plasmas. Full article

As a key component of a high-power microwave (HPM) system, a multi-gap gas switch (MGS) has recently developed insulation failure due to surface flashover. Although design criteria for surface insulation have been put forward, it is still not clear how the insulation in this case deteriorated under long-term repetitive microsecond pulses (RMPs). In this paper, flashover experiments under RMPs were carried out on various dielectric surfaces between parallel-plane electrodes in SF₆ and air atmospheres, respectively. Based on tests of the surface insulation lifetime (SIL), an empirical formula for SIL prediction is proposed with variations of insulator work coefficient λ, which is a more suitable parameter to characterize SIL under RMPs. Due of the accumulation effect, the relationship between E/p and pt_delay varies with the pulse repetitive frequency (PRF) and SIL recovery capability decreases with an increase in PRF and surface deterioration is exacerbated during successive flashovers. It is concluded that the flashover path plays a crucial role in surface insulation performance under RMPs due to the photoemission induced by ultraviolet (UV) radiation, signifying the necessity of reducing surface paths in future designs as well as the improvement of surface insulation. Full article

We report on novel observations of directed re-deposition of ablation debris during the ultrafast laser micro-structuring of stainless steel in the air with multi-beams in close proximity on the surface. This interesting phenomenon is observed with both 10 ps and 600 fs NIR laser pulses at 5 kHz repetition rate. Ablation spot geometries could be altered with the use of beam splitting optics or a phase-only Spatial Light modulator. At low fluence (F ~ 1.0 J cm⁻²) and pulse exposure of a few hundred pulses, the debris appears as concentrated narrow “filaments” connecting the ablation spots, while at higher fluence, (F ~ 5.0 J cm⁻²) energetic jets of material emanated symmetrically along the axes of symmetry, depositing debris well beyond the typical re-deposition radius with a single spot. Patterns of backward re-deposition of debris to the surface are likely connected with the colliding shock waves and plasma plumes with the ambient air causing stagnation when the spots are in close proximity. The 2D surface debris patterns are indicative of the complex 3D interactions involved over wide timescales during ablation from picoseconds to microseconds. Full article

The unique biological effects stimulated by short pulsed electric field have many applications in tumor treatment, such as irreversible electroporation, electrochemotherapy, gene transfection and immune therapy. These biological effects require high voltage pulses with different pulse width in the range from nanoseconds to hundreds of microseconds. To fulfill this requirement, a compact high voltage pulse generator has been designed based on a switchable capacitor array and a SiC MOSFET switching array. The proposed pulse generator has one output channel with an adjustable pulse width from 100 ns to 100 µs, an amplitude range from 0 kV to 2 kV, a repetition rate less than 1.2 kHz and a voltage drop less than 5%. The mechanism of the stacked switches circuit was investigated, in connection with a switchable capacitor array. The introduction of a switchable capacitor array extends the pulse width from nanosecond scale and microsecond scale compared with other similar design methods. The pulse generator has been designed in simulation and implemented in experiment. The developed pulse generator provides a convenient and economical tool for the further studies of the unique biological effects stimulated by different pulsed electric fields for tumor treatment. Full article

In this work, we have investigated the feasibility of sub-microsecond range irreversible electroporation (IRE) with and without calcium electroporation in vivo. As a model, BALB/C mice were used and bioluminescent SP2/0 myeloma tumor models were developed. Tumors were treated with two separate pulsed electric field (PEF) pulsing protocols PEF1: 12 kV/cm × 200 ns × 500 (0.006 J/pulse) and PEF2: 12 kV/cm × 500 ns × 500 (0.015 J/pulse), which were delivered with and without Ca²⁺ (168 mM) using parallel plate electrodes at a repetition frequency of 100 Hz. Both PEF1 and PEF2 treatments reduced tumor growth and prolonged the life span of the mice, however, the PEF2 protocol was more efficient. The delay in tumor renewal was the biggest when a combination of IRE with calcium electroporation was used, however, we did not obtain significant differences in the final mouse survival compared to PEF2 alone. Anti-tumor immune responses were also investigated after treatment with PEF2 and PEF2+Ca. In both cases the treated mice had enlarged spleens and increased spleen T cell numbers, lower percentages of suppressor cell subsets (conventional CD4⁺CD25⁺ Treg, CD4⁺CD25⁻DX5⁺ Tr1, CD8⁺DX5⁺, CD4⁺CD28⁻, CD8⁺CD28⁻), changed proportions of Tcm and Tef/Tem T cells in the spleen and increased amount of tumor cell specific antibodies in the sera. The treatment based on IRE was effective against primary tumors, destroyed the tumor microenvironment and induced an anti-tumor immune response, however, it was not sufficient for complete control of tumor metastasis. Full article

A simple, economic and successful design for distance and cable length detection is presented. The measurement system is based on the continuous repetition of a pulse that endlessly travels along the distance to be detected. There is a pulse repeater at both ends of the distance or cable to be measured. The endless repetition of the pulse generates a frequency that varies almost inversely with the distance to be measured. The resolution and distance or cable length range could be adjusted by varying the repetition time delay introduced at both ends and the measurement time. With this design a distance can be measured with centimeter resolution using electronic system with microsecond resolution, simplifying classical time of flight designs which require electronics with picosecond resolution. This design was also applied to position measurement. Full article