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Electrode erosion caused by dense plasma in constrictive discharge channel is one of the fundamentally detrimental effects existing in pulsed discharge switches. An enhanced plasma expansion in pseudospark discharge assisted by a magnetic switch (MS) is observed from ICCD images in this paper, accompanied by reduced commutation loss, and the mechanisms are revealed by experiments and simulations. The characteristics of the discharge waveforms and channel images of the pseudospark discharge with and without a series-connected MS are compared, and the influence of the number of magnetic cores is studied. As the loop current increases, the discharge channel expands radically and reaches the maximum as the current rising rate reaches the maximum. As the number of magnetic cores increases from 0 to 8, the maximum diameter of the discharge channel increases from 16 mm to about 38 mm, and the commutation loss is reduced from 30 mJ to 11 mJ. The electrode erosion rate of the case with a MS is lower than that without a MS. A particle in cell/Monte Carlo collision model coupling to nonlinear external circuit elements is established. The simulation results fit well with the experiment phenomena, including the discharge waveforms and the profiles of the discharge channel. The distribution of ions shows more diffused features than that of electrons, while the distribution of electrons is more similar to the discharge channel observed in experiments. The enhanced plasma expansion is mainly caused by the higher radial acceleration component of the charged particles during the magnetically delayed time. [ABSTRACT FROM AUTHOR]

Electrode erosion caused by dense plasma in constrictive discharge channel is one of the fundamentally detrimental effects existing in pulsed discharge switches. An enhanced plasma expansion in pseudospark discharge assisted by a magnetic switch (MS) is observed from ICCD images in this paper, accompanied by reduced commutation loss, and the mechanisms are revealed by experiments and simulations. The characteristics of the discharge waveforms and channel images of the pseudospark discharge with and without a series-connected MS are compared, and the influence of the number of magnetic cores is studied. As the loop current increases, the discharge channel expands radically and reaches the maximum as the current rising rate reaches the maximum. As the number of magnetic cores increases from 0 to 8, the maximum diameter of the discharge channel increases from 16 mm to about 38 mm, and the commutation loss is reduced from 30 mJ to 11 mJ. The electrode erosion rate of the case with a MS is lower than that without a MS. A particle in cell/Monte Carlo collision model coupling to nonlinear external circuit elements is established. The simulation results fit well with the experiment phenomena, including the discharge waveforms and the profiles of the discharge channel. The distribution of ions shows more diffused features than that of electrons, while the distribution of electrons is more similar to the discharge channel observed in experiments. The enhanced plasma expansion is mainly caused by the higher radial acceleration component of the charged particles during the magnetically delayed time. [ABSTRACT FROM AUTHOR]

To reduce the commutation loss of the pseudospark switch, we connect a magnetic switch in series to the anode of the pseudospark switch. Herein, the typical discharge process and switching characteristics of a pseudospark switch in series with a magnetic switch are experimentally investigated at a different number of magnetic cores, gas pressures, and applied voltages. The results show that the addition of the magnetic switch extends the duration of the hollow cathode discharge of the pseudospark. Moreover, the rapid increase of the loop current and the fast decrease of the discharge voltage are separated, which is beneficial for reducing the commutation loss. With an increasing number of magnetic cores, the magnetic delay time increases and the commutation loss significantly decreases. However, the trigger delay time hardly changes since the magnetic switch does not affect the development of the pre-breakdown phase. At a fixed applied voltage, the trigger delay, magnetic delay time, and commutation loss decrease with increasing gas pressure. Furthermore, at a fixed gas pressure, the trigger delay of the pseudospark switch in series with a magnetic switch first decreases and then remains unchanged with increasing applied voltage, which is similar to that of a pseudospark switch without a magnetic switch because of the strong enough trigger injection. The magnetic delay time decreases and the commutation loss almost linearly increases with increasing applied voltage. [ABSTRACT FROM AUTHOR]

Used widely in microwave devices and material processing, triggered pseudospark discharge can generate an electron beam, the adjustability of which is affected greatly by the working gas pressure and the applied voltage of the pseudospark discharge. This paper reports the development of an electron-beam source based on a single-gap triggered pseudospark discharge device. Its working performance, as measured by the peak beam current, charge quantity, and pulse width, was studied experimentally, and the results showed that the peak beam current increased initially and then decreased with increasing gas pressure. This was because the beam current was limited by the low plasma density at low pressures, while the electron scattering and ion neutralization during drift decreased the beam current at high pressures. The pulse width of the beam current was consistent with that of the loop current and was determined by the damping coefficient of the discharge circuit. With increasing gas pressure, the pulse width decreased gradually. When the gas pressure was constant, the peak beam current increased significantly with increasing applied voltage because of the enhanced electron-emission current and weakened electron scattering. The charge quantity of the beam current also increased with increasing applied voltage, but the pulse width decreased slightly. Based on the above results, this paper also reports qualitative analysis of the change of the discharge process corresponding to the variation of the electron-beam parameters with gas pressure and applied voltage. [ABSTRACT FROM AUTHOR]

Pseudospark discharge is a special low pressure discharge and has been widely used in the gas switch and electron beam sources. From experiments, when the work pressure is relatively low, the electron beam current or loop current generated by a pseudospark discharge usually has two or more peaks, which has not been fully explained. In this paper, a single-gap pseudospark discharge model is established using 2D kinetic plasma simulation code VSim to study this phenomenon. According to the simulation results, when the anode voltage is 20 kV and the helium pressure is 100 Pa, the current has two peaks, which is similar to the experimental results, accompanied by the stepwise penetration of the virtual anode. This is mainly related to the formation and disappearance of the potential barrier in the cathode hole region. The formation of the potential barrier is caused by the consumption of ions at the cathode hole, and the disappearance is caused by the increase in electrons in the cathode cavity. By classifying the electrons, it is found that the increased electrons are generated by secondary emission caused by ion bombardment on the wall of the cathode. The simulation results also show that the stepwise penetration of the virtual anode can be suppressed or eliminated by increasing the working gas pressure, the secondary electron yield of the cathode material, or the trigger intensity. [ABSTRACT FROM AUTHOR]

The avalanche transistor-based Marx circuit has great advantages in generating high-voltage nanosecond pulses. The introduction of auxiliary triggering topology (ATT) can reduce the damage probability of transistors in M × N-stage Marx bank circuits (MBCs). However, as the number of modified stages increases, the output voltage drops significantly, which makes it not achievable to adopt ATT at each stage. The transistors in nonmodified stages still have a certain failure rate when operating at a high repetition rate. Moreover, the previous ATT is only applicable in the negative polarity MBC. In this article, an improved ATT is proposed to solve the above problems. First, its operating principle is analyzed, and the feasibility of its application in both negative and positive MBCs is verified by simulations. Then, the effects of the improved ATT on M × N-stage MBCs are experimentally studied, and the results show that the improved ATT can be used in all stages of MBC without reducing the output voltage. It can also reduce the minimum operating voltage of transistors and widen the regulating range of output voltage. In addition, it can improve the synchronous conduction of parallel-connected transistors. Finally, two nanosecond pulse generators with positive and negative polarity, respectively, are developed and used to drive the atmospheric pressure plasma jet. The amplitude of output voltage can be adjusted in the range of 5–10 kV, measuring at the open end of a coaxial cable with 75 Ω impedance, and the maximum repetition rate is up to 7 kHz. [ABSTRACT FROM AUTHOR]

All-solid-state linear transformer drivers (LTDs) are widely used in high-voltage repetitive nanosecond-pulsed generators, and only a few LTD generators can output bipolar rectangular waves currently. Furthermore, owing to the large reverse overshoot when the output pulse width is long, fewer LTD generators can achieve a rectangular wave output with a microsecond pulse width. In this study, a bipolar LTD circuit topology based on a push–pull circuit is proposed for irreversible electroporation. In this topology, a single-stage LTD module has four push–pull branches in its primary winding to achieve a bipolar output and a short-circuited winding with two resistor–capacitor–diode snubbers to suppress forward/reverse overshoot. A single-stage LTD module and a 12-stage LTD have been tested, and the results show that they can output bipolar rectangular pulses with variable parameters. When the output pulse width is 100 ns to 1 µs, the maximum output voltage amplitude is 5.74 kV, the rise time is 29.1 ns, and the reverse overshoot at 1 µs is 2.9%. When the output pulse width is 1–8 µs, the maximum output voltage amplitude is 2.93 kV, the rise time is 24.3 ns, and the reverse overshoot at 8 µs is 11.3%. [ABSTRACT FROM AUTHOR]

Avalanche transistor (AT)-based repetitive nanosecond pulse generators with high amplitude, fast rise time, narrow pulse width, and low jitter have been widely developed and applied in numerous fields. However, relatively little research has been carried out on the positive nanosecond pulse generation with high flexibility in a wide range of output voltage amplitude. In this article, a novel AT-based Marx circuit (MC) topology adopting base-triggering method is proposed to avoid the formation of current filamentation inside the transistors and resolve the contradiction between high-voltage output and high repetition rate operation. A $6\times 10$ -stage MC prototype is implemented with optimized parameters to validate the feasibility of the proposed topology. The conduction processes of transistors show that, with the injection of additional base current, the switched-ON modes are transformed, which contributes to reliable conduction of transistors even without sufficient overvoltage ramp. With the adoption of inner triggering loops, the minimum working voltage of the prototype is extended to 1150 V. The operation characteristics of the generator in the whole operation range are investigated in depth. Experimental results illustrate that, at the 75- $\Omega $ match-ended coaxial cable, the prototype is capable of generating positive pulses with an adjustable voltage amplitude in the range of 6.50 to 12.39 kV, a basically consistent rise time of 3.6 ns and pulse width of 19.5 ns. Over 107 successive pulses are generated at a maximum repetition rate of 1 kHz without any device failure. [ABSTRACT FROM AUTHOR]

Trigger injection and its influence on performances of a single-gap pseudospark switch were studied by using a high-dielectric trigger unit and two types of pulse generators, where pulse parameters, polarity effect, anode voltage, and electrode geometries were varied. It is found that pulses with the rising edge of nanosecond scale can cause diffused electron emission in a large region of the trigger unit, while the slower pulses result in a localized surface flashover. The trigger injection is strongly polarity-dependent, and the way of applying the negative pulse voltage to the outer electrode is more favorable. Not all injected electrons can contribute to the triggering process, and the only factors that can lead to higher charge quantity and energetic components of electrons with a short injection delay are helpful to reduce the trigger delay. A model considering the potential penetration is presented to explain the influence of trigger injection, anode voltage, and electrode geometries. When trigger injection is strong enough or the ratio of cathode hole diameter to its thickness is relatively large, the trigger delay remains constant at different anode voltage; otherwise, it decreases as the anode voltage increases. [ABSTRACT FROM AUTHOR]

Avalanche transistor Marx bank circuits (MBCs) are widely used in high voltage repetitive nanosecond pulse generators, but problems exist with respect to increasing the output voltage due to the limited pulsed current. Accordingly, a novel topology based on an avalanche transistor MBC combined with a linear transformer driver is proposed, the latter of which exhibits advantageous stress distribution and modular structure. A four-module prototype with four units in each module is developed in the laboratory. The output characteristics are investigated by varying important parameters such as the main capacitance, the number of conducting units, the number of cascaded modules, and the trigger signal time delay. The test results verify the validity of the proposed topology. For a 50 Ω resistive load, the prototype can generate pulses with an amplitude of 10.9 kV, a rise time of 3.3 ns, and a voltage superposition efficiency of 89%. The topology proposed in this paper may help to provide a method to further improve the output performance of avalanche transistor MBCs. [ABSTRACT FROM AUTHOR]

Avalanche transistor-based Marx bank circuit (MBC) is widely used to generate nanosecond pulses with high amplitude, high repetition rate, fast rise time, and low jitter. Recently, the problem that avalanche transistors in the first several stages of $M \times N$ -stage MBC fail frequently has been alleviated by adopting auxiliary triggering topology (ATT). However, the reasons for improvement and the optimal design method have not been fully understood. In this article, a flexible $4\,\,\times10$ -stage MBC is developed to further investigate the output characteristics under different modified stages, capacitors, and loads through both experiments and simulations. The results show that on the one hand, adopting ATT will cause energy loss of the main capacitors to auxiliary triggering capacitors, which can decrease the output voltage; and on the other hand, transistors operating in the auxiliary triggering switching-ON mode have lower residual voltage than in the lower overvoltage switching-ON mode, which can increase the output voltage. This implies that there exists an optimal value of modified stages, which is 5 for the $4\,\,\times10$ -stage MBC. Besides, by adopting ATT, the waveform of output pulse changes, and the rising edge becomes faster. Finally, a repetitive nanosecond pulse generator is developed based on the optimized parameters, which can produce pulses with an amplitude of 9.04 kV, a rise time of 3.4 ns, a pulse width of 18.8 ns, and a maximum repetition rate of 1 kHz at the 75- $\Omega $ open-ended cable. Also, the feasibility of this modified MBC is validated by the preliminary experiments of driving atmospheric pressure plasma jet (APPJ). [ABSTRACT FROM AUTHOR]

Marx bank circuits (MBCs) based on avalanche transistors are widely used to generate nanosecond pulses with high power, high repetition rate, and low jitter. However, it was observed that avalanche transistors in the first and second stages of traditional M×N-stage MBCs have high failure rates. This paper proposes an auxiliary triggering topology (ATT) for improving the reliability of high-power MBCs based on avalanche transistors. The operation principles of ATT and failure mechanisms of traditional MBCs are analyzed. By adopting ATT, the auxiliary triggering pulse is generated between the base and emitter of the transistor, and the switching mode is changed from “overvoltage switching-on” to “triggering switching-on.” The combined effect of overvoltage ramp and auxiliary triggering pulse significantly reduces the failure rate and prolongs the lifetime of easily damaged transistors. Two high-power nanosecond pulse generators, 3 × 12-stage and 6 × 10-stage MBCs adopting ATT, are developed to verify the feasibility of the proposed configuration. The output amplitude and the rising time are 8.5 kV/6 ns and 17 kV/6 ns at the open end of coaxial cable with 75-Ω impedance, respectively. Both generators are tested continuously at the repetition rate of 2 kHz for 60 min and no failure occurs, which shows much better performance in repetition rate and reliability than traditional M×N-stage MBCs. [ABSTRACT FROM AUTHOR]

High power pseudospark switches (PSSs) can control pulsed high voltage, high peak current discharges. In this paper, a PSS in series with a saturable inductor at the anode is investigated. The commutation process is analyzed with a time-resolved method. And the effects of the number of magnetic cores and anode voltages on discharge characteristics are studied experimentally. At the beginning of commutation process, when the anode voltage is 10.4 kV with four magnetic cores, the inductor is unsaturated, and there is a flat plateau of the forward voltage drop after the anode voltage drops rapidly to about 200 V, at the same time the current rise rate is 2.8 kA/ $\mu \text{s}$. When the inductor is saturated, there is a voltage spike that occurs at the end of the flat plateau, and the current simultaneously rises rapidly at the rate of 6.4 kA/ $\mu \text{s}$. The current sharp rise lags behind the voltage drop of the pseudospark gap, and the electrode erosion and energy dissipation reduce significantly. When the anode voltage is 15.6 kV, the commutation losses reduce by about 30% with seven cores in series at the anode. As a result, it is evident that the discharge process will speed up if the anode voltage is higher. And the trigger delay decreases by up to 8% when the anode voltage varies from 5.2 to 15.6 kV. Finally, in order to obtain a reproducible pseudospark discharge by the use of a saturable inductor, the number of magnetic cores has to be optimized for different anode voltages. [ABSTRACT FROM AUTHOR]

By triggering the switches in sequence and controlling the brick charging voltage independently, a pulsed power system can generate flexible current waveform in the load, which could allow high-accurate magnetic pressure in experimental materials in the study of EOS data and dynamic response of material characterization. Currently, a 1.5-MA driver with 20 independent bricks has been developed, and a programmable 256-brick parallel discharge driver which is able to generate shaped current greater than 6.5 MA is under construction in the China Academy of Engineering Physics. In this paper, for these two multibrick parallel discharge drivers, the method of optimizing the output current waveform of the driver is studied by using the genetic algorithm and the corresponding optimization results of different target current waveforms are obtained. An experimental platform is set up for studying the gas switch breakdown characteristics. The self-breakdown and triggering breakdown experiments of the switch are conducted, and its breakdown characteristics are researched. Meanwhile, the effect of breakdown delay and jitter on the output current waveform is studied by simulation. This paper is of great significance to the actual operation of the multibrick parallel discharge drivers. [ABSTRACT FROM AUTHOR]

Isentropic compression experiments (ICEs) are an important method to study the dynamic characteristics of materials at pressures, temperatures, and stress or strain rate not attainable in conventional shock experiments. Yet it makes high demands toward the precise current pulse tailoring capability of the source. The China Academy of Engineering Physics is now developing a 20-brick parallel discharge driver and a 256-brick parallel discharge driver (CQ-7) to improve the performance and flexibility of compact magnetic compression devices. The load current of ICEs is tailored by optimizing the switch trigger sequence and the brick charging voltage. This paper establishes the equivalent circuit model of the 20-brick parallel discharge driver, taking the distributed inductance, resistance, and capacitance of transmission plates into account based on the partial-element equivalent circuit method. Besides, the equivalent circuit model of discharge brick based on the switch arc resistance model is also established to study the influence of key structural parameters on the output current. Furthermore, by comparing the simulation and experimental results of simultaneous discharge of 20-brick driver, the accuracy of simulation model is confirmed. This paper is of great significance to the design and optimization of multibrick parallel discharge drivers. [ABSTRACT FROM AUTHOR]

This paper presents the design and development of a trigger with a high repetition rate, low jitter, and compact structure for the pseudospark switch (PSS), which includes an improved Marx generator based on avalanche transistors and a corona-plasma trigger unit. The generator adopted a novel 3 × 12-stage Marx circuit based on avalanche transistors in which the failure rate of transistors in the first and second stages was significantly reduced by connecting the parallel capacitors compared to the previous similar generator. The reason for the improved performance was also discussed. The main parameters of output pulses were an amplitude of −7 kV, rise time of 6 ns, jitter of 0.2 ns, and repetition rate of 2 kHz. The corona-plasma trigger unit adopted BaTiO3 ceramics with high εr as the dielectric and was arranged in the hollow cathode of the PSS. The experiments of triggering a PSS prototype were conducted. The influence of anode voltage and pressure on the trigger delay and jitter was studied, and the minimum trigger jitter achieved <1 ns. This trigger worked for 107 shots at the repetition rate of 2 kHz continuously without obvious performance degradation and any failure of the generator. The main advantage of this trigger is the simultaneous combination of the high repetition rate, low jitter, long lifetime, and great simplicity in a compact structure. [ABSTRACT FROM AUTHOR]