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.

High-Power Nanosecond Pulse Generators With Improved Reliability by Adopting Auxiliary Triggering Topology

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).

Further Investigations on a Modified Avalanche Transistor-Based Marx Bank Circuit

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.

Discharge Characteristics of a Pseudospark Switch in Series With a Saturable Inductor

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.

Influence of Trigger Injection on Performances of a Single-Gap Pseudospark Switch

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.

High voltage nanosecond pulse generator based on avalanche transistor Marx bank circuit and linear transformer driver.

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 er 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.

Saikang Shen

Papers

A novel trigger for pseudospark switch with high repetition rate, low jitter, and compact structure

High-Power Nanosecond Pulse Generators With Improved Reliability by Adopting Auxiliary Triggering Topology

Further Investigations on a Modified Avalanche Transistor-Based Marx Bank Circuit

Discharge Characteristics of a Pseudospark Switch in Series With a Saturable Inductor

Influence of Trigger Injection on Performances of a Single-Gap Pseudospark Switch