A low-jitter self-triggered spark-discharge pre-ionization switch: primary research on its breakdown characteristics and working mechanisms

MV pulsed switch plays a key role as the transfer switch in large electromagnetic pulse simulators. To broaden the range of self-triggering time, a novel spark-discharge pre-ionization switch, in which the main gap electric field is superposed at the trigger gap to let the electrons in its spark channel also become initial electrons, is proposed and tested. The design idea is: as electrons in the spark channel of the trigger gap always exist after its breakdown, the injection time of pre-ionization should have a more negligible effect on reducing the switch jitter. The experiment results under pulses with a rise time of ~100 ns support the above assumptions.When the operating voltage is from ~300 to ~800 kV and the self-triggering time is ~45% to~75% of the peak time, the breakdown time delay jitter is less than 2 ns, and the breakdown voltage jitter is smaller than 1.25%. Under specific self-triggering time, the breakdown time delay jitter is less than 1.5 ns, and the breakdown voltage jitter is smaller than 0.8%.

DIFFUSE VOLUME-DISCHARGES WITHOUT PRE-IONIZATION FORMED IN SF_6 AND C_2H_6 MIXTURES

It has been shown that a volume discharge is forming in non-uniform electric-field rough cathode and even anode without pre-ionization in SF6 and C2H6 mixtures. The discharge is presented with many diffuse channels attached to bright circular cathode spots that diverge towards the anode, with the channels overlapping, form a spatially uniform glow discharge. Self-Initiated Volume Discharge(SIVD) has been performed at a total mixture pressure up to 8 kPa and energy deposition up to 200 J/L. The experimental results indicate that Self-Sustained Volume Discharge(SSVD) in SF6 and C2H6 mixtures develops in the form of SIVD, which is promising for creation of high energy and pulse-periodic HF laser.

Microwave-assisted pre-ionization experiments on GLAST-Ⅲ

GLAss Spherical Tokamak(GLAST-Ⅲ)is a spherical tokamak with an insulating vacuum vessel that has a unique single-passage capability for incident microwaves.In this work,electron cyclotron resonance heating(ECRH)-assisted plasma pre-ionization in GLAST-Ⅲis explored for three radio-frequency(RF)polarizations(the O-,X-,and M-modes)at different toroidal-field(TF)strengths and filled gas pressures.The optimum hydrogen pressure is identified for efficient plasma pre-ionization.A comparison of the plasma pre-ionizations initiated by the O-,X-,and M-modes shows prominent differences in the breakdown time,location,and wave absorption.In the case of O-mode polarization,microwave absorption occurs for a relatively shorter duration,resulting in a bell-shaped electron-temperature(Te)temporal profile.Microwave absorption is dominant in the case of the X-mode,leading to a broader Te temporal profile.The M-mode discharge contains features of both the X-and O-modes.Efficient plasma pre-ionization is achieved in the X-mode polarization for the intermediate TF strengths(with a central toroidal magnetic field B0=0.075 T).Traces of the electron-number density show a similar tendency,as revealed by Te.These results suggest that the X-mode is the best candidate for efficient plasma pre-ionization at low filled gas pressures(10-2 Pa)in small tokamaks.

LONGITUDINAL DISCH.CO_2 LASER WITH PULSED PRE-IONIZATION

A novel pre-ionization scheme of helical transverse-pulsed pre-ionization in a longitudinal discharge CO2 laser is presented. The laser tube is made of glass with inner diameter of 7.5mm and discharge length of 50cm. The laser performance characteristics as functions of parameters, such as pressure, charging capacitance and applied voltage, are investigated. Compared with the same laser structure without pre-ionization, the maximum pulse energy improves by 23%, the optimum electro-optical efficiency increases by 31%, and the specific output energy reaches 26 J/(L·atm).

Comparative study of pulsed breakdown processes and mechanisms in self-triggered four-electrode pre-ionized switches

This work investigates the pulsed breakdown processes and mechanisms of self-triggered preionized switches with a four-electrode structure in nitrogen through intensified charge coupled device photographs.The diameter of the trigger plane hole mainly determines the switch’s electric field distribution.Two configurations with minimum and maximum trigger plane holes are adopted for comparison.In the switch with a minimum trigger plane hole,the maximum electric field distributes at the surfaces of the main electrodes.Although charged particles in the triggering spark channel cannot drift out,homogeneous discharges can be stimulated from both the cathode and anode surfaces through ultraviolet illumination.Two sub-gaps are likely to break down simultaneously.In the switch with a maximum trigger plane hole,the maximum electric field locates near the trigger electrodes.Discharges in both sub-gaps initiate from the trigger electrodes in the form of a positive or negative streamer.Due to the lower breakdown voltage and electric field threshold for discharge initiation,the cathode side sub-gap breaks down first.The analysis of two extreme examples can be referenced in the future design and improvement of self-triggered four-electrode switches with different trigger electrode structures.

A two-dimensional air streamer discharge modified model based on artificial stability term under non-uniform electric field at low temperature and sub-atmospheric pressure

In this paper, an improved air discharge fluid model under non-uniform electric field is constructed based on the plasma module COMSOL Multiphysics with artificial stability term, and the boundary conditions developed in the previous paper are applied to the calculation of photoionization rate. Based on the modified model, the characteristics of low temperature subatmospheric air discharge under 13 kV direct current voltage are discussed, including needle-plate and needle-needle electrode structures. Firstly, in order to verify the reliability of the model, a numerical example and an experimental verification were carried out for the modified model respectively. Both verification results show that the model can ensure the accuracy and repeatability of the calculation. Secondly, according to the calculation results of the modified model, under the same voltage and spacing, the reduced electric field under low temperature subatmosphere pressure is larger than that under normal temperature and atmospheric pressure. The high electric field leads to the air discharge at low temperature and sub atmospheric pressure entering the streamer initiation stage earlier, and has a faster propagation speed in the streamer development stage, which shortens the overall discharge time. Finally, the discharge characteristics of the two electrode structures are compared, and it is found that the biggest difference between them is that there is a pre-ionization region near the cathode in the needle-needle electrode structure. When the pre-ionization level reaches 1013 cm-3, the propagation speed of the positive streamer remains unchanged throughout the discharge process, and is no longer affected by the negative streamer. The peak value of electric field decreases with the increase of pre-ionization level, and tends to be constant during streamer propagation. Based on the previous paper, this paper constructs the air discharge model under non-uniform electric field, complements with the previous paper, and forms a relatively complete set of air discharge simulation system under low temperature and sub atmospheric pressure, which provides a certain reference for future research.

Comparative study on breakdown characteristics of trigger gap and overvoltage gap in a gas pressurized closing switch

Gas pressurized closing switches are one of the most important elements in FLTD-based systems. Improving the trigger performance of gas switches is useful for optimizing the output parameters and the reliability of the FLTD. In this paper, the breakdown characteristics of the trigger gap and the overvoltage gap are studied experimentally. The reasons for the different breakdown performance of the two gaps are also investigated. The results show that the breakdown delay of the trigger gap is more influenced by the trigger voltage, while the breakdown delay of the overvoltage gap is more influenced by the working coefficient and always higher than that of the trigger gap. The jitter of the trigger gap is more influenced by the trigger voltage and accounts more than 60% of the total switch jitter, while the jitter of the overvoltage gap is hardly changed with the trigger voltage as well as the working coefficient and maintains less than 1.4 ns. It is proved that the discharging product from the trigger gap can effectively reduce the breakdown delay and jitter of the overvoltage gap. Based on that, the effect and improvement of pre-ionization on the two gaps are also studied. It is concluded that the jitter of the trigger gap reduces obviously when the pre-ionization is added, while the pre-ionization almost has no effect on the jitter of the overvoltage gap. The jitter of the overvoltage gap is about two times higher than the trigger gap in the pre-ionizing switch.