Breakdown Processes and Mechanisms of Preionized Nitrogen Spark Gaps Under Sub-Microsecond Pulses

Self-triggered preionized switches with four-electrode structures are widely used in large pulsed power generators. Reducing its time delay jitter is an essential requirement. To understand the time delay, this article investigates its breakdown processes and mechanisms. The switch is divided into two symmetrical sub-gaps with a sphere-plate structure. The breakdown processes of the sub-gaps in nitrogen are photographed by an intensified charge coupled device (ICCD) camera. It is observed that two sub-gaps can be separately preionized by the spark discharge. Uniform discharge can be stimulated from the main electrodes of two sub-gaps. Increasing the gap distance or postponing the injection time of preionization makes the uniform discharge easier to transfer to streamer discharge. Combining the discharge phenomena and the spectra of trigger gap discharge, it is inferred that the initial electrons are generated through photoemission and cascade ionization. Furthermore, a large number of excited molecules generated by preionization will accelerate the breakdown process. Under the same voltage pulse, a uniform-like discharge channel with relatively low resistance (
$\text{k}\Omega $
class) first forms in the cathode side sub-gap. In a practical four-electrode switch whose grading resistors are in
$\text{k}\Omega $
to 10
$\text{k}\Omega $
class, the anode side sub-gap will withstand overvoltage and break down first after the channel in the cathode side forms.