Note on the Breakdown Process and Preionization Mechanism in a Four-Electrode Self-Triggered Switch

The breakdown process of a self-triggered preionized switch with the four-electrode structure under nanosecond pulses is studied based on optical diagnosis. The breakdown of both subgaps initiates from the main electrode surface in the form of uniform discharge. A uniform-like low-impedance discharge channel forms first in the cathode side subgap. Then several streamer-like high-impedance discharge channels form later in the anode side subgap. Therefore, the anode side is under overvoltage after the cathode side discharge channel forms. The intensity of discharge channels indicates that the anode side subgap first breaks down. When the injection time of preionization is postponed, the uniform discharge in the cathode side converts into streamer. It is also proved that the trigger discharge can preionize each subgap separately, indicating that both photoemission and photoionization mechanisms contribute to the preionization process.