Characteristics and breakdown initiation of triggered spark gaps with uniform applied field at very high voltages

The characteristics of the trigatron spark gap, in which the main electrodes are shaped to give an approximately uniform applied field, have been investigated. It is shown that this form of trigatron spark gap operates over a wider range of voltage and with shorter and more consistent time lags to breakdown than conventional trigatrons having spherical or hemispherical electrodes. The luminous phenomena which occur during breakdown initiation have been studied using an image convertor as an electro-optical shutter. Breakdown occurs in three stages. Firstly, a burst of corona-like discharge, sometimes with a bright stem at the origin, was initiated at the earthed electrode and was directed towards the opposite electrode. Secondly, the corona-like discharge was followed by a bright filamentary discharge (referred to as a leader stroke), usually starting from the anode and propagating across the gap towards the cathode. Finally, an even brighter discharge occurred (referred to as a main stroke) starting from the cathode and tracing back the route of the leader stroke towards the anode. Leader-stroke velocities fell within the range 5×107¿6×108cm/s, depending on experimental conditions. Main-stroke velocities were about 108¿109cm/s. It is shown that the phenomena occurring during breakdown can be explained by a mechanism of the streamer type.