The borehole phase of the pseudospark discharge-a transition between hollow cathode and high current phase

The borehole phase is one of the five phases in the development of a pseudospark discharge. In chronological order, the borehole phase follows the low current predischarge and the hollow cathode phase with currents up to some 100 A. This discharge phase makes the transition between the hollow cathode phase and the high current phase which is connected to the appearance of cathode spots. The transition is fast and is characterised by a sudden decrease of the switch impedance. One problem in understanding the borehole phase is the cause of the high current density of more than 10/sup 4/ A/cm/sup 2/ and the mechanism responsible for the emission of such a high density of electrons. Self sustained self sputtering of cathode material, thermionic field emission and the emission caused by impact of discharge gas ions are discussed as possible processes. Different optical and spectroscopic measurements show that secondary emission by gas ion bombardment is the main reason for the high current density. During the borehole phase, only neutral atoms and single ionised ions from the cathode material can be detected, which seem not to have enough energy to extract electrons from the surface. However, bulk ions (i.e. hydrogen) have enough energy to generate secondary emission of electrons.