A Cyclical Explosive Model of the Cathode Spot

A fundamental role of microexplosions on the cathode in discharge initiation and development is established on the basis of the author's earlier studies of pulsed vacuum discharges. Upon analyzing some methods of vacuum arc initiation, the conclusion is reached that all of them are based in the end on the occurrence of a microexplosion on the cathode. Self-maintenance of an arc is due to the fact that each preceding microexplosion creates the conditions for appearance of one or a few new microexplosions. It is shown that microexplosions occur in a cyclical manner, i.e. they appear due to fast heating of the cathode microarea and then cease because this area cools in consequence of the decreased current density, the heat conductance, the intensity of electron emission, and evaporation. It is suggested that new microexplosions necessary for self-maintenance of a vacuum arc occur due to the explosion of a metal neck which forms on the detachment of a liquid drop from a point, when a current density of up to l09 A · cm-2 flows through the neck. The drops form due to the high-pressure plasma (up to 109 Pa) that affects the liquid metal. The suggested model explains many wellknown experimental results: the high current density in a cathode spot, the cathode spot random motion, the emission of microdrops from the cathode spot, the threshold arc current, etc.