Investigation of Vacuum Breakdown in Pulsed DC Systems

Vacuum breakdown in the pulsed DC experiment at CERN was investigated by means of electrical measurements, SEM and STEM imaging of the surface and cross sections of the copper electrodes, and numerical modeling. The breakdown sites comprise one or more craters. There is plastic deformation beneath the craters. The crater(s) represent the epicenter of the breakdown, where electron emission and vaporization of the metal occur, and the plasma ball is formed. The only possible mechanism of ionization of neutral vapor seems to be ionization by the emitted electrons. The pressure exerted by the plasma ball is responsible for the formation of the crater. Simulations have been started with the aim to describe the mechanisms of vacuum breakdown on copper cathodes with protrusions of different geometries. First results show that, while Joule heating is initially a minor effect, together with the Nottingham effect, it may lead to a rapid increase of the protrusion temperatures up to the critical temperature, which may be indicative of a microexplosion. Very thin protrusions are not critical for the initiation of a microexplosion and subsequent breakdown; rather, the breakdown may be initiated by significantly wider ridge-like structures. The numerical model will be further developed so as to describe the whole evolution of the breakdown in vacuum.