What Heats the Cathode Spot of a Vacuum Arc?

According to the existing vacuum arc (VA) models, the cathode spot should be heated up to 5000 K. The processes to heat the cathode are ion bombardment, joule heating inside the cathode, and flux of the hot plasma electrons back to the cathode. Heating by ions is considered as the most important process. Analysis of the emission process of the VA shows that at an experimentally observed current density of 10(8) A/cm(2), the heat delivered to the cathode by ion current is way below that necessary to heat up the cathode spot. Joule heating could heat the cathode only if the current density is substantially above 10(8) A/cm(2). The role of plasma electrons in cathode heating is discussed.