A mechanism for nanosecond cathode spot operation in vacuum arcs

A model for nonstationary cathode spot operation in vacuum arcs is proposed. The model takes into account increasing spot current during the spot life time. Calculations for a copper cathode indicate that the spot current density increases when the rate of rise of the spot current dI/dt increases and can exceed 10/sup 8/ A/cm/sup 2/ for dI/dt about 10/sup 11/ A/s. The plasma density is 10/sup 20/-10/sup 21/ cm/sup -3/. The plasma electron temperature T/sub e/ is about 3-4 eV when the cathode potential drop U/sub c/=15-20 V and T/sub e/ reaches 7-8 eV when the U/sub c/ fluctuates and increases up to 40-50 V. The calculated spot parameters are in the range measured in recent experiments with nanosecond time resolution. The effect of different mechanisms of electron emission (thermo-and thermo-field emission) was investigated. The self-sustained solution shows that the spot current density changes by factor 2 by using different emission mechanism in the system of equation.