Current density and the cathode spot lifetime in a vacuum arc at threshold currents

Abstract

The autograph method was used to investigate the arc spot motion over a virgin tungsten cathode under a magnetic field of H=0.1-0.7 T in the current range 2-50 A. The cathode spot (CS) has been found to start dividing at a current of a few amperes. The current density measured at the moment of CS death is (1.2+or-0.5)*1011 A m-2 for a cold cathode and (3+or-1)*1010 A m-2 for a hot one. The CS lifetime is 25-50 ns for a cold cathode and it increases to 150-200 ns for a heated one. Neither the current density nor the CS lifetime depends on current. Estimates have shown that the erosion rate decreases with current and increases with temperature. The backflow electron current from the CS plasma is invoked to account for the experimental evidence.