Structure and dynamics of high-current arc cathode spots in vacuum

Experiments are reported on the number and displacement velocity of are spots on CuCr and Cu cathodes in the current range 40-1500 A. The spot number was found to increase linearly with current. The average current per resolvable spot amounted to 100 +/- 30 A for CuCr and 150 +/- 70 A for Cu. For times greater than or equal to 3 ms after ignition random spot displacement R was observed, having mean square values of [R(2)]/Delta t = (1 +/- 0.3) x 10(-3) m(2) s(-1) for Cu and [R(2)]/Delta t = (4 +/- 1) x 10(-4) m(2) s(-1) for CuCr. The Cu spots showed brightness fluctuations with intervals of T-f = (17 +/- 3) mu s. Because of this obvious dynamics, the theoretical models of the spot plasma must be time-dependent. A self-consistent theoretical description of Cu and Cr plasmas is given, yielding the cathode temperature, plasma density, electric field strength, current density and plasma velocity in the time range 10 ns to 3 ms.