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

TLDR

The authors measured arc spot number and displacement velocity on CuCr and Cu cathodes over 40–1500 A and developed a self‑consistent model that predicts cathode temperature, plasma density, electric field, current density, and plasma velocity from 10 ns to 3 ms. The measurements reveal that spot number increases linearly with current, that each resolvable spot carries a characteristic current on CuCr and Cu, that post‑ignition spot displacement becomes random with material‑dependent mean‑square values, and that Cu spots show brightness fluctuations, implying that spot‑plasma models must be time‑dependent.

Abstract

Experiments are reported on the number and displacement velocity of arc 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 for CuCr and for Cu. For times after ignition random spot displacement R was observed, having mean square values of for Cu and for CuCr. The Cu spots showed brightness fluctuations with intervals of . 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.