Time-Resolved Spectroscopy of Single Cathode Spots: Comparison of Cathode and Anode Position

Optical emission of a single cathode spot in vacuum was studied to improve the understanding of cathode spot initiation and dynamics. Discharges with high reproducibility were generated by applying rectangular voltage pulses of 100-ns length to a liquid Ga-In-alloy cathode. In the first pulse, the cathode spot is ignited in vacuum, and its radiation is delayed by several tens of nanoseconds. In the second pulse of opposite polarity, the cathode spot at the former anode emits strong radiation almost immediately after the inversion of the current because of the ignition in the plasma-filled gap. Atomic and ionic line radiation is observed with slightly different delay of their intensity maxima in the first and the second spot ignition.