Temporal and spatial behaviour of the single cathode spot ignition in vacuum

Optical emission of a single cathode spot was studied to improve understanding of cathode spot initiation and dynamics. A single cathode spot was generated with high reproducibility in a gap under UHV conditions at a liquid Ga-In-alloy cathode. Discharges were initiated by rectangular voltage pulses of 100 ns length produced by a cable generator. The spot radiation was recorded in time with a high temporal and spectral resolution by a spectrograph coupled with a streak camera and a CCD camera. Cathode spot light radiation was found to be as weak in the discharge beginning as the equipment set records nothing. Cathode spot radiation becomes bright when cathode spot plasma approaches closely to the anode. Unlike spot initiation in a pure vacuum gap, a cathode spot starts as a bright flash when the gap is filled with plasma.