Study of a single cathode spot in vacuum produced by short voltage pulses

Summary form only given. The optical emission of single cathode spots is studied to improve the understanding of the vacuum spot initiation and dynamics. A single cathode spot was generated with high reproducibility in a gap under UHV conditions with a liquid metal cathode of Gain alloy. Discharges are initiated by rectangular voltage pulses of 100 ns produced by a cable generator. By varying the electrode distance the current pulse can be controlled because of the different commutation time of the cathode and anode plasma. Applying a distance of 1 mm the current starts to rise significantly with a delay of about 15 ns after the voltage rise and reaches over 100 A within 40 ns. The electrical setup enables an abrupt end of the current pulse within less than 10 ns. The optical radiation of the spot has been observed by a 0.5m spectrograph in combination with a streak and an intensified CCD camera. Setup and triggering enable the study with temporal resolution in the ns scale and a jitter of much less than 10 ns. Typically, the spot radiation of at least 50 pulses was accumulated to obtain a sufficient intensity for the distinguishing of several lines in the spectrum. Spectral lines of Ga and In atoms and single as well as double charged ions have been observed simultaneously. From preceding spectroscopic investigations of the spark and arc stages of the cathode spot it is already known that the emission is dominated by ionic lines in the beginning whereas the highest intensity of the atomic lines follows with delay of few hundred ns. In the study additional details are revealed like a radiation delay after the current rise and the continuation of the line radiation after the current pulse.