Spectroscopic Study of a Single Vacuum-Arc Cathode Spot

The cathode-spot plasma in spark and arc stages of a vacuum discharge was studied spectroscopically. A single spot was generated with high reproducibility in a gap under ultrahigh-vacuum conditions with a liquid-metal cathode of GaIn alloy. The self-breakdown of the vacuum gap resulting in a short discharge of less than 100 ns as well as a discharge over and up to 3 mus has been considered. The combination of a 0.5-m spectrograph with a streak camera enabled observation of spot evolution with a time resolution in the nanosecond range. Applying the streak camera as an image converter, time-integrated spectra resolved in the direction along the arc axis have been obtained. Limits concerning wavelength and time resolution as well as the emission intensity are discussed. Spectral lines of Ga and In atoms and single- and double-charged ions have been observed simultaneously. At the beginning of the discharge, ionic lines of higher charge state and wide line broadening dominate the spectrum. With a delay of several hundreds of nanoseconds, atomic lines appear and fall down in intensity to a much lesser degree than the ionic lines. Hence, atomic lines finally dominate in the arc stage of the discharge.