Spectrally and spatially resolved imaging of an anode flare in the initial stage of a vacuum arc discharge

Dynamics of the anode spot generation in the initial stage of vacuum arc with a short gap with gallium-indium liquid-metal electrodes was investigated. Using a spectrometer combined with an intensified CCD camera, spatial and spatio-spectral pattern of plasma emission from the discharge gap were registered with a temporal resolution of 10 ns. Generation of the anode flare begins with the intensive evaporation of anode material in the spark (high-voltage) stage of the discharge. In this period, only lines of neutral atoms are present in the spectrum measured at anode position although higher charge states can be found in the cathode spectra. Intensive ionization of anode flare atoms starts when the discharge changes from the spark to an arc (low-voltage) stage. Lines of single and double charged ions consistently appear in the spectrum. At the subsequent expansion of anode flare, the highest expansion velocity was observed for double charged ion lines, and the lowest one was found for neutral atoms. Expansion velocity was increasing with increasing a voltage and current.