THE DYNAMICS OF ARC CATHODE SPOTS IN VACUUM

High-speed emission pictures of are cathode spots in vacuum have been obtained in a spectrally narrowed range. It has been found that the total radiation was delayed with respect to the are current by 8.5 ns for Ti cathodes because of the limited transition probability of excited states. This effect sets lower bounds to spatial and temporal resolution. By selecting a spectral range with prevailing continuum radiation, the resolution could be improved down to 5 mu m and 2 ns, respectively. In this way a typical size of the spot plasma of about 10 mu m was observed for nanosecond discharges as well as for durations up to 100 mu s. The spots showed random displacements with a diffusion constant D = 4 x 10(-4) m(2) s(-1). The spot brightness was subjected to periodical fluctuations with discrete intervals T-n, n = 0-3. The measured values obey the relation T-n/T-o = (D/4 kappa)(n), with kappa being the thermal diffusivity of the cathode material. The smallest time T-o is assumed to be the elementary spot lifetime tau(s). It shows a distribution according to a 'survival law' with an average value of 14.2 ns for Ti. These results agree with studies of laser absorption pictures and surface craters. They are in line with explosive-spot models. The occurrence of fluctuations at discrete intervals T-n is explained by heat accumulation at the cathode surface during the random walking of the spots.