WHY VACUUM-ARC CATHODE SPOTS CAN APPEAR LARGER THAN THEY ARE

The visual appearance of are cathode spots in vacuum is studied experimentally and theoretically, Emission photographs of the spots taken with line radiation have a broad light profile with a rather flat slope (proportional to r-(beta) with beta approximate to 2, r being the distance from the spot center), while photographs taken in absorption are small, having a sharp edge with a steep slope of the profile (beta greater than or equal to 4). Emission photographs from the continuum are similar to absorption photographs. Theoretical analysis shows that the emitted line radiation cannot stem from the locus of excitation. As a consequence, the particles are excited at the edge of the dense spot core, but they radiate at a greater distance due to the finite lifetime of the excited levels and the plasma expansion. Thus, emission photographs from line radiation indicate a greater spot size (about 100 mu m) than corresponding to the active spot radius which amounts to less than or equal to 10 mu m. This statement holds for discharge durations from 10 ns up to at least 100 mu s. The spots exist not only at ignition but during the whole time of the discharge, the location varying due to the spot movement. Absorption photographs show a small size of 10-20 mu m still 200 mu s after ignition.