Sequential modes of drawn vacuum arcs between butt contacts for currents in the 1 kA to 16 kA range

The development of vacuum arcs on Cu-Cr butt contacts was studied using high-speed photography for currents up to 11 kA rms, with separation in the first 1.5 ms of the cycle. Upon separation at a current I_s, one or two initial bridge column arcs formed after the rupture of the molten metal bridge. The arc development can be divided into three sequences, based on the peak current and the average separation current per bridge column, I_s*. For I_s*<~5.5 kA, the bridge arc first expanded while retaining its localized appearance. The expansion continued into a mode which had a diffuse appearance with cathode spots spreading outward. If the peak current was below, ~6 kA, the arc became fully diffuse. If the peak current was above ~6 kA, the arc abruptly collapsed into a diffuse columnar mode before the current peak. In the third sequence, a single bridge arc with I,>~6 kA remained stable as it developed into a diffuse column. The results are compared to earlier studies with and without applied axial magnetic fields