Observation of an anode spot shell at the high-current vacuum arc

Stable and bright shells were observed at the anode of a high-current vacuum arc burning for 10 ms at current of up to 15 kA in peak. A shell covers a hot spot at the anode. Substance inside a shell is dark. Light from a shell is emitted mostly by neutral atoms. A distinct bright shell is surrounded by substance emitting light in ion lines. Appearance of shells looks to be a result of interaction between cathode and anode jets. Shell dimensions depend inversely on the arc current. Anode spot shells were recognized on copper-chromium electrodes to be a stable object. However, similar shells on pure copper are much less stable and appear only at heat-insulated liquid protrusions and droplets flying in a gap. This fact indicates that the evaporation rate plays a key role in appearance of shells. In addition, evaporation rate of a plane copper-chromium surface under high-current vacuum arc is supposed to be as high as that of heat-insulated liquid copper protrusions and droplets.