Refraction and absorption shadow imaging of the vacuum arc cathode spot at an atomic resonance line of cathode vapors

The resonance laser interference and absorption shadow imaging combined with the high-speed IMACON 468 camera was used for studying the dynamics of metal vapors supply into the cathode spot plasma at operation of low-current vacuum arc discharge with liquid-metal cathode. The experiments were performed with a capillary-type liquid-metal cathode made of gallium-indium alloy. The data are analyzed in terms of cyclicity of cathode spot operation. Two distinct modes of cathode spot operation were discerned in images, which are the explosive mode and the steady mode. The explosive mode takes the 2%-part of the spot operation cycle. It follows by the steady mode (∼20% of the spot cycle time). The rest time the spot is transparent at the resonant line of cathode vapors. At a discharge current above 50A, the spot is always in the transparent mode.