Investigation of cathode spots and plasma formation of vacuum arcs by high speed microscopy and spectroscopy

Though vacuum arc cathode spot has been under investigation for more than a century, its structure and operation is still discussed controversially. Experimentally measured parameters and descriptions of the visible structures given by different authors are far from fitting into an uniform picture. In the present paper, the wide range of findings, parameter values, and phenomenological descriptions will be explained as different semblances of a complex structured object. The random behavior of the spot related parameters, mostly stated as a factor complicating their accessibility, gives the key for a better understanding of its nature. It will be shown that the arc spot may be characterized by the statistical self-similarity of its structure. Recognizing fractal structures as one of the main features of the arc spot, its similarity on different spatial scales is understandable. Scale dependent values of structure dimensions and time constants are inevitable consequences of that “fractal picture.”