Vacuum Arc Cathode Spots
ByAndré Anders
Pages 7

Vacuum arcs produce copious amounts of metal plasma at cathode spots. Arcs are discharges characterized by relatively high current (usually from tens of amperes to kiloamperes) and low burning voltage (typically 20 V and somewhat higher for high currents). The formation of cathode spots is related to the energetically most efficient means of emitting electrons and producing the plasma needed for current transport between electrodes. Cathode spots are the location of intense electron emission facilitated by locally high temperature and electric field. The concentration of power in spots leads to phase transitions of the solid metal, ultimately producing fully ionized plasma containing multiple ion charge states. The dense plasma expands from cathode spots and moves with a velocity of about 104 m/s, corresponding to several tens of electronvolts of kinetic ion energy. Cathode spots exhibit a substructure and tend to group—features that are best described by a fractal model. Cathode spots are fractals, i.e., they are self-similar objects that are approximately invariant to changes of scale. Physical fractals have a lower scale cutoff, which, according to the ecton model of explosive electron emission, is determined by a minimum action required to cause explosive electron emission. Therefore, ecton and fractal concepts of spots are complementary.