The Phenomenon of a Cathode Spot in an Electrical Arc: The Current Understanding of the Mechanism of Cathode Heating and Plasma Generation

A vacuum arc is an electrical discharge, in which the current is supported by localized cathode heating and plasma generation in minute regions at the cathode surface called cathode spots. Cathode spots produce a metallic plasma jet used in many applications (microelectronics, space thrusters, film deposition, etc.). Nevertheless, the cathode spot is a problematic and unique subject. For a long time, the mechanisms of spot initiation, time development, instability, high mobility, and behavior in magnetic fields have been described by approaches that caused some controversy. These spot characteristics were discussed in numerous publications over many years. The obscurity and confusion of different studies created the impression that the cathode spot is a mysterious phenomenon. In the present work, a number of typical representative publications are reviewed with the intention of clarifying problems and contradictions. Two main theories of cathodic arcs are presented along with an analysis of the experimental data. One of the approaches illustrates the cathode heating by Joule energy dissipation (volume heat source, a sharp rise in current density, etc.), nearly constant cathode potential drop, and other certain initial conditions. On the other hand, a study using a mathematically closed approach shows that the spot initiation and development are determined not by electron emission current rise but by a rise in arc power density, affecting heat sources including the energy of ion flux to the cathode (surface heat source).