Mysteries of the arc cathode spot: A retrospective glance

The properties and the behavior of arc spots on cathodes, especially in vacuum arcs-the purest form of electrical discharges-are most surprising and very difficult to comprehend and to disclose at first glance. Their experimental exploration and theoretical interpretation took several decades and the task is not yet completed, in spite of many efforts and much progress. It remains an exciting problem and a challenge for future physical investigations, mainly due to the highly complex nature of arc spot operation, which calls for joint endeavors of several disciplines of physics, mathematics, chemistry, and material science. The effort is justified also by the technical applications and their requirements. I point out as a personal view some of these characteristic problems in the understanding of arc spots without claim to completeness. The main issues discussed are the cathode as the decisive electrode for the existence of electrical discharges, comparison with the anode, the strange properties of vacuum arc spots, their relation to glow cathodes and high pressure arc cathodes, the necessarily limited parameter range of arc spots, the astonishing, and the miraculous interplay of very different processes in arc spots cooperating in such a way to implement all the conditions to enable vacuum arcs, the conditions within the expanding cathodic plasma cloud (only briefly discussed) and, finally, arc spots as complex thermodynamic systems and self-organizing dissipative structures.