Cathodic are spots: Ignition probability as a a fundamental concept to describe spot types, phases, and motion

Cathode spot operation is initiated by a thermal rnnway process of an electron emission site, leading to explosive destruction of the site. Arcs are based on cyclic processes, and perpetual ignition of spots is critical. By considering the probability of ignition as depending on surface condition and field enhancement, many properties of arc spots such as spot type, plasma composition, voltage fluctnations, etc. can be consistently described. The actual surface field and ignition probability is mainly determined by local field enhancement whose nature is very different for "contaminated" and "dean" surfaces. The azimuthal symmetry of ignition probability is broken when a transverse niagnetic field is applied.