A mechanism for refractory cathode spot operation in a vacuum arc is proposed, based on the presence of a finite electric field Ep at the cathode sheath–plasma interface. The proposed model determines the relationship between the electron current and the total spot current density j and includes a system of equations solved for cathode plasma containing a mixture of ion states. The calculation shows that Ep reaches a value, ∼106V/cm, and j is 104–108A/cm2, when the electron current fraction s increases from 0.5 to 0.9. It is discovered that for a refractory cathode, ions with high ionicity (from 2 to 4) can be generated in the spot region for a relatively small s, 0.7–0.9, in contrast to ionization in the plasma jet, as predicted previously for copper cathodes.

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