What Plasma Density and Temperature Can Support Unipolar Arc?

The unipolar arc (UA) consists of two parts: the emitting electrons cathode part and the anode part collecting electrons. The central (emitting) part operates as a vacuum arc with relatively well-established experimental parameters: cathode voltage drop, current density, and minimal arc current ( Imin). The current distribution in the background plasma was considered to find the condition of the UA existence, which is the ability of the background plasma to provide the return current at a level above Imin. Two situations were considered: with and without external magnetic field. It was found that in the magnetic field absence, the condition for the UA existence is determined mostly by the plasma temperature. At the same time, the magnetic field dramatically shifts the domain of the unipolar existence toward higher temperatures and higher plasma densities.