Three modes of plasma expansion in a vacuum arc

A steady-state expansion of current-carrying plasma into vacuum has been studied theoretically. Such a situation arises in the vacuum arc discharge where the dense plasma streams from a cathode spot region with a super-sonic velocity. It has been shown that three characteristic modes of plasma expansion into the vacuum ambient may be distinguished in accordance with the conical, parabolical and cylindrical shape of plasma flaw. These modes correspond to the cases of expansion with electron cooling, isothermal expansion and expansion with electron heating. The parameters controlling formation of the different modes have been found.