Vacuum arc plasma jet interaction with neutral ambient gas

Abstract

Vacuum arc plasma jet (VAPJ) propagation in a neutral nitrogenatmosphere has been calculated numerically on the basis of a hydrodynamicdescription. It was assumed that the VAPJ was emitted isotropically from apoint source located z0 = 20 mm in front of the entrance of a straight duct100 mm in diameter in which an axial magnetic field of 0.02-0.05 T wasimposed. Plasma densities of n0 = 1018 and 1019 m-3 and neutralgas densities nn0 = 0.001-0.1×n0 were used as initial conditionsat the duct entrance. A three-fluid model was used, with the ion temperaturehaving an initial value at the duct entrance of Ti = 104 K, while constanttemperatures of Te = 3×104 K and Tn = 103 K were assumed for theelectrons and neutrals, respectively.It was found that the plasma jet decelerates down to the critical point, i.e. where the jet velocity is equal to the local speed of sound. The distance ofthe critical point from the entrance cross section decreased from zc = 4×R (R = radius of duct) for nn0 = 0.001×n0 down to zc = Rfor nn0 = 0.1×n0. It was found that increasing the magnetic field alsodecreased zc. It was also found that the electrons and ions have rotationalmotion components, in opposite directions, and that friction with the neutralgas decreases the rotational velocities.