Vacuum arc plasma jet interaction with neutral ambient gas

Vacuum arc plasma jet (VAPJ) propagation in a neutral nitrogen atmosphere has been calculated numerically on the basis of a hydrodynamic description. It was assumed that the VAPJ was emitted isotropically from a point source located z₀ = 20 mm in front of the entrance of a straight duct 100 mm in diameter in which an axial magnetic field of 0.02-0.05 T was imposed. Plasma densities of n₀ = 10¹⁸ and 10¹⁹ m^-3 and neutral gas densities n_n0 = 0.001-0.1×n₀ were used as initial conditions at the duct entrance. A three-fluid model was used, with the ion temperature having an initial value at the duct entrance of T_i = 10⁴ K, while constant temperatures of T_e = 3×10⁴ K and T_n = 10³ were assumed for the electrons 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 of the critical point from the entrance cross section decreased from Z_c = 4×R (R = radius of duct) for n_n0 = 0.001×n₀ down to z_c = R for n_n0 = 0.1×n₀. It was found that increasing the magnetic field also decreased z_c. It was also found that the electrons and ions have rotational motion components, in opposite directions, and that friction with the neutral gas decreases the rotational velocities.