Characteristics of Cold Argon Arc-Jet Plasma Flowing Along Open-Field-Line and the Effects of Collisions on Deceleration

We report basic characteristics of a cold supersonic argon arc-jet plasma accelerated along open-field-line. The arc jet is generated by arc discharge, and ejected through an anode-nozzle into a rarefied gas wind tunnel with a uniform longitudinal magnetic field. The plasma further expands at the end of magnets, which we refer to as a magnetic-nozzle, where we examine variation in plasma parameters using a para–perp type Mach probe and a four-tip type Mach probe. Our experiments show that plasma potential drops at the magnetic-nozzle area and that the ion flow velocity increases up to about Mach 3, followed by deceleration into subsonic transition. To discuss the mechanism of acceleration and deceleration, we assume a 1-D quasi-neutral flow, and constructed a model to describe the deceleration because of friction between ions and neutral particles at the end of electromagnets, where charged particles are accelerated but neutrals are not. We find reasonable agreement of the experimental results with our modeling of the acceleration and deceleration, although a few issues are left for us to improve.