Critical condition for plasma confinement in the source of a magnetic nozzle flow

The existence of a theoretically predicted critical magnetic field strength for efficient plasma confinement in helicon plasma thrusters is verified experimentally in the source of a magnetic nozzle (MN) flow. Control of the plasma confinement is crucial for enhancing the mass utilization efficiency of electric propulsion systems that employ MNs. Langmuir probe measurements of the density at the MN throat of a helicon plasma thruster as a function of the applied magnetic field strength indicate a transition from a low-confinement operation mode, in which a majority of the plasma diffuses to the solid walls of the plasma source before emerging from the thruster, to a high-confinement operation mode, in which the plasma preferentially exhausts downstream through the MN. This transition is shown to be governed by the anisotropic Péclet number, Pe_an, which is defined as the ratio of the advective (field aligned) to diffusive (cross field) mass transport rates. Experimental estimations of the mass utilization efficiency of the plasma source for various magnetic field strengths and plasma source lengths are shown to support an analytically derived scaling law, and suggest Pe_an ≥ 1 as a design criterion for MN plasma sources.