Performance Scaling of Magnetic Nozzles for Electric Propulsion

Abstract

The use of magnetic nozzles (MNs) in electric propulsion (EP) systems is investigated analytically and experimentally. MNs have the potential to efficiently accelerate propellant without the restrictions of electrodes, however, their measured performance has been poor compared to existing EP technology. A theoretical model was developed to understand the requirements for efficient operation. Analytical scaling laws were derived for the mass utilization efficiency, channel efficiency, and MN thermal and divergence efficiencies, in terms of dimensionless parameters that describe the relevant collisional processes in the channel and the radial plasma structure at the MN throat. In comparison to previous MN thrusters, performance levels comparable to state of the art EP systems are only possible if three conditions are met: (1) the thruster operates in a high confinement mode, (2) the plume divergence is significantly reduced, and (3) electron temperatures are increased by an order of magnitude. The final requirement implies these thrusters should be operated with heavy propellants such as xenon to limit the specific impulse to reasonable values.

An experiment was designed to investigate the fundamental dynamics of plasma flow through a MN. The experiment consists of a helicon plasma source and two electromagnetic coils. The plasma parameters are determined at a variety of locations using electric probes mounted on a positioning system. The existence of a critical magnetic field strength for high confinement and the predicted scaling of the mass utilization efficiency were verified. Electron cooling in the magnetically expanding plasma was observed to follow a polytropic law with an exponent that agrees with theory. With decreasing magnetic field, a transition from a collimated plume to an under-collimated plume was found, where an under-collimated plume is defined such that the plume divergence is greater than the magnetic field divergence. This transition was accompanied by the disappearance of an ion-confining potential well at the plasma periphery. Using the potential well as a metric for plasma confinement downstream, it is shown that confinement is lost when the electrons demagnetize via finite Larmor radius effects. The ambipolar ion response to this demagnetization determines the difference between the two plume modes.