Investigation of an ECR plasma thruster and plasma beam interactions with a magnetic nozzle

Citation

Kaufman, David A. (1995) Investigation of an ECR plasma thruster and plasma beam interactions with a magnetic nozzle. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3gr2-3q55. https://resolver.caltech.edu/CaltechETD:etd-07102007-131210

Abstract

The results of an experimental study of an electrodeless electric propulsion device using electron cyclotron resonance (ECR) heating for plasma production are presented. The effects of pressure, propellant flow rate, and microwave input power on the plasma properties were examined. In addition, the effect of a magnetic nozzle on a plasma beam were examined experimentally and computationally.

A laboratory ECR thruster was operated with argon propellant in a vacuum tank at pressures in the 10(-5) torr range using a 2.115 GHz microwave beam at power levels up to several kilowatts. Several movable plasma diagnostics were used to measure the spatial variation of various plasma properties in the plume of the thruster.

At low pressures, ion flux profiles showed an unexpected annular plasma plume with a depressed ion flux along the thruster axis. The ion energy measurements indicate that the ion kinetic energy is invariant with input microwave power. However, increases in pressure cause the plasma to lose kinetic energy due to friction with the background neutrals in the vacuum tank. Propulsion parameters were calculated from the ion flux and energy data. The results are greatly affected by additional ion flux due to entrainment of the background gas. The plasma potential and electron temperature both decreased with increasing pressure in the tank but were invariant with changes in microwave power. Microwave power reflected from and transmitted through the ECR region was measured and the results indicate that inefficient absorption may contribute significantly to energy losses in the laboratory device.

Plasma detachment from the magnetic nozzle was identified as a critical issue for ECR and other applied-field thrusters. A collisionless model was used to calculate the trajectories of plasma rings in a magnetic nozzle. The code predicted that the plasma will detach under certain conditions and that the acceleration due to the force on the dipole moments of the electrons is inconsequential to the plasma trajectories. The radii of 90° deflection were calculated for several plasma initial conditions, and it was shown that the nozzle configuration can be manipulated to reduce beam divergence and increase the useful radius of the thruster.

An attempt was made to experimentally examine detachment using an ion thruster in an applied magnetic nozzle. The magnetic field could then be varied independent of the ion energy. The grids of the thruster were masked to extract a thin ring of plasma coaxial with the magnetic nozzle, and the ion flux density profile was measured to determine the effect of the magnetic field on the trajectory of the annular plasma. A radial electric field was established in presence of the magnet nozzle that caused the annulus to spread and decrease in radius making detachment unobservable. It is believed that the magnetic field inhibited neutralization of the ion beam causing the electric field to develop.

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