Characterization of the Magnetic Nozzle Region of High Powered Electric Propulsion Thrusters Using Numerical Simulation, RF Interferometry and Electrostatic Probes.

Abstract

Experimental results are presented from the plume of a high powered (200 kW) DC plasma gun emitting into an applied magnetic nozzle. The plasma source operated on helium and hydrogen and was attached to a large (3 m x 5 m) vacuum chamber kept at low background pressure (< 2x10-6 Torr). Density profiles, electron temperature and ion velocity are measured in a region where the ratio of plasma kinetic pressure to magnetic pressure was Beta = 0.2 – 20. Numerical simulations are employed to compare experimental results with theoretical predictions of plasma detaching from magnetic fields. Significant particle deviation from confining magnetic fields was found for conditions approximating Beta > 1 in accordance with magnetic detachment theory.

Unique accomplishments of this research include detailed measurements of propulsion-appropriate plasmas exiting a magnetic nozzle and transitioning from Beta < 1 to Beta > 1. This region is of particular interest for magnetized plasma thrusters since inefficient magnetic detachment may result in a serious efficiency penalty for their use in proposed in-space propulsion systems. Nozzle efficiency estimates are provided based on simulated and measured experiment conditions. In particular, an optimized magnetic nozzle condition is found that theoretically improves nozzle efficiency by 10% over the standard magnetic dipole condition.

Plasma diagnostics are utilized, including microwave interferometers and Langmuir triple probes. Diagnostic theory is reviewed for these tools, specifically for the conditions found in this experiment. Prior theory was sometimes found inapplicable to the experimental conditions, particularly in the case of a Langmuir triple probe in a flowing plasma. To make up for inadequacies in standard theory, numerical simulations were conducted to find calibration factors for the appropriate experimental conditions.

In addition, a new measurement methodology is developed utilizing electrostatic probes and microwave interferometers in tandem. Detailed density profiles were collected using this method, and a comprehensive error analysis was conducted. The error in density measurements was determined to be much lower than the error in electrostatic probe measurements, and on the order of microwave interferometer uncertainty – as low as 10%.