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Plasma Structure and Behavior of Miniature Ring-Cusp Discharges

  • Author(s): Mao, Hann-Shin
  • Advisor(s): Wirz, Richard E
  • et al.
Abstract

Miniature ring-cusp ion thrusters provide a unique blend of high efficiencies and millinewton level thrust for future spacecraft. These thrusters are attractive as a primary propulsion for small satellites that require a high delta V, and as a secondary propulsion for larger spacecraft that require precision formation flying, disturbance rejection, or attitude control. To ensure desirable performance throughout the life of such missions, an advancement in the understanding of the plasma structure and behavior of miniature ring-cusp discharges is required.

A research model was fabricated to provide a simplified experimental test bed for the analysis of the plasma discharge chamber of a miniature ion thruster. The plasma source allowed for spatially resolved measurements with a Langmuir probe along a meridian plane. Probe measurements yielded plasma density, electron temperature, and plasma potential data. The magnetic field strength was varied along with the discharge current to determine the plasma behavior under various conditions. The structure of the plasma properties were found to be independent of the discharge power under the proper scaling. It was concluded that weaker magnetic fields can improve the overall performance for ion thruster operation.

To further analyze the experimental measurements, a framework was developed based on the magnetic field. A flux aligned coordinate system was developed to decouple the perpendicular and parallel plasma motion with respect to the magnetic field. This was done using the stream function and magnetic scalar potential. Magnetic formulae provided intuition on the field profiles dependence on magnet dimensions. The flux aligned coordinate system showed that the plasma was isopycnic along constant stream function values. This was used to develop an empirical relation suitable for estimating the spatial behavior and to determine the plasma volume and loss areas.

The plasma geometry estimates were applied to a control volume analysis on the plasma electrons. Balancing the plasma electron generation and loss yielded nominal values used in miniature ion thrusters. This result was ultimately used to develop a design tool for miniature discharges. This tool was used to perform a parametric evaluation on the magnet field configuration of the research mode. By understanding the plasma behavior, significant improvements over the baseline configuration were obtained with relatively minor changes, thus revealing the importance of plasma structure on the performance of miniature ring-cusp discharges.

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