A high-speed probe positioning system for interrogating the discharge plasma of a 30 cm ion thruster
A method for delivery of a symmetric double probe into the discharge chamber of a 30-cm diameter ringcusped ion thruster is discussed. Motivation for direct access of the electrostatic probe to the discharge chamber stems from the need to characterize the discharge plasma to better understand the possible cause of discharge cathode assembly (DCA) erosion. A symmetric double probe allows electron temperature and number density measurements with minimal perturbation to the discharge plasma and thruster operating conditions. The High-speed Axial Reciprocating Probe positioning system (HARP) was used to further minimize thruster perturbation during probe insertion and minimize heating of the probe. Integration issues and selection criteria for hardware used for interrogation of the discharge plasma are reviewed. Preliminary plasma parameters were measured 2.6 mm downstream of the keepered DCA at various discharge cathode operating conditions. The electron number density was found to range from 8.9×1010 - 2.1×1012 cm-3 with the maximum occurring at DCA centerline for each operating condition. Electron temperature ranged from 3.2 - 4.8 eV, consistent with predictions for ion thrusters incorporating a ring-cusped design. Improvements for increasing the accuracy of future mappings of the discharge plasma are suggested. © 2002 by Daniel A. Herman.
