A Comparison of Nude and Collimated Faraday Probes for Use with Hall Thrusters

* Presented as Paper IEPC-01-020 at the 27 International Electric Propulsion Conference, Pasadena, CA, 15-19 October 2001. † Copyright © 2001 by Richard Hofer. Published by the Electric Rocket Propulsion Society with permission. The effectiveness of collimated Faraday probes in obtaining the ion current density profile in Hall thrusters is investigated. A collimated probe design is attractive because it offers the possibility of obtaining the true ion current density profile regardless of the facility pumping speed. The collimator is intended to act primarily as a filter for low energy ions introduced in the plume by charge exchange collisions. Experiments were conducted using a traditional nude Faraday probe and a collimated probe. The probes are evaluated using a 5 kW Hall thruster in a facility capable of maintaining background pressures of 10 Torr for flow rates of 5-10 mg/s. Detailed examination of the results has shown that the probe is interacting strongly with the plasma inside the collimator. Further study is required to bring the collimated Faraday probe to a maturity level sufficient for use in evaluating Hall thruster plumes.