Facility Effects Associated with Ion Beam Neutralization*

Gridded ion thrusters utilize a separate electron source to neutralize the thrust producing positive ion beam. The neutralizer coupling voltage drives electrons to the beam. The coupling voltage is dependent on the relative ease for electrons to reach the beam as well as facilitate the neutralization process therein. The neutralization process however is complex and remains poorly understood. Neutralization processes that take place in ground tests can differ appreciably from those that occur in space owing to the presence of a relatively denser charge exchange plasma and the proximity to ground potential surfaces. Variations in neutralizer-chamber wall voltage coupling, neutral background pressure, and external magnetic fields are important facility effect considerations that must be considered when attempting to replicate space conditions. In this particular study, we characterize electron transport from the neutralizer to three potential loss surfaces—the beam, the chamber walls, and the thruster plasma screen. Through such a study, we aim to better clarify the facility effect impacting neutralizer beam coupling and to pose potential solutions for better mimicking space conditions in ground test facilities.