AIAA 2001-3356 Hall Thruster Plume Effects and Sputtering of Spacecraft Surfaces

The use of plasma thrusters on board of commercial and military spacecraft introduces a number of integration challenges due to sputtering and contamination of critical spacecraft surfaces by energetic ions. Although erosion rates for various elements, predominantly metals in the periodic table, have been measured in the past, important spacecraft materials exposed to Xe ions in the appropriate energy range between 50 and 300 eV had never been characterized. In order to accurately predict the effects of the Hall thruster plume on these surfaces during the course of a mission, an extensive database of material sputter rates and optical properties was developed at the Lockheed Martin Space Systems Company. Several samples of spacecraft materials were exposed to a Xe ion source and erosion rates were measured at two different facilities as a function of ion energy and the ion beam incident angle. In addition, changes in transmissivity, absorptance, and emissivity of these materials were measured and characterized as a function of the ion incident angle and the erosion depth. Micrograph images of the exposed surfaces suggested that the observed changes in optical properties could be due to the variation in the surface roughness topology with the ion beam incident angle. Introduction Integration of Hall thrusters on geostationary communication satellites requires a detailed understanding of the potential effects induced by the thruster on satellite operations. The effects related to the thruster plume include a) erosion and sputtering of spacecraft surface materials due to ion impingement, b) re-deposition of sputtered materials onto critical surfaces and surface contamination, c) transmission of bus and payload signals through the ionized plume, d) electromagnetic interference due to thruster radiated and conducted emissions, and e) attitude control disturbances due to thruster body torques and plume impingement on spacecraft surfaces. A series of papers Copyright © 2001 by Lockheed Martin Space Systems Co. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission will be presented to report the work pursued at the Lockheed Martin Space Systems Company in evaluating and characterizing a number of these effects: measurements of Hall thruster plume properties [1], numerical modeling of the plume characteristics including background chamber effects [2], sputter model correlation to the test chamber data [3], and the development of a code for plume impact analysis on satellite communications [4]. This paper addresses experimental work pertaining to the development of a sputter rate database and measurements of the changes in optical/thermal properties of spacecraft materials exposed to a simulated thruster plume. Proper assessment of the plume impacts on the performance of critical spacecraft components, such as solar arrays and optical reflectors, requires the knowledge of sputter rates and changes, if any, in relevant surface properties (transmissivity, absorptance, and emissivity) as a function of ion impingement angle and energy. Although large quantities of sputter rate data exist in the literature, most of the reported rates are for pure metals only, and with coupons exposed to relatively high-energy ions [5], [6], [7], [8]. This work is the first attempt to characterize important spacecraft materials exposed to Xe ions at the energy levels that are observed in the wings of the Hall thruster plume. It is these low-energy ions that do most of the damage to the exposed surfaces [2]. The measurements of material sputter rates were performed on multiple spacecraft materials at ion energy levels ranging from 50 eV to 2000 eV and at ion incident angles (relative to the normal) ranging from 0 degrees to 85 degrees. These measurements were made on discrete material samples at two separate test facilities using controllable ion sources and following two different test protocols. Some measurements were duplicated at the two facilities on a subset of materials in order to identify possible sources of measurement uncertainty and to achieve a high level of confidence in the resulting empirical relations for the sputter rates as a function of incident angle and ion energy. Some of the representative results of these measurements and the 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.