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414 Chapter 8 contamination are thruster material sputtered by energetic ions, as well as spacecraft material sputtered by the main thruster beam. In ion thrusters, sputter erosion of grid material not only limits thruster life, but the sputtered grid material may be a significant source of contamination to spacecraft surfaces. This was recognized early in the development of commercial ion thrusters [16], and as a result, a third grid was added to reduce the amount of sputtered grid material coming from the thruster and to shield the spacecraft from grid sputter products. The third grid has the added benefit of dramatically reducing the grid sputter rate by preventing charge-exchange ions made downstream of the third grid from hitting the accelerator grid [17]. For ion thrusters with metal grids, the problem of contamination in the absence of a third grid can be quite important. Only a few monolayers of a metallic contaminant can make large changes to the optical, thermal, and electrical properties of spacecraft surfaces. For Hall thrusters, the situation can be quite different. The plume from Hall thrusters normally has about twice the angular divergence of an ion thruster, and so sputtered thruster material comes out at large angles. However, early in life most of the contamination comes from sputter erosion of the ceramic channel wall. Although this can produce a substantial flux of sputter products, the products are mainly insulating molecules. Deposition of sputtered insulators, such as Hall-thruster channel ceramic or solar-cell cover glass materials, has little effect on the spacecraft surface optical and thermal properties. More problematic is the sputtered metallic material from the late life erosion of Hall-thruster magnetic pole pieces. In the same manner as with ion thrusters, very thin layers of the deposited metal can radically change the properties of spacecraft surfaces. One effect discovered with Hall thrusters, but common to both ion and Hall thrusters, is that surfaces can experience net deposition of sputter products or can be eroded away by energetic beam ions, depending on their location with respect to the thruster ion beam [13]. As shown in Fig. 8-13, the plume of sputtered products coming from the thruster is normally much narrower than the main ion beam. For surfaces at small angles with respect to the thrust vector, sputtering from the beam ions is greater than the deposition of thruster erosion molecules. These surfaces will erode over time. However, surfaces located at large angles to the thruster vector are contaminated by thruster erosion products faster than they can be sputter away by energetic beam ions. Over time, sputtered thruster material will accumulate on these surfaces. For the SPT-100 Hall thruster, the dividing line between erosion and deposition is about 65 deg [13].PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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