## 3 Experimental Investigations of Hall Thruster Erosion
### Thruster Tests  
The Fakel SPT-70 and SPT-100 Hall thrusters successfully operated on Soviet Union satellites in the 1970s and 1980s, but interest in lifetime tests was renewed in the early 1990s as the life of geostationary spacecraft improved and thruster life requirements grew from 700-2000 h to 4000-6000 h [12]. Early available data on thruster erosion therefore comes from workers in the United States and the former Soviet Union states attempting to qualify the 1.35 kW SPT-100 for longer-term operation with xenon propellent. The SPT-100 discharge chamber has a length of 2.5 cm, inner radius of 3.5 cm, and outer radius of 5 cm, resulting in a total discharge chamber volume of approximately 100 cm\({}^{3}\)[7]. The initial thicknesses of the inner and outer annulus walls are approximately 11 mm and 5.5 mm, respectively [14; 15]. Erosion rates were reported as the loss of thickness or volume per unit time and were determined by periodically measuring changes in either the wall thickness or mass. Four notable results were garnered from these tests. First, erosion was most prominent in the 10 mm closest to the thruster exit plane (where ionization and ion acceleration are most prevalent). Second, the erosion rate decreased exponentially with time from \(\sim\)10 \(\upmu\)m/h to \(\sim\)1 \(\upmu\)m/h. Third, the outer wall eroded more rapidly than the inner wall. Fourth, thrust and specific impulse decreased by 5-8% over the first few hundred hours of thruster operation, and discharge electrical oscillations were not consistent throughout thruster life [14; 15; 44]. Later tests of the Keldysh Research Centre 1.3 kW T-100 [45] and KM-series thrusters [19], Snecma 1.3 kW [16; 17] and 2.5 kW [20] variants of the PPS-1350, Pratt & Whitney 10 kW T-220 [46], NASA 50 kW T-400M [47], and Aerojet Rocktedyne 4.5 kW BPT-4000 [18] found similar trends in erosion rate and performance loss but did not universally observe more rapid erosion of the outer wall. The T-220 campaign reported variation of erosion rate with azimuthal position, and the BPT-4000 test campaign measured no erosion after 5600 h of operation. The BPT-4000 result was explained as a \"magnetic shielding\" effect and led to the development of the Zero-Erosion(tm) thruster line [48]. Hall thrusters are typically operated and characterized with xenon propellent. Assessment of the impact of alternative propellants on the SITAEL 5 kW HT5k performance found that use of krypton and krypton-xenon mixtures, compared to pure xenon, resulted in higher wall erosion rate [49]. Other efforts have quantified wall erosion in thrusters operating with krypton propellent, but more work is required to compare krypton and xenon erosion trends and to understand the erosion characteristics of other alternative propellants, such as argon and iodine [47; 50]. Other notable work to experimentally characterize plasma-induced erosion in Hall thrusters includes that of [PERSON] et al. [51], who tested the relative erosion rates of Hall thruster channels constructed from various grades of BN composites, and [PERSON] et al. [52], who found erosion rate increases with discharge voltage. The importance of pressure facility effects on erosion rate was recently examined by [PERSON] et al. [53] and [PERSON] et al. [54]. In parallel, other researchers [46; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64] have developed in-situ erosion rate measurement techniques, such as telemicroscopy and spectroscopy. The most surprising erosion result from thruster life tests is the formation of mm-scale sawtooth erosion patterns in both the inner and outer annulus walls. First reported in the literature by [PERSON] al. [65], so-called anomalous erosion ridges are a ubiquitous presence in Hall thruster walls and can be detected after fewer than 100 h of operation [13]. Erosion ridges are clearly discernable in Figure 6, for example, which shows the BPT-4000 [18], PPS-1350 [66], and SPT-100 [15] after life tests.