## 5 Hall Thruster Erosion Reduction Concepts  
Experimental and modeling efforts have shown that Hall thruster erosion rate increases with ion energy and ion flux to the wall. Attempts to reduce erosion have therefore centered on the development of Hall thruster variants that utilize different electric and magnetic field configurations to shield thruster walls from higher energy ions. A prominent example, with origins that can be traced back to the 1960s, is the thruster with anode layer (TAL). This variant uses metallic walls, features a shorter discharge chamber, and employs metallic guard rings to provide a voltage bias that reduces electron wall losses. The bulk of the ionization and ion acceleration regions, where the most wall erosion occurs in an SPT, is pushed beyond the thruster exit plane. The TAL has extensive laboratory test and flight heritage in Russia but has not gained widespread acceptance as a flight thruster in the United States and Europe [101; 3; 102; 103]. As noted above, the BPT-4000 extended life test reported an undetectably low wall erosion rate after 5600 h of operation [18]. Reduction in wall thickness caused by erosion early in thruster life changed the wall surface location relative to the magnetic field. This, in turn, altered the electric field near the wall and resulted in near-wall plasma conditions that more closely resembled those in the bulk of the discharge. The effect of this so-called \"magnetic shielding\" was to reduce the magnitude of the potential gradient in the sheath and to shift the electric field direction at the wall from pointing towards the wall to pointing along the wall [104; 105; 106; 107; 108]. The baseline technology of the BPT-4000 was licensed from Busek, and the BPT abbreviation represents \"Busek-Primex Thruster.\" Aerojet Rocketdyne performed the qualification test that demonstrated low wall erosion, and NASA performed the extended lifetime test and much of the simulation and analysis work to provide the theoretical basis for magnetic shielding. Aerojet Rocketdyne captured the use of the low-erosion magnetic field topology in its Zero-Erosion(tm) Hall thruster line, and NASA utilized the concept to develop its own set of magnetically-shielded thrusters. The combined efforts of Aerojet Rocketdyne and NASA resulted in the 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) that will serve as the Advanced Electric Propulsion System (AEPS) thruster for the Lunar Gateway in the Artemis program. Extensive qualification testing shows the expected lifetime of the thruster exceeds 50,000 h and that its performance is comparable to that of a standard SPT [11]. Thruster life is expected to be limited by magnetic pole, rather than discharge chamber wall, erosion [108; 109]. Figure 8 shows an image of the AEPS Engineering Test Unit 1. Less technologically mature concepts include the cusped-field plasma thruster, the cylindrical Hall thruster, the wall-less Hall thruster, and the external discharge plasma thruster. These designs utilize unique magnetic field configurations to reduce ion flux to the wall. The cusped-field plasma thruster discharge chamber can be conical or cylindrical (as opposed to annular) and utilizes magnets in the chamber wall along the thruster axis to form magnetic cusps that limit electron diffusion to the anode [110]. The cylindrical Hall thruster features a cylindrical discharge chamber and places magnets near the thruster anode and in the thruster wall [111; 112]. Removal of the inner wall in both the cusped-field plasma thruster and cylindrical Hall thruster causes these designs to have inherently lower wall surface-to-volume ratio compared to the SPT, which may explain their reduced wall erosion rates. The wall-less Hall thruster and external discharge plasma thruster utilize magnetic field configurations that push the discharge plasma away from thruster walls. The wall-less Hall thruster resembles an SPT, except that the anode is moved from the thruster base to the thruster exit plane.