# Future Directions for Electric Propulsion Research
## 4 Challenges for Electric Propulsion Development
### Moderate-Power Thrusters
#### 4.2.1 Hall Effect Thrusters  
The Hall effect thruster has emerged as one of the most popular EP solutions to achieve relatively high thrust-to-power and efficiency above 1 kW. These devices in this power range are at the top of their s-curve with growing flight heritage, as shown in Figure 8, and are naturally evolved from the low-power Hall thrusters discussed in Section 4.1.1. For example, the Fakel SPT-100, Safran PPS-1350, and Aeroject Rocketdyne XR-5 are all moderate-power Hall thrusters with a low-power lineage. Since the introduction of Hall thrusters to the West in the early 1990s, there have been significant improvements in the efficiency and longevity of this technology. [PERSON] et al. studied the plasma lens magnetic field topography, proving that Hall thruster anode efficiency can reach nearly 70% with specific impulses exceeding 3000 s [49]. Figure 9 shows laboratory thrusters that resulted from this research. Further, the application of magnetically-shielded topographies has reduced thruster channel erosion by several orders of magnitude, eliminating this historical life-limiting process in Hall thrusters [50]. The power of these devices has steadily increased over the years as well, culminating in the flight of the 5 kW XR-5 [51] and SPT-140 [52], the near-term development of NASA's Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5 kW thruster to support cislunar activities [53], the study of 20 kW Hall thrusters at SITAEL (shown in Figure 10) and CNRS in support of ESA goals [54; 55], and the ground demonstration of a nested Hall thruster at 100 kW [56] (to be discussed in Section 4.3.1). Nearly one hundred Hall thrusters have flown on satellites in geostationary orbit [57], one has flown in lunar orbit [58], over 400 are flying as part of the incomplete SpaceX Starlink constellation (as of 2020), and soon several low-power articles will be employed in the deep space mission to visit the minor planet 16 Psyche [59]. Despite the growing flight heritage of Hall thrusters, there remain a number of technical challenges related to their operation. For example, it has long been acknowledged that ground testing of in-space propulsion systems may not be entirely representative of flight performance, in large part due to differences in pressure and the presence of conductive and sputterable chamber surfaces. Early work by [PERSON] et al. identified background pressure and chamber sputtering as having an effect on SPT life testing [60]. Typical operating pressures for electric propulsion testing on the ground are \(\sim\)1 uTorr at best due to finite pumping speed; this pressure is still several orders of magnitude higher than low Earth orbit [61]. Studies of thruster performance as a function of facility pressure have highlighted that indeed many aspects of Hall thruster operation are sensitive to pressure. For example, [PERSON] et al. showed that thrust decreases with pressure [62], which invites the disappointing possibility that Hall  
Figure 9: The H9 (left), a 9 kW magnetically-shielded Hall thruster, and the H6 (right), a 6 kW unshielded Hall thruster. Both were developed by the Jet Propulsion Laboratory, the Air Force Research Laboratory, and the University of Michigan. thrusters are less effective in space than on the ground. Similar studies by [PERSON] at the University of Michigan yielded trends of decreasing thrust with pressure for a wide range of discharge voltages [63]. Aside from pressure, the electrical configuration of the thruster during ground tests also constitutes a facility effect. A thruster operated in a vacuum chamber is enclosed in a grounded vessel, such that the thruster plume interacts with the vessel and is influenced by it. For instance, with a grounded chamber and/or beam dump, a nonzero amount of beam current is sunk into the chamber, requiring current to be sourced elsewhere; thus, the shape and material of the chamber walls and how they communicate with the thruster plasma may impact performance.