# Future Directions for Electric Propulsion Research
## 4 Challenges for Electric Propulsion Development
### Low-Power Thrusters
#### 4.1.1 Sub-Kilowatt Hall Thrusters  
In Figure 3, the s-curve for Hall thrusters inflected several decades ago and has been progressively maturing since then. Traditionally, Hall effect thrusters have been operated above 500 W to maintain moderate current densities while minimizing the ratio of wetted surface area to volume of plasma. For magnetic field topographies that have field lines intersecting the channel walls nearly perpendicularly, the walls serve as an energy sink that stymies the development of a narrow, hot acceleration region to which this technology owes its efficiency. For this reason, most modern, high-performance Hall thrusters that are flown tend to be \(>\)1 kW. Moreover, until recent decades, the demand for very low-power Hall thrusters has been weak since these thrusters were often first used for satellite stationkeeping with \(>\)1 kW available. Historically, however, there are many notable development efforts that have focused on sub-kW devices, including the groundbreaking development of the Fakel Stationary Plasma Thruster family [3], the Central Research Institute of Machine Building (TsNIIMASH) D-55 Thruster with Anode Layer (TAL) [4], the Safran PPS series of  
Figure 3: Notional s-curves for various low-power electric propulsion technologies categorized with the color scheme of Figure 1. Shown are: RJ, resistojet; HET, Hall effect thruster; PPT, pulsed plasma thruster; GIT, gridded ion thruster; ES, electrospray; and RFMN, radio frequency power paired with a magnetic nozzle. thrusters [5; 6], and the Busek BHT family [7]. Figure 4, for example, shows a low-power Hall thruster throttleable down to nearly 500 W developed at the University of Michigan based on the SPT-70. The low-power entries in the SPT series of thrusters ultimately yielded the SPT-100, a moderate-power device (discussed in Section 4.2.1) that has been flown extensively (the first discussed in [8] and more recently [9]) and has been subject to considerable investigation. As shown in Figure 3, these early sub-kW Hall thrusters matured fairly rapidly several decades ago, yet the poor performance of these devices at very low-power (\(<\)100 W) due to geometric scaling issues still leaves room for advancement. Recently, there has been renewed interest in very low-power Hall thrusters. For example, these devices are being developed privately by Orbion [10], Exoterra [11], Exotrail [12], Satrec Initiative [13], and Apollo Fusion [14], and considerable research on the magnetic shielding of this class of Hall thruster was performed by UCLA and the Jet Propulsion Laboratory with the Magnetically-Shielded Miniature (MaSMi) Hall thruster [15], shown in Figure 5 along with Exotrail's 50 W Hall thruster. The modern challenges for sub-kW Hall thrusters remain many of the obstacles identified for this technology by the 1990s. For example, the fundamental issue of growing wall losses with smaller geometries remains a problem, although magnetic shielding reduces these losses somewhat [16]. Figure 4: The PEPL-70 (**right**), a low-power Hall thruster developed at the University of Michigan in the 1990s, compared to the H9 (**left**), a state-of-the-art Hall thruster developed from 2015–2017, roughly to scale. Figure 5: A 50 W Hall thruster, part of Exotrail’s ExoMG-nano propulsion platform (**left**); reproduced from [12], courtesy of Dr. [PERSON], Exotrail. An engineering model of the Magnetically-Shielded Miniature (MaSMi) Hall thruster for the ASTRAEUS program (**right**); reproduced from [14], courtesy of Dr. [PERSON], JPL. The power demands of the magnetic circuits of these devices also have proven challenging for integration on small spacecraft; designs incorporating permanent magnets have been explored [17] but are yet to fly.