# Future Directions for Electric Propulsion Research
## 4 Challenges for Electric Propulsion Development
### High-Power Thrusters
#### 4.3.1 High-Power Hall Effect Thrusters  
Figure 13 indicates that Hall thrusters at high power, although moderately mature, are advancing relatively slowly compared to their moderate-power equivalents. In the first decade of the Twenty-first Century, it was quickly realized that physically scaling up moderate-power Hall thrusters was impractical; for example, designs were drawn for a \"NASA-1000M\" thruster that would be an unprecedented 1 m in diameter and operate at 150 kW, yet this device was never constructed [105]. Since then, several ideas have been explored to scale Hall thrusters to higher power while keeping them competitive in terms of footprint and mass. For example, clustering of Hall thrusters has been studied at length, including the performance of clusters sharing a cathode [106] and the interaction of the plumes of clustered thrusters [107]. In fact, the current Lunar Orbital Platform-Gateway concept Power and Propulsion Element involves a cluster of 13.3 kW Hall thrusters to form a 50 kW system with a \(\alpha\) of roughly 0.3 kW/kg [108].  
Another approach has been the nesting of Hall thrusters, studied at the University of Michigan and culminating in the two-channel X2 [109], three-channel X3 [56], and the magnetically-shielded two-channel N30 [110] at power levels of 10 kW, 100 kW, and 33 kW, respectively. Figure 14 shows these three thrusters. By sharing a magnetic circuit and capitalizing on the traditional Hall thruster annular shape, these thrusters have a reduced mass and footprint compared with equivalent clustered and monolithic systems [111].  
Despite the impressive headway made in researching high-power Hall thrusters, many challenges have emerged that have impeded their development. Foremost, ground testing facilities are largely incapable of handling the high gas throughput of these devices, bringing into question the role of facility effects as described in Section 4.2.1. For example, the X3 was only recently tested to a record-setting 100 kW [56], but these experiments were short duration and had to be conducted in Glenn Research Center's Vacuum Facility 5, a chamber far more capable than those to which most researchers have access. And yet this facility was barely able to maintain a background pressure during these tests within the standard of [PERSON] et al. [60]--assuming this standard is even still applicable to such large devices. Additionally, such tests become increasingly uneconomical for higher power thrusters due to the cost of the most common propellant, xenon; for this reason, more plentiful gases like krypton are being explored.  
Further, there remain many outstanding physical questions regarding nested Hall thrusters. For example, the study of hot interaction between channels has only been preliminary [112]. Although there have been recent strides in optimizing magnetic field topography for different multi-channel operating modes, this too is still an area ripe for investigation.  
Figure 13: Notional s-curves for various low-power electric propulsion technologies categorized with the color scheme of Figure 1, including magnetoplasmadynamic thrusters (MPD) and nuclear thermal propulsion (NT).