# Future Directions for Electric Propulsion Research
## 5 Strategies for Addressing Technical Challenges for Electric Propulsion Development
### Power and Propellant Improvements  
Another priority in advancing electric propulsion technology is improving the physical and electrical properties of these systems to minimize the mass-to-power ratio \(\alpha\) and maximize available discharge power. For the former, improvements are mainly by way of more sophisticated and efficient power processing technology. For example, as mentioned in Section 4.1.4, research into nozzleled ECR thrusters has accelerated due to the prevalence of compact, light, and inexpensive microwave electronics. Aside from the power processing systems themselves, more direct methods of power conditioning are currently being researched and could play a key role in enabling high-power EP. For example, so-called \"direct drive\" architectures are being considered in which a propulsion system is designed to operate at the solar panel array voltage. For instance, [PERSON] et al. operated a 6 kW Hall thruster directly from a solar panel array with only minimal filtering electronics involved [147], demonstrating that many of the power processing systems conventionally paired with flight Hall thrusters may be eschewed for lower \(\alpha\) without degrading performance.  
Another practical improvement could be in exploring and utilizing advanced or novel propellants. Many higher power EP systems use noble gases like xenon as the propellant, which are expensive, relatively difficult to pump in ground facilities, and cannot readily be stored in solid form onboard a spacecraft. However, other propellants have been investigated that address some of these difficulties, including lithium [148], bismuth [149], iodine [150], and cesium and mercury [151]. Further, electrodeless technologies like PITs are propellant agnostic, which is a significant practical advantage for advancing the state of electric propulsion technology.  
One major factor in guiding the direction of future electric propulsion is the available power source. Currently, all EP systems are \"solar electric propulsion\" (SEP)--meaning they are powered with solar panels--but with the maturation of in-space fission reactors, for example NASA's Kilopower project [152], the bottlenecks associated with SEP may disappear. In particular, nuclear electric  
Figure 19: An example of data-driven optimization of an ECR thruster from [146], in this case with the purpose of maximizing the plume floating potential. Multiple trials are conducted according to a data-driven optimization algorithm (**right**), leading to a map of floating potentials (**left**) with a peak identified when frequencies are identically 1650 MHz.  
propulsion (NEP) may enable outer Solar System or extra-Solar missions where traditional solar power is scarce. Figure 20 demonstrates the breadth of applications for NEP in showing that available solar power in the outer Solar System declines orders of magnitude below the capabilities of modern solar panels. Conversely, advances in solar power technology may direct greater attention to SEP, which naturally may emphasize inner Solar System or manned missions that require high thrust. In all likelihood, both roads may be traversed to some extent, where the mileage will be decided by technological developments yet to happen.