# Diversity of Physical Processes: Challenges and Opportunities for Space Electric Propulsion
## 2 Variety of Physical Processes to Integrate into a Single Design  
Novel physical principles are now being explored to create thrust in space [35; 36], along with intense research efforts to further advance the existing types of space thrusters [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. The pace propulsion systems illustrated in Figure 1 using plasma [40; 41; 42; 43] and ion jets [30; 41; 42; 43; 44; 45; 46; 47] are among the most promising systems due to numerous advantages [48; 49; 50], including high specific impulse [51; 52] and long service life [53; 54] in space. Currently, space technology, and in particular space electric propulsion systems are undergoing rapid development as part of many advanced space programs [55], including NASA's Mars and Moon exploration programs [56] which will require high-powered electric propulsion systems [57; 58], and a  
Figure 1: The major types of space electric propulsion thrusters spanning the input power to the thruster. The circled icons represent the basic electrical schematics of each type of thruster. This image illustrates the wide spread of physical complexity that is manifested by the numerous propulsion devices employed over five orders of magnitude of power to the thruster. large number of deep space exploration projects [59; 60]. Table 1 lists several key parameters (including efficiency and operational time) for the major types of space electric propulsion technologies. However, despite many exciting and promising developments, we are yet to realise propulsion systems that would provide these missions with the necessary level of efficiency, reliability and lifetime [61]. This is particularly true for miniaturised thrust-generating systems that need to deliver a sufficiently high level of performance under stringent power, mass and volume constraints. These constraints also apply to the supporting subsystems (e.g., propellant and energy storage), which are only allowed to take up limited space within the satellite. When compared to conventional thermochemical thrust systems, electric propulsion platforms feature a substantially increased specific impulse, i.e., exhaust velocity of propellant. There is a growing demand for exclusively electrically powered, all-electric satellites [62; 63], with both industry and Universities allocating significant funds and efforts to increase the efficiency and reliability of electrostatic and electrodynamic thrusters [64] and associated ancillary sub-systems. These include cathodes for plasma generation and neutralisation [31], control systems, propellant feed and storage systems [65] and the materials from which these systems are made [15]. There is also a significant overlap with research efforts that exploit the physical and chemical processes generated in plasmas to fabricate and assemble materials [66] and devices [67; 68; 69; 70; 71; 72] for space technologies, as well as for other applications [73; 74; 75; 76; 77]. This means that the modifications of, for example, Hall-type thrusters can be used for the synthesis of materials and even for the growth of materials directly in the thruster channel (e.g., nanodiamonds, graphene flakes and others, see details in [78]). A comparison with currently available propulsion technologies, both well-developed (e.g., chemical rockets) and emerging ones (e.g., solar sails), suggests that at present, plasma thrusters represent one of the most practical and technologically realizable means of propulsion for missions requiring high specific impulse, delivered total impulse, thrust precision, and spacecraft control authority.