# Diversity of Physical Processes: Challenges and Opportunities for Space Electric Propulsion
## 2 Variety of Physical Processes to Integrate into a Single Design
These are the missions that currently drive the exploration and exploitation of near-Earth space and deep space and, eventually, will drive the creation of permanent extra-terrestrial outposti  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **Resistolet** & **Arjet** & **GIE** & **HEI/HEMT** & **PFT** & **MPDT/ECK** \\ \hline Type & Electothermal & Electothermal & Electrostatic & Electrostatic & Electromagnetic & Electromagnetic \\ Achievable thrust, & 0.5–6000 & 50–6800 & 0.01–750 & 0.01–2000 & 0.05–10 & 0.001–2000 \\ mN & 150–850 & 130–2200 & 1500–10,000 & 600–3000 & 1400–2700 & 200–3200 \\ Efficiency, \% & 30–110 & 25–60 & 30–90 & 20–70 & 5–30 & 20–70 \\ Thrust-to-power ratio, \(\mathrm{mN/KW}\) & 450–700 & 150–600 & 20–250 & 150–300 & 50–200 & 150–500 \\ Operational time & \begin{tabular}{c} Month \\ NH\({}_{3}\), hydrazine, \\ H\({}_{2}\), Xe, N\({}_{2}\) \\ \end{tabular}  & \begin{tabular}{c} Month \\ H\({}_{2}\), N\({}_{2}\), NH\({}_{3}\), \\ hydrazine \\ \end{tabular}  & \begin{tabular}{c} Xe, Kr, Ar, \(\mathrm{l}_{2}\) \\ \(\mathrm{l}_{2}\), \(\mathrm{H}_{2}\)O \\ \end{tabular}  & \begin{tabular}{c} Xe, Kr, Ar, \(\mathrm{l}_{2}\) \\ \(\mathrm{l}_{2}\), \(\mathrm{H}_{2}\)O \\ \end{tabular}  & \ \begin{tabular}{c} Type \\ Xe, Kr, Ar, \(\mathrm{l}_{2}\) \\ \(\mathrm{l}_{2}\), \(\mathrm{H}_{2}\)O \\ \end{tabular}  &
\begin{tabular}{cFigure 1 exemplifies 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. Moreover, even within the same type of thruster technology, a wide spread of the parameters can be observed, e.g., a large collection of experimental material has been accumulated on anode efficiency, which is one of the most important characteristics of the Hall-type space thrusters [79]. A wide spread of the characteristics for the same type of thrusters is an apparent signature of the complexity of the processes, their intertwined characteristics and their mutual interference. Among the schematics shown in Figure 1, two types of thrusters are the most developed with respect to current application in space: Hall thrusters and ion thrusters, shown in Figures 2 and 3, respectively. These thrusters rely on different acceleration processes, each suitable for a particular range of available electric power. Not surprisingly, a small variation in the design, operational modes, materials and geometry of the thruster results in a dramatic change in its efficiency. On the other hand, optimization of electric and plasma thrusters implies the multi-parametrical optimization of a large number of electric, magnetic, thermodynamic and other concatenated processes. As a consequence, optimization of space thrusters is considered to be one of the most time-, labour-, and fund-consuming stages of the design. The optimization of parameters of the existing types of thrusters is the first challenge in space electric propulsion. Figure 2: Various types of space electric propulsion systems utilize a vast variety of interdependent physical processes and design solutions. As an example, the electrostatic system is illustrated: Hall thrusters are among the major candidates for powering future spacecraft. Integration of electric propulsion thrusters into satellites represents an even more complex task due to the possible interactions of the thruster and plasma jets with sub-systems integrated within the satellite. For more details about the operation principles of Hall thrusters, see e.g. [23; 24]. While optimization challenges need to be solved to improve the characteristics of existing types of engines, the development of new types of thrusters is always a current demand. The necessity to simplify the thruster design to lower power consumption, remove plasma-contaminating elements, improve the performance characteristics and decrease the mass of the device are among the leading incentives for the development.