# Diversity of Physical Processes: Challenges and Opportunities for Space Electric Propulsion
## 2 Variety of Physical Processes to Integrate into a Single Design
This process follows a well-known routine, where a scrupulous analysis of the design and operational principles of the most developed subject-related engineering solutions results in synthesis of a new device. One of the promising approaches is the transition from stationary flows and electric/ magnetic fields, which are features of Hall and gridded ion thrusters, to rotational configurations. Several types of electromagnetic plasma thrusters were recently developed by various research groups [80; 81; 82]. One of the characteristic examples of the rotational system (which is yet quite far from space testing) is the miniaturized Rotamak-type device, which was initially suggested for the experiments with hot plasma for thermonuclear fusion [83]. A rotating magnetic field (RMF) is engaged to drive the azimuthal plasma current in a plasma torus, which is sustained in equilibrium by a steady axial field, applied externally as RMF. A schematic of the setup is shown in Figure 4. An external antenna generates oscillating electric fields to produce plasma. Unlike the common ICP discharge, where the external oscillating field does not penetrate into the plasma because of the plasma's high conductivity, the implementation of RMF allows the introduction of the external electric field into the Rotamak plasma. In this case, a synchronous rotation of plasma electrons occurs, while ions form a static background of positive charge, creating a shield. In the device, the ponderomotive force accelerates the plasma particles in the same direction, unlike the ion or Hall thrusters, and the field of the force is created by a static magnetic and non-uniform high frequency field. Figure 3: Gridded ion thruster. The gridded ion thruster is a second promising candidate 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. The design features the advantages of the electrodeless thrusters for plasma propulsion, such as: (i) thrust density and acceleration are not limited by Hall parameters or electrical screening; (ii) there is an absence of neutralizer; (iii) the thruster can be throttled between the higher thrust and specific impulse at the constant power; (iv) the electrode erosion and plasma contamination are absent. Integration of several complex physical processes in a single system enabled the new generation of space thrusters of Rotamak type. This approach could be promising for other efforts on integration of various physical mechanisms to design novel space thruster systems. One of the big challenges in the modern propulsion industry is miniaturization, which means the development of small thrusters suitable to be employed in the booming market of CubeSats. Indeed, miniaturized plasma thrusters are often considered a propulsion means of choice for small space assets, e.g., small satellites and CubeSats. Yet, a thruster designed for a satellite with centimetre to tens of centimetres dimensions would experience significant power losses in ultra-miniaturized plasma sources, and for this reason cannot provide the desired level of efficiency. Currently, there are active research efforts to combat this issue, with some of the potential solutions outlined [23, 61]. Unfortunately, the direct approach of scaling down the well-proven Hall and ion thrusters does not lead to a satisfactory result, since the ratio of the plasma-generating volume (\(\
eg r^{3}\)) to the plasma-decay surface (\(\
eg r^{2}\)) decreases with a decrease in the thruster size, which leads to a drop in performance characteristics. It should be mentioned that the characteristics of the Rotamak-type setup allow it to be considered for application as a small space thruster. In general, the development of new types of miniaturised plasma thrusters to meet the current requirements of space exploration missions is the challenge. Nowadays, miniaturization of the thrusters is still much in demand.