The effects of collisions and oscillating fields on the thrust in electric propulsion*

The study of electric propulsion involves interesting basic plasma physics issues. The dependece of the efficiency on the particle energy distribution and implications to air-breathing electric propulsion [1] will be discussed. A major figure of merit in electric propulsion is the thrust per unit of deposited power, the ratio of thrust over power. Thrust over power is important for air-breathing propulsion where propellant saving is not the major issue. As we have demonstrated [2],  for a fixed deposited power in the ions, the momentum delivered by the electric force is larger if the accelerated ions collide with neutrals during the acceleration. Operation in the collisional regime can be advantageous for air-breathing propulsion, and its practicality should be examined. In a magnetic nozzle it is the magetic pressure whch provides the thrust [3]. The relation between the magnetic field configuration and the height of the potential hill which delivers energy to the accelerated ions will also be discussed. In electrodeless electric thrusters, oscillating electromagnetic fields are used to deliver energy to either electrons of ions while momentum is delivered by the magnetic pressure. Mechanism where electromagnetic waves themselves deliver momentum to ions are interesting [4]. We have recently cosidered such a mechanism that relys on standing waves [5]. Unfortunately, It is diffcult to achieve a considerable acceleration by this mechanism, but the mechanism could be of interest for other applications.

[1] F. Romano et. al., Acta Astronautica 16, 476 (2020).

[2] G. Makrinich and A. Fruchtman, Phys. Plasmas 20, 043509 (2013).

[3] K. Takahashi et. al., Phys. Rev. Let. 107, 231001 (2011).

[4] S. Shinohara et. al., Plasma Phys. and Controlled Fusion 61, 014017 (2018).

[5] A. Fruchtman and G. Makrinich, 36th Intern. Electric Propulsion Conf., IEPC-2019-514, Vienna (2019).