Magnetic nozzle rf plasma thruster: fundamental physics and current performance

Low-pressure radiofrequency discharge under magnetic fields can provide highly ionized dense plasmas approaching 10¹⁹ m-3, where the rf power is coupled with electrons via capacitive-, inductive-, and wave- modes. In a magnetic nozzle configuration, the high-density plasma is transported along the field lines and expands along the magnetic nozzle; the electron thermal energy is converted into the plasma flow energy via electrostatic or electromagnetic acceleration processes. An electric space propulsion device utilizing this acceleration process has recently been investigated from the viewpoint of fundamental physics and development. A brief overview of the magnetic nozzle rf plasma thruster R&D will be shown in the presentation [1]. We have also demonstrated that the thruster can be applied for contactless removal of space debris by having two open exits, where one plasma plume impinges and decelerates the debris, leading a re-entry to the Earth atmosphere by lowering their altitude. Another plasma plume ejected in the opposite direction to the debris is used to maintain zero net thrust exerted on the thruster [2]. A recent fundamental experiment for mounting the thruster for debris removal will also be shown.

Interestingly, although the instability generally induces a performance degradation of plasma devices, a beneficial influence of a plasma instability/wave has been discovered in a recent experiment, where anomalous transport induced by the wave provides an inward electron transport leading to an electron detachment from the magnetic nozzle structure. This finding will open a new perspective for the role of waves and instabilities in plasmas [3].

Regarding the electric propulsion devices, the development of peripheral components such as gas supply system and the rf generator are also required to minimize size and weight and to maximize efficiency. The operation with tiny impedance matching circuit and a frequency-tunable rf generator will be presented.

[1] K. Takahashi, Reviews of Modern Plasma Physics, 3, 3 (2019).

[2] K. Takahashi, C. Charles, R.W. Boswell, and A. Ando, Scientific Reports, 8, 14417 (2018).

[3] K. Takahashi, Christine Charles and Rod W. Boswell, Scientific Reports, 12, 20137 (2022).