7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Magnetic nozzle rf plasma thruster:
performance improvement and electron detachment
Kazunori Takahashi1,3,4, Christine Charles2, and Rod W. Boswell2
1
Department of Electrical Engineering, Tohoku University,
2
Space Plasma, Power and Propulsion Laboratory, Research School of Physics,
The Australian National University
3
Interdisciplinary Research Center for Non‑equilibrium Plasma, Tohoku University
4
National Institute for Fusion Sciencee-mail (speaker): kazunori.takahashi.e8@tohoku.ac.jp
During the plasma expansion in the divergent magnetic
field resembling the nozzle, the momentum conversion
processes can frequently occur as results of interaction
with the spontaneously formed electric fields and themagnetic fields [1,2]. The electrostatic ion accelerationby the electric fields does not increase the total axial
momentum [3], where the electron pressure is converted
into the ion dynamic momentum [4], while the Lorentz
force arising from the internal plasma current and theapplied field enhances the axial momentum of theplasma flow [5-7]. These processes are useful for theelectric propulsion device called a magnetic nozzle rf
plasma thruster. The thrust generation physics has been
vigorously investigated over the last several years; thethruster performance has been improved based on thescientific understanding of the physical processes [8-10]
Here the highest thruster efficiency of the magnetic
nozzle rf plasma thruster, approaching 30% thruster
efficiency, will be presented [11]. In the experiment, acusp magnetic field, which isolates the plasma from thesource wall, is applied inside the source, while
maintaining the magnetic nozzle structure downstream ofthe source, as shown in Fig.1. The result can bequalitatively well explained by considering the change inthe loss area and the plasma volume in a global model ofthe plasma production and a magnetic nozzle model.
When operating it in space, the plasma has to be finallydetached from the magnetic nozzle inevitably forming
closed magnetic field lines further downstream;otherwise, the plasma will return to the thruster along theclosed fields. The unmagnetized ions having a larger
gyro-radius than the plasma scale can be easily detachedfrom the field lines, while the electrons are magnetizedin typical magnetic field strength. In the thruster
configuration, a coherent fluctuation around 40 kHz has
been detected at the peripheral region of the magneticnozzle, where the ion deviation from the magnetic fieldlines are significant. The inward cross-field transport ofthe electrons has recently been discovered in ourlaboratory experiment for the first time [12], which isinduced by the spontaneously excited electrostatic mode
wave in the magnetic nozzle. Comparison between thewave-induced electron flux and the steady-state ion flux
suggests that the cross-field inward electron transport
significantly contribute to neutralize the ion flux
deviating from the magnetic field lines. Spontaneously
excited plasma waves often degrade the performance ofFig.1: Calculated magnetic field lines and measured ion
saturation current of the magnetic nozzle rf plasma
thruster providing 30 % efficiency.

the plasma-based device (e.g., fusion reactors), while thepresent study shows the beneficial effect on the thruster
operation in space. Therefore, the result will open thenew perspective for the role of the plasma waves in theplasma application technology.
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