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

Effect of a cusp magnetic field provided by a permanent magnet arrayon a magnetic nozzle plasma thruster performance
Y. Nakahama1, K. Takahashi1,2,3
1
Department of Electrical Engineering, Tohoku University
2
Interdisciplinary Research Center for Non-equilibrium Plasma, Tohoku University
3
National Institute for Fusion Sciencee-mail (speaker):nakahama.yugo.r6@dc.tohoku.ac.jp
A magnetic nozzle (MN) radio frequency (rf) plasma
thruster has been actively investigated over the last fewdecades [1,2]. Because no electrodes are exposed to theplasma, it is expected to provide a longer lifetime even ata higher power level than mature electric propulsion
devices. The thrust efficiency of the MN rf thrusters has
been improved year by year and recently increased toabout 30% [3].
In the MN rf plasma thrusters, the thrust is generated
when the gaseous propellant ionized by the rf plasma
source is spontaneously accelerated by the expansion
process in the MN and ejected at high velocity into space[2]. The physics and thrust generation mechanisms within
the MN and plasma sources have been investigated [2],and it has been found that energy and momentum losses
at the plasma source wall cause the poor thrust efficiency[2,4]. Therefore, it is essential to suppress the wall losses
to improve performance. Recent experiment showing the30 % efficiency has utilized a cusp magnetic field, which
can be formed by locating the additional upstreamsolenoid in addition to the downstream solenoid, tosuppress the plasma loss to the source wall [3].
Replacing the solenoid with permanent magnets would
reduce power and weight and simplify the structuraldesign for practical application of the MN rf plasma
thrusters [5]. In this study, the upstream additionalsolenoid is replaced by a permanent magnet array, while
maintaining the downstream solenoid providing the MN.
The thrust and the ion saturation current profiles aremeasured under the following three conditions of “no cuspfield”, “cusp field by the two solenoids”, and “cusp fieldby the permanent magnets array” to verify theeffectiveness of the permanent magnets array.
Figures 1 shows the schematic diagrams of theexperimental setup tested here. The source tube iscontinuously attached to a cylindrical stainless steel
diffusion chamber. The plasma source configuration withno cusp field and with cusp field by two solenoids are
drawn in Fig.1(a) together with a pendulum target platefor qualitative assessment of the impulse bit. The
calibration procedure of the target technique follows theprevious thrust measurement [6], where the axial
displacement of target is measured by a light emitting
diode (LED) displacement sensor. The impulse bit isobtained from the calibration coefficient and the LEDsensor output. When pulsing the rf power with the pulse
width δt, the thrust can be obtained by dividing themeasured impulse bit by δt. Figure 1(b) shows the plasma
source configuration with a cusp field by the permanent

magnets array and the Langmuir probe. The permanent
magnets array consists of NdFeB magnets and is designed
to have an axially movable structure.
The results show the increase in the thrust by locating thecusp field inside the source even for the permanent magnet
array, providing the better system efficiency than the twosolenoids configuration. Detailed results will be describedin the presentation.

Figure 1. Schematic diagram of (a) the plasma source
configuration with “no cusp field”, with “a cusp field bytwo solenoids”, a pendulum target plate, (b) the plasma
source configuration with “a cusp field by permanent
magnets array”, and the LP.
References[1]C. Charles, J. Phys. D:Appl. Phys. 42, 163001 (2009).
[2]K. Takahashi, Rev. Mod. Plasma Phys. 3, 3 (2019).
[3]K. Takahashi, Sci. Rep. 12, 18618 (2022).
[4]T. Lafleur, Phys. Plasmas. 21, 043507 (2014).
[5]K. Takahashi et al, Plasma Sources Sci. Technol. 19,025004 (2010).
[6]K. Takahashi et al., Rev. Sci. Instrum. 86, 023505(2015).