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

Effects of high-energy electrons on the low- and high-temperatureplasma expansion in the divergent magnetic fields

Kazuma Emoto,1 Satoshi Togo,1 Yoshinori Takao,2 Kazunori Takahashi,3,4 and Mizuki Sakamoto1
1Plasma Research Center, University of Tsukuba, 2 Division of Systems Research,
Yokohama National University, 3 Department of Electric Engineering, Tohoku University,
4National Institute for Fusion Sciencee-mail (speaker): emoto@prc.tsukuba.ac.jp
Convergent and divergent magnetic fields are
employed to extract directional plasma flow from thesource. Recently, the application of electric thrusters has
actively been studied and developed, which are expected
to be mounted on space satellites and probes and impart
the thrust as a reaction force [1]. In the thrusters, theconvergent and divergent magnetic fields have a similarrole to the de-Laval nozzle. The plasma expands in thedivergent magnetic fields, where the diamagnetic current
is induced and the Lorentz force is exerted on the plasma[2,3]. In considering the plasma expansion, the density
and temperature were discussed with the polytropic
index in recent papers [4,5]. It was implied that thepotential structure in the divergent magnetic fieldsignificantly affected to the plasma expansion [5].
Because the potential structure modifies the electron
trap or loss, the plasma expansion is also expected todepend on the electron energy, where many laboratory
experiments have shown non-Maxwellian electronenergy distribution [6]. To analyze the plasma expansion
depending on electron energy, fully kinetic simulations
were conducted using particle-in-cell and Monte Carlocollision techniques. As a result, the high-energy freeelectrons showed adiabatic expansion, whereas thelow-energy trapped electrons indicated isothermalexpansion [7]. It was numerically shown that thehigh-energy free electrons expanded more efficiently inthe divergent magnetic field without heat dissipation.
More recently, the effect of the high-energy freeelectrons on the formation of density profiles wasnumerically discussed [8]. In this paper, it was reported
that the high-energy electrons were generated in theperipheral region by the RF heating, and the magnetizedhigh-energy electrons generated the hollow density
region at the center of the convergent and divergent
magnetic field. These results are consistent with previous
experimental work [9]. Therefore, the high-energy freeelectrons have an important role in the convergent anddivergent magnetic field.
To further investigate the role of high-energy electrons,
we have started to build a fully kinetic simulation ofhigh-temperature plasma expansion. GAMMA 10/PDX
is modeled to kinetically simulate the high-temperatureplasma expansion, which is the largest tandem mirror
device and has a divergent magnetic field at the ends ofthe device [10]. It can be investigated how the role ofhigh-energy electrons changes between low- andhigh-temperature plasmas. In GAMMA 10/PDX,
additionally, the plasma temperature can be actively

controlled by ion cyclotron radio frequency and electron
cyclotron resonance heating [11,12]. Therefore, theenergy dependency on plasma expansion can bevalidated using experimental data.
In the conference, the numerical studies on the electric
thrusters using the divergent magnetic field will bediscussed, and the recent progress of fully kineticanalysis for the high-temperature plasma expansion will
be reported.
This work was supported by research grant program ofThe FUTABA Foundation, JSPS KAKENHI Grant
Number JP23H05442, and JST FOREST Program (Grant
Number JPMJFR212A, Japan). The computer simulation
was performed on the A-KDK computer system at
Research Institute for Sustainable Humanosphere, Kyoto
University. This research is partially supported byInitiative on Promotion of Supercomputing for Young or
Women Researchers, Information Technology Center,
The University of Tokyo.
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