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

Experimental demonstration of diamagnetism enhanced by energetic electrons
in a magnetic nozzle plasma
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
Diamagnetism is a fundamental and inherent
characteristic of magnetized plasmas and distorts theexternal magnetic fields by reducing the field strength
inside the plasma. This can be clearly observed for ahigh-beta condition, where the plasma pressure is larger
than the magnetic pressure of the external magnetic
fields. It plays important role in a momentum conversion
process in a magnetic nozzle utilized for a developmentof a magnetic nozzle plasma thruster [1,2], where theplasma beta is typically much lower than unity. The
identity of the magnetic field distortion is thespontaneously excited internal azimuthal plasma current.
Several laboratory experiments in low-beta plasmas
have observed that the distortion of the magnetic field issmaller than the expected one calculated from the plasmabeta. The effect of radial electric fields [3], magnetic
field penetration and dissipation [4], and neutral pressure[5] have bee discussed to explain the observed magnetic
field distortion. A recent laboratory experiment has
shown that the azimuthal current can be given by thesum of the electron diamagnetic and ExB drift currents,
validating the reduction of the diamagnetism by thepresence of the electric field, where the electron energy
distribution is assumed to be Maxwellian [6]. However,
the electron energy distribution in the low-pressureplasma is indeed non-Maxwellian, e.g., having adepleted tail or energetic tail electrons [7,8]. Therefore,
the measured azimuthal current seems to include theeffects of both the electric fields and the non-Maxwellianelectrons.
Here an electron-beam plasma source presented
previously [9,10] is employed to produce a plasma
having zero plasma potential, allowing us to purely
investigate the diamagnetism under zero electric field.
The two-dimensional mapping of the plasma-induced
axial magnetic field is measured by using a B-dot probe,
providing the internal plasma current by taking a rotationof the measured magnetic fields as shown in Fig.1 [11].
Furthermore, the electron energy probability functions(EEPFs) are taken over four orders of magnitude,
showing the presence of the energetic tail electrons. The
comparison between the azimuthal current calculated
from the magnetic fields and the EEPFs are in goodagreement when considering the energetic tail electrons.
The results demonstrate that the tail electrons are major
contributors to the diamagnetism of the plasma even if
their density is a small fraction of the total density.

Fig.1: Two-dimensional profile of the internal azimuthal
plasma current density. Reproduced from permission
from Phys. Rev. Research 5, L022029 (2023). Copyright2023, Authors, licensed under a Creative Commons
Attribution 4.0 International license.
References[1]K. Takahashi, Rev. Mod. Plasma Phys. 3, 3 (2019).
[2]K. Takahashi et al., Phys. Rev. Lett. 110, 195003(2013).
[3]R.L. Stenzel and J.M. Urrutia, Phys. Plasmas 7, 4450
(2000).
[4]C.S. Corr and R.W. Boswell, Phys. Plasmas 14,122503 (2007).
[5]A. Fruchtman and S. Shinohara, Phys. Plasmas 24,103523 (2017).
[6]K. Takahashi et al., Plasma Sources Sci. Technol. 25,055011 (2016).
[7]K. Takahashi et al., Phys. Plasmas 14, 114503 (2007).
[8]K. Takahashi et al., Appl. Phys. Lett. 94, 191503(2009).
[9]K. Takahashi et al., Phys. Rev. Lett. 120, 045001(2018).
[10]K. Takahashi et al., Phys. Rev. Lett. 125, 165001(2020).
[11]K. Takahashi et al., Phys. Rev. Research 5, L022029(2023).