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

Comparison of Internal Magnetic Structure during
Field-Reversed Configuration Merging Process with Resistive MHD Simulation
D. Kobayashi1, T. Seki1, T. Asai1, T. Takahashi1, H. Suda1, M. Sasaki2, and S. Dettrick3
1
College of Science and Technology, Nihon University
2
College of Industrial Technology, Nihon University
3
TAE Technologies, Inc., Foothill Ranch, CA 92610, USAe-mail (speaker): kobayashi.daichi@nihon-u.ac.jp
Internal magnetic measurements during the collisional
merging
formation
processoffield-reversed
configurations (FRCs) have been conducted on the FRC
amplification via translation-collisional merging (FATCM) device at Nihon University.[1] An FRC is amagnetically confined plasma, characterized by thehighest beta (~1) and is categorized as a compact torus. A
single FRC state can be formed through the collision andmerging of FRC-like plasmoids at super Alfvénic/sonic
speed.[1,2] Despite the disruptive perturbation experienced
by the plasmoids during the collision and merging phases,
the plasmoid, after merging, relaxes into the FRC-likeequilibrium state within several tens of microseconds.
This phenomenon suggests the robustness of FRC.
Thetwo-dimensional
axisymmetricresistive
magnetohydrodynamics (MHD) simulation[3] of thecollisional merging formation of FRCs has been attempted
to obtain the detailed internal magnetic structure ofplasma during the collision and merging phase. The
simulation successfully reproduced major quantities andglobal behaviors of the plasmoid, such as density,
temperature, volume, and translation speed.[4] Figure 1
shows the radial profile of the magnetic field in the x, y,and z directions, as observed by the internal magnetic
probe array, around the midplane of the device.
Comparable contour maps generated from the simulation
result are shown in Fig. 2. Note that this MHD model does
not incorporate a toroidal field (represented as Bx). In theexperiments, the magnetic field was locally perturbated to(a)the extent that it disturbed the FRC-like reversed fieldstructure during the timeframe in which the plasma radius
expanded due to the collision of plasmoids (t ~ 15–30 µs),
as shown in Fig. 1. A similar perturbation was also
observed in the simulation as shown in Fig. 2. On the otherhand, fast oscillation due to plasma bouncing was notobserved experimentally. Furthermore, the slower
oscillation, which induces radial expansion, decays
significantly faster in the experiment allowing theplasmoid to relax into the FRC-like equilibrium state more
quickly. Though the inserted internal magnetic probearray may have an invasive effect, oscillations on theplasma radius were not observed even without the probe.
Therefore, a mechanism might exist to expedite the decayof these oscillations, which cannot be solely explainedwith this MHD model, and it may facilitate the rapid
merging of the FRC. More detailed analyzed results will
be reported in future work.
This work was partially supported by JSPS KAKENHI
Grant No. JP20H00143, Nihon University, College of
Science and Technology, Grant for the Promotion ofLeading Research, Grant for Project Research, and theresearch grant of Nihon University.
References[1] T. Asai et al., Nuclear Fusion 61, 096032 (2021).
[2] D. Kobayashi et al., Phys. Plasmas 28, 022101(2021).
[3] Y. Mok et al., Bull. Am. Phys. Soc. 55, GP9.00097(2010).
[4] D. Kobayashi et al., Rev. Sci. Instrum. 92, 053515(2021).

(a)

Fast oscillation(b)

Slower oscillation

(b)

(c)

Radial expansion

Figure 1. Radial magnetic profile of FRC observed by theinternal magnetic probe array in the experiment. Grey dashedline denotes the plasma radius.

Figure 2. Radial magnetic profile of FRC obtained from thesimulation. Grey dashed line denotes the plasma radius. Note
that, there is no Bx in this simulation result.