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

Simultaneous 3-view measurement of toroidal flow using IDS systemin the FAT-CM device
R. Kikuchi1, K. Araoka1, K. Kiyota1, T. Seki1, D. Kobayashi1, T. Takahashi1, T. Asai1
1
College of Science and Technology, Nihon University
e-mail (speaker):csry23009@g.nihon-u.ac.jp
A Field-Reversed Configuration (FRC) plasma is
one of the magnetic confinement systems characterized by
its remarkably high confinement efficiency. It comprises
a compact torus, formed solely by a poloidal magnetic
field and a toroidal current. Another salient feature of FRC
is its capacity for axial translation , attributable to theabsence of interlinked structures within the plasma ring. It
has been experimentally observed that the FRC, formedby the field-reversed theta pinch system spins up in thetoroidal direction immediately after its formation. Thisrotation induces the rotational instability with the toroidal
mode number n = 2, resulting in the deformation of thetoroidal cross-section into an elliptical shape. [1]We have conducted collisional merging
experiments on this FRC system. Figure 1 shows aschematic diagram of the FAT-CM (FRC Amplification
via Translation-Collisional Merging) device employed inthis experiment. FRCs are generated by the formation
regions at each end of the device and are translated at a
range of Alfvén speed by producing a magnetic pressure
difference, achieved by using solenoid-like coils ofdifferent diameters. These two FRCs subsequently collide
and merge within the confinement section located at thecenter of the device.[2] Following the impact of thecollision, initial FRCs undergo axial compression andsubsequently exhibit considerable expansion in bothradial and axial directions. The toroidal spin up has been
observed on the collisional merged FRC [3].
In this study, our goal is to assess the axial
distribution of the toroidal flow of the FRC during andafter the collisional merging processes, through the use ofsimultaneous multi-point measurement of the Doppler
shift on the FAT-CM device. [3]Figure 2 illustrates the an ion Doppler spectroscopy(IDS) measurement, designed to measure the Doppler
shift induced by the toroidal flow, at the central crosssection as viewed from the axial direction, where 𝑟
denotes the distance from the geometrical axis and 𝑟!
representes the separatrix radius.[4] In this measurement,
the mid-plane and cross-sections at V1 and R1 within theconfinement section are designated as the measurement
points. The axial distribution of toroidal flow is derived
by concurrently measuring the Doppler shift of the FRC
within each cross-section, with impact parameter 𝑟varying.
The spectral line of doubly ionized carbon (C III:229.69 nm), present as an impurity in collisional merging
FRC, is measured with a spectrometer. An IDS system,possessing a high time response on the order ofmicrosecond, is employed to accurately measure the spinup, which occurs on the scale of tens of microseconsds.
This work was partially supported by the JSPS
KAKENHI under Grant No. JP20H00143 and Nihon
University, College of Science and Technology, Grant for
Project Research, Grant for the Promotion of Leading
Research and the research grant of Nihon University.
References[1] Y. Fujikawa et al., Plasma Fusion Res. 7, 22402130(2012).
[2] T. Asai et al., Nucl. Fusion, 61, 096032 (2021).
[3] K. Araoka et al., Rev. Sci. Instrum. 93, 123512(2022).

θzFRC
Collimator

r
2

13rs
Measurement Points①:Center Cross-section (z = 0m),②:R1 Cross-Section (z = -0.3m),③:V1 Cross-Section (z = 0.3m)

Figure.1 Schematic of the FAT device with thepositions of the measurement points. The mid-plane isthe origin of z axis, with measurement points placed
0.3m apart on either side.

confinement section

Figure.2 Schematic of measurement of toroidal flow
with IDS at cross-section of FAT-CM device