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

Effect of Local Helical Coil Field on Vertical Position Displacement
Caused by Uniform Horizontal Field in TOKASTAR-2
S. Morizawa, T. Fujita, A. Okamoto, S. Kato, T. Oshiro, and H. Kidu
Department of Applied Energy, Nagoya University
e-mail (speaker): morizawa.shunsuke.y1@s.mail.nagoya-u.ac.jp :coil current was increased further to 4.8 kAT.
Visible light images of the plasma were then takenusing a high-speed camera. The image was comparedwith the shape of the plasma obtained by the filament
method code. They approximately agreed confirming thevalidity of the filament method code calculation.
This study was supported by the NIFS General
Collaboration NIFS23KIPP029.
References[1] K. Yasuda, T. Fujita, A. Okamoto, et al, Physics ofPlasmas, 28 (2021) 082108.

ZJ [m]RJ [m]k

ISC [AT]

Figure 1 ULT and SC coils

Ip [kA]The vertical position of the tokamak plasma is
considered to be stabilized by the application of a helicalmagnetic field. In a previous study, we succeeded instabilizing the vertical position instability in an elongatedplasma by using two local helical coils (ULT coils), at
the top and the other two at the bottom of the plasma in
TOKASTAR-2. The SC coils, which are used to generate
elongated plasma, and the ULT coils, are shown in Fig 1.
Comparing the horizontal component of the effective
magnetic field from the local helical coils and thehorizontal component of the two-dimensional magnetic
field (quadrupole magnetic field) that makes theelongated plasma, it was confirmed that the position
instability tends to be stabilized when the former is larger
than the latter, but the boundary between the stability andinstability remains unclear [1]. The horizontal
component of the quadrupole magnetic field isdependent on the vertical position of the plasma, and its
evaluation is affected by the accuracy of the analysis of
the vertical position and the reproducibility of thedischarge.
In this study, the SC coils are connected in theopposite directions to generate a nearly uniform
horizontal field. In addition, by reducing the number ofturns of the winding used, the inductance is reduced, anda rapid horizontal field can be applied in response to thetime variation of the plasma current. With the change in
the SC coil connection, the filament method code that
calculates the and the shape of the plasma was modified
to allow the direction of the SC coil current to bevariable.
A uniform horizontal magnetic field was applied
during the discharge to investigate how the response of
the plasma vertical position to the uniform horizontal
field depends on the presence and magnitude of the local
helical coil field.
First, an experiment was conducted to investigate theeffect of a uniform horizontal magnetic field underconditions where no local helical coil field was applied.
When the charging voltage of the SC coil capacitor wasincreased, a downward vertical displacement event(VDE) occurred at a peak current value of about 1.1 kAT.
Then, the ULT coil field was applied. Its magnitude wasnearly constant during the tokamak discharge. The
results are shown in Fig. 2. When the ULT coil current
was 1.8 kAT, the speed of vertical displacement wasreduced and the plasma current duration was increased;
when the ULT coil current was 3.6 kAT, VDE did notoccur and the plasma current duration was increased toabout 0.5 ms. No change was observed when the ULT

1200

IULT

800w/o1.8 kAT
3.6 kAT
4.8 kAT

4000
1.4
1.2
1.0
0.8
0.6
0.14
0.12
0.10
0.08
0.08
0.04
0.00
-0.04
-0.08
2.5
2.0
1.5
1.0
0.5
0.0
3.5

3.6

3.7

3.8
3.9
Time [ms]

4.0

4.1

Figure 2 Time evolution of SC coil current, ellipticity,
plasma R position, plasma Z position, plasma current ofdischarges with and without ULT coil field