23P039-P
Magnetic Island Experiment in Tohoku University Heliac II
東北大学ヘリアック装置における磁気島生成実験 II
Sumio Kitajima, Yu Sato, Keiichi Ishii, Mamoru Kanno, Jo Tachibana, Yasuhiro Suzuki1),
Masayuki Yokoyama1), Shigeru Inagaki2), Kiyohiko Nishimura1), Hiromi Takahashi1),
Atsushi Okamoto and Mamiko Sasao
北島純男，佐藤優，石井啓一，菅野守，立花丈，鈴木康浩1)，横山雅之1)，稲垣滋2)
西村清彦1)，高橋裕己1), 岡本敦，笹尾真実子
Department of Quantum Science and Energy Engineering, Tohoku Univ., Sendai 980-8579, Japan1)
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan2)
Research Institute for Applied Mechanics, Kyusyu Univ., Kasuga 816-8580, Japan東北大学大学院量子エネルギー工学専攻 〒980-8579 仙台市青葉区荒巻字青葉6-6-01-21)
核融合科学研究所 〒509−5292 岐阜県土岐市下石町322-62)
九州大学応用力学研究所〒816-8580 福岡県春日市春日公園6-1
Effects of island width on a poloidal flow were surveyed in the Tohoku University Heliac (TU-Heliac).
The poloidal flow was driven externally by a hot cathode biasing and m = 3 magnetic islands wereproduced by two pairs of external perturbation field coils. The electrode current required for the poloidal
flow jumping increased with growing the island width. The effect of the degradation of plasma
performance to this dependency was also surveyed in limiter configurations. These dependencies suggestthat the magnetic island located at the plasma periphery affects the poloidal flow as a drag term.

1. Introduction
Study of magnetic island effects on thetransport is important, because it leads to theadvanced control method for a plasma periphery ina fusion reactor. The perturbation field effects on
the transport have been surveyed widely in LHD
and DIIID [1-3]. For the research on island effects
on confinement modes, Tohoku University Heliac(TU-Heliac) has advantages that (1) the island
formation can be controlled by external
perturbation field coils; (2) a radial electric fieldand particle transport can be controlled by theelectrode biasing [4]. The purpose of this work is tosurvey the relationship between the threshold of theexternal driving force required for a plasma flow
jumping and an island width. We also compared thethreshold of the external driving force required forthe plasma flow jumping in limiter configurations.
2. Experimental Setup
Experiments were carried out in the TU-Heliac.
The He plasma produced by the low-frequency
joule heating (f = 18.8 kHz, Pout ∼35 kW) in themagnetic field is B0 = 0.3 T and the discharge timewas 10 ms, which was time of flattop of coil current
for confinement. The LaB6 electrode was inserted inplasma from a low field side for plasma biasing.
To generate m = 3 islands at the plasma periphery,
we used external perturbation coils. Four pair ofupper and lower coils located at the toroidal angles
φ = 0°, 90°, 180° and 270° that generated a cuspfield at each toroidal angle [5]. In this experiment,
we used only two pairs of external perturbation
field coils at the toroidal angle φ = 0° and 180° toproduce magnetic islands at fixed poloidal positions.
Using these coils, m = 3 island was generated onmagnetic configuration that has a rational flux
surface (n/m = 5/3) at ρ ∼ 0.5 [6].

Fig.1. Typical time evolution of (a) LaB6
electrode current, (b) electrode voltage, (c)electron temperature, (d) electron density
measured by a triple probe and (e) Machprobe current ratio

1.5

IET [A]

1.5
1.0
0.5
0.0-90LCSF

Island-80
Z-70-60[mm]-50

LimiterFig.3. Dependency of the limiter position on
the electrode current required for thetransition IET

electrode current required for the transition IET
almost independent on the limiter position. In Fig. 3
the limiter position Zlimiter = - 60 mm corresponds tothe inside edge on the m = 3 magnetic island andZlimiter = -85 mm corresponds to the last closed flux
surface. The electrode current IE was proportional toa plasma driving force. These dependencies shown in
Fig. 2 and 3 suggest that the magnetic island locatedat the plasma periphery affects the plasma poloidal
flow as the drag term.

2.0

1.0
0.5
0.0

2.0

IET [A]

3. Experimental Results
In order to study the effects of magnetic islands
on plasma poloidal flow, we externally controlled theflow velocity by changing the electrode current withthe current control power supply. Figure 1 shows (a)LaB6 electrode current, (b) electrode voltage, (c)electron temperature, (d) electron density measured
by a triple probe and (e) Mach probe current ratio.
We adopted a sawtooth function for the current
control power supply for the electrode. The
electrode current started at t = 3 ms and was ramped
up linearly up to 3 A at t =10 ms. It is clear that theMach probe current ratio (Fig. 1(e)) increased
suddenly at t ∼ 6 ms. It is also clear that after thistime the temperature fluctuation was significantly
suppressed (Fig. 1(c)) and, the electron density
increased by a factor of 3 (Fig. 6(d)) which suggests
the improved mode transition. Therefore we0510Iex1/2 [A1/2]

1520Fig.2. Dependency of the island width Iex1/2
on the electrode current required for thetransition IET

adopted the electrode current required for thetransition IET in Fig. 1 as the index for the islandeffect on the poloidal flow. Before t = 0 ms weapplied the pre-biasing for the pre-ionization.
The dependency of the island width Iex1/2 on theelectrode current required for the transition IET wasshown in Fig. 2. It is clearly seen that the electrodecurrent required for the transition IET increased withgrowing the island width. However for theevaluation of this dependency we should takeaccount of the degradation of plasma performance byislands. In order to study the effect of the degradationof plasma performance to the dependency of theisland width on the transition, we also tried thebiasing experiments in limiter configurations. The
limiter was inserted from the bottom side of theplasma. We surveyed the dependency of the limiter
position on the electrode current required for thetransition IET, which was shown in Fig. 3. The

4. Summary
We surveyed the relation between thethreshold of the external driving force required for aplasma flow jumping and an island width. The
electrode current required for the transition
increased with growing the island width at theplasma periphery. We also surveyed the effect of thedegradation of plasma performance in the limiter
configurations. The electrode current required for thetransition IET almost independent on the limiter
position.
Acknowledgments
This work was supported by the LHD Joint
Planning Research programme at the National
Institute for Fusion Science.
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[2] N. Ohyabu et al.: Phys. Rev. Lett. 88 (2002) 55005.
[3] T.E. Evans et al.: Phys. Rev. Lett. 92 (2004) 235003.
[4] S. Kitajima, et al.: Nucl. Fusion, 46 (2006) 200-206.
[5] S. Kitajima, et al.: Plasma Fusion Res. 3 (2008)
S1027.
[6] S. Kitajima et al.: Plasma Fusion Res. 5 (2010)
S2041.