P070-P
2323P070-P
Experimental Investigation of Influence of Externally Applied
Resonance Magnetic Perturbation
on Magnetic Structure during Tokamak Disruption.
外部共鳴摂動磁場印加がディスラプション時の磁場構造へ与える影響の実験的検証
Yuki Kanazawa, Yoshihide Shibata, Masaaki Okamoto*, Noriyasu Ohno, Kiyomasa Watanabe**,Shin Kajita***, Yusuke Kikuchi**** and Hirohiko Tanaka**
金沢由樹, 柴田欣秀, 岡本征晃*, 大野哲靖, 渡邊清政**,梶田信***, 菊池祐介****, 田中宏彦**
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan*Ishikawa National College Technology, Kitachujo, Tsubata, Kahoku-gun, Ishikawa 929-0392, Japan**
National Institute for Fusion Science, Oroshi-cho, Toki, Gifu 509-5292, Japan***
Eco Topia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan****
Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan
名古屋大学工学研究科，〒464-8603 名古屋市千種区不老町*
石川工業高等専門学校, 〒929-0392 石川県河北郡津幡町北中条タ1**
自然科学研究機構 核融合科学研究所, 〒509-5292 岐阜県土岐市下石町322-6***
名古屋大学エコトピア研究機構，〒464-8603 名古屋市千種区不老町****兵庫県立大学工学研究科, 〒671-2280 兵庫県姫路市書写2167
Tokamak disruptions are one of the most critical issues for the ITER and demo-reactors, because an intenseheat load on plasma facing components and large electromagnetic forces on the vacuum vessel andin-vessel components are generated during the disruption. Therefore, the disruption control technology isnecessary for the fusion tokamak devices, and many studies have been carried out until now. In this study,
we investigated the effect of externally applied resonance magnetic perturbation on the disruption control
in the small tokamak device HYBTOK-II.

1. Introduction
The tokamak disruption, which is accompanied
by an intense heat load on plasma facing
compornents during thermal quench and a large
electromagnetic force on in-vessel components
during current quench, is one of the most crucial
issues for next generation tokamak devices such as
ITER [1]. Disruption control and avoidance arenecessary from the viewpoints of device protection,and many studies have been done in many tokamak
devices [1]. In HYBTOK-II, which is a small
tokamak device in Nagoya Univ., the disruption
experiment had been carried out to understand theoccurrence mechanism of disruption [2]. In Ref. 2,a growth of m / n = 3 /2 tearing mode, where m andn are poloidal number and toloidal number
respectively, was observed just before thedisruption. And then, a rapid change of sudden
pump-out of the core plasma current, which wasinduced and shifted the plasma center position tovacuum vessel inner side, has been observed. The
critical issues in the tokamak disruption such asheat load have been investigated in many devices[3,4]. An example is a study on a resonancemagnetic perturbation (RMP) applied to tokamak
plasma. RMP can generate the ergodic magnetic
region in the plasma egde region [5]. Therefore
RMP can change the plasma current density
distribution drastically during the disruption. In thisstudy, we investigated the effect of externally RMP
on the plasma current density distribution during
the disruption in order to control the disruption.
2. Waveform of disruption in HYBTOK-II
HYBTOK-II is a small tokamak device with alimiter configuration. The major radius R is 40 cm,and the minor radius of the vacuum vessel and thelimiter radius a are 12.8 and 11 cm, respectively [6].
Hydrogen is used as the working gas in theexperiments and the discharge duration is 20 ms.
The RMP coils are composed of two sets of local
helical coils, which have an IGBT invertor power
supply [7]. The RMP coils are installed outside thevacuum vessel and poloidal and toroidal numbers of

RMP coils are m = 6 and n = 1, respectively. Themaximum coil current in RMP system is 150 A, andthe frequency changed from 1 to 30 kHz.
The typical waveform of disruption discharge inHYBTOK-II is shown in Fig. 1. Disruption wasdriven by ramping up the plasma current, Ip, toreduce the plasma surface safety factor, qa. In theexperiment, the positive spike of plasma current
appeared at qa < 3, and then a negative spike of theplasma loop voltage Vloop and an inward shift of theplasma column were observed.

Fig. 1. Time evolution of (a) plasma current Ip, andplasma safety factor qa (b) plasma loop voltage Vloop, and(c) the distance from horizontal plasma current centre ∆x.

w/o RMPw/ RMPFig. 2. Histogram of the plasma current decay rate,∆Ip/∆t, (a) without RMP and (b) with RMP

The analytic interval in this study corresponds a
hatched area in Fig.1 because the plasma center andplasma cross-section didn't change in this region.
The current decay rate, ∆Ip/∆t, was evaluated bylinear approximation of plasma current in ananalytic interval. The RMP was applied from t = 13
to 20 ms.
3. Experimental result
Figure 2 (a) and (b) show the histogram of theplasma current decay rate, ∆Ip/∆t, without RMP andwith RMP during disruption. The number of shot is
120 in Fig. 2 (a) and (b), respectively. The averagevalue of plasma current decay rate, ∆Ip/∆t, in Fig 2(a) is -4.98 kA/ms, while the average value of thatin Fig. 2 (b) is -4.43 kA/ms. However the effect ofRMP on the current density distribution during thedisruption was observed in this study.
4. Discussion and Summary
We investigated the influence of RMP on thecurrent decay during disruption in the HYBTOK-II.
However, we couldn't obtain the significant effectof RMP for the disruption.
In order to understand the reason why RMP did
not act on the current decay during the disruption,
we argued over the penetration time of RMP. The
diffusion of magnetic field is determined by theresistive diffusion time τR,(1)τ R = µ0 L2 / η ,where µ0 is the permeability of vacuum and L is ahalf of the plasma limiter radius, which is 0.055 mbecause the magnetic islands exist in HYBTOK-II.
The plasma resistivity η is calculated using theclassical Spitzer formula [8]. If the effective charge
Zeff is assumed to be1, the plasma resistivity η ~7.1
7.1×10-4 Te-3/2 [Ωm]. In the current quench phase,the electron temperature, Te, in the region where themagnetic islands exist is 30~50 eV. Therefore, theresistive diffusion time τR is 0.6~1.9 ms. Because
the RMP coil current frequency was 1~30 kHz, theplasma couldn't respond against the RMP.
For the future, we are planning to use the direct
current power supply for RMP coils due to reduce
the skin effect of vacuum vessel, and continue toinvestigate the influence of RMP on the current
decay during the disruption.
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