23P069-P
Study of pressure gradient MHD instabilities in LHD plasmas
ＬＨＤにおける圧力駆動型MHD不安定性の成長率の研究
WATANABE Kiyomasa, TAKEMURA Yuiki1, SAKAKIBARA Satoru, Ohdachi Satoshi,
OKAMOTO Masaaki2, NARUSHIMA Yoshiro and LHD experiment group
渡邊 清政, 武村勇輝1, 榊原 悟, 大舘 暁, 岡本 征晃2, 成嶋吉朗, LHD実験グループ
National Institite for Fision Science, Oroshi-cho, Toki, Gihu 509-5292, Japan
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
1
Department of Fusion Science, Graduate University for Advanced Studies, Oroshi-cho, Toki, Gihu 509-5292, Japan
総合研究大学院大学 核融合専攻，〒509-5292 岐阜県土岐市下石町322-6
2Ishikawa National College Technology, Kitachujo, Tsubata, Kahoku-gun, Ishikawa, 929-0392 JAPAN
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

The growing behavior of the typical MHD activities in LHD, the rotating and the non-rotating modes, arestudied in order to resolve the difference of the influence of the modes on the confinement performance.
The growth rates of the rotating and the non-rotating modes, which are the typical MHD activities in theLHD experiments, are studied. The growth rate of the rotating mode is same order with the predicted one.
On the contrary, that of the non-rotating mode is much slower than the predicted one.

1. Introduction
In the recent LHD experiments, the high beta
with more than 5% as the volume averaged toroidal
beta value, <β>, which is relevant to that of aheliotron reactor design, are sustained for a periodof ~100 times of the energy confinement time
without disruptive phenomena. However, LHD has
been considered to have a disadvantage with respect
to pressure driven magneto-hydrodynamics (MHD)
instabilities because the magnetic hill region exists.
In the LHD experiments, some kind of MHD
activities are observed. Two among them are
typical [1]. One is characterized as the rotating
modes, which means that the instability has a real

frequency. The modes are observed in the wide
regime of the plasma parameters where theinterchange instabilities are predicted by the linear
MHD stability analysis. And the amplitude of thefluctuation increases with the increase of the beta
gradient and the decrease of the magnetic Reynolds
number. The effect on the confinement performanceis a little. Figure 1 shows a wave form and a beta
profile in the typical discharges with the rotating
modes of m/n=1/1. Here m and n are the poloidal
and troidal mode numbers. In Te profile enclosedby the circles, the fine structures are observed asshown in Fig.1(b). Another typical MHD activity is
characterized as the non-rotating mode. The mode

Fig.1 (a) Magnetic fluctuation behavior of low-n
mode in a high beta discharge. (b) Pressure profile ofthe high beta plasmas with the magnetic fluctuation.

Fig.2 (a) Time evolution of the beta value in the lowmagnetic shear and the magnetic hill plasma with acollapse. (b) The time evolution of the electron
temperature profile during the colapse.

are observed in the low magnetic shear and themagnetic hill configuration with the relatively lowbeta value and the collapse behavior, which meansthat the core pressure rapidly decreases. Figure 2
shows the wave form and the electron temperature
profile in the typical discharges with the nonrotating modes. The averaged beta value and thecentral temperature decrease by one third due to theFig.3 (a) Time evolution of the beta and the line
averaged density in a discharge with the appearance/disappearance of the rotating magnetic fluctuation.
(b) Contour of the correlation of the magnetic
fluctuation. (c) Time evolution of magnetic
fluctuation amplitude of m/n=1/1 mode.

Fig.4 (a) Time evolution of the beta, the toroidal
plasma current and the line averaged density in adischarge with a collapse. (b) The detail time
evolution of the beta, the non-rotating m/n=1/1
magnetic fluctuation amplitude and the line averageddensity. (c) Time evolution of ECE signals forvarious minor radial location.

collapse.
In this paper, the growing behavior of the MHD
activities are studied in order to resolve thedifference of the influence of the modes on theconfinement performance.
2. Time evolution of the fluctuation in the phaseof appearance of MHD instabilities
Figure 3 shows the time evolution of the beta, theelectron density, the correlation of the magnetic
fluctuation in the frequency space and the magnetic
fluctuation amplitude of m/n=1/1 mode in adischarge with the appearance/ disappearance of therotating magnetic fluctuation. In Fig.3(b), the white
regions correspond to the frequency of the low-ninstabilities, that around 1.5kHz to the m/n=1/1
mode and those around 3kHz, 4.5kHz, 6kHz to thehigher harmonics. At t~1.75s, the exited m/n=1/1
mode disappears once and it appears again 20mslater. The growth rate in the first phase is ~1/0.6[ms-1], changes to ~1/1.7[ms-1] later, and finally themode amplitude is saturated. According to a linear
stability analysis, the growth rate of the resistive
interchange mode is predicted ~1/0.1[ms-1], whichis same order to that in the initial phase of theinstability growth.
Figure 4 shows the time evolution of the beta, thedensity, the non-rotating magnetic fluctuation
amplitude and ECE signals in a discharge with thenon-rotating mode. The growth rate of the nonrotating mode is ~1/100 [ms-1], which is muchslower than the predicted one. Any precursors has
not been found yet. In Fig.4(c), the ECE signals areshown in the just after the instability occurs. Someburst signals with the rapid growth rate ~0.1 [ms] isfound. The 0.1 ms is almost same with the predicted
linear growth rate.
3. Summary and discussion
The growth rates of the rotating and the nonrotating modes, which are typical MHD activities inthe LHD experiments, are studied. The growth rateof the rotating mode is same order with thepredicted one. On the contrary, the non-rotating
mode much slowly grows than the predicted one.
In an early papers, the strong relation between
the influence of the confinement and the mode
width was suggested [2]. However, the growth rate
might be another main player of the confinement
influence. To resolve it is one of our future subject.
Reference[1] S.Sakakibara et al., Plasma Phys. Control. Fusion, 50(2008) 124014.
[2] K.Y.Watanabe et al., 17th Int. Toki Conf., I-13 (2007).