23P048-R
Measurement of the structure of low-order MHD modes in LHD
ＬＨＤにおける低次MHDモード構造計測
Hayato Tsuchiya, Yuuki Takemura1, Yoshio Nagayama, Daisuke Kuwahara2,Kiyomasa Watanabe and Satoru Sakakibara
土屋隼人,武村勇輝1,長山好夫,桑原大介2,渡邊清政,榊原悟
National Institute for Fusion Science1
SOKENDAI, 2Tokyo Tech.
322-6,Oroshi,toki city, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
This study reports the comparison between the magnetohydrodynamic (MHD) mode structure measured byECE (Electron Cyclotron Emission) and SX (Soft-Xray) in Large Helical Device (LHD). The MHD mode,
whose frequency is 0.7~2.7kHz, is enhanced around the area of ρ = 0.8, ι = 1. The fluctuation datameasured by the SX, which is included the integral effect along the line-of-sight, cannot show the mode
structure clearly. At the same discharge, ECE data can identify the mode width using the displacement
amplitude profile.

1. Introductions
It is well known that the edge resonant
low-order modes have some effects on the radialstructures such as local flattening in density ortemperature profiles which may cause degradationof plasma performance. Therefore, it is quite
important issue to obtain the data of magnetic
fluctuations and instability mode structure, which
are basic data for MHD instability study and its
control [1]. In this paper, the results of thecomparison between the mode structures measured
by SX and ECE (Electron Cyclotron Emission) [2]
are reported.
2. Experimental set up and Diagnostics
This study is performed in Large Helical
Device (LHD) [3]. The MHD fluctuations aremainly measured by magnetic probe (MP) [4],Soft-Xray (SX) detectors array [5] and radiometerof electron cyclotron Emission (ECE) [2]. The
frequency resolution of ECE is 1GHz. It
corresponds to the spatial resolution (∆ρ∼0.02).
MHD or high-beta experiment are conducted in thecondition of low magnetic filed B t< 1.0T in orderto clear the effect on the magnetic field. In thatcondition, ECE measurement cannot be usedbecause of the cutoff in LHD. Instead, SX radiation
measurement is a helpful tool for fluctuation studies.
The SX detector array’s spatial resolution isapproximately 3mm (∆ρ∼0.06) on the mid plane.
Here, we have to note that the SX emission
intensity is included the integral effect along theline-of-sight. Therefore, it is difficult to convert theline-integrated fluctuation data into local fluctuation
value. The experiment is conducted in the conditionof Rax = 3.6m, Bt = 1.5T, which is suitable for ECE

to be measured in 0.1 < ρ < 1. If the result ofidentification of mode structure used by SX is
supported by the ECE measurement, this study will
helps the confinement studies of the low fileddischarge.

Fig. 1 a) Power spectrum of Te fluctuations(ρ=0.84) measured by ECE b),c)Coherence
evolution among ECE measurement, SX andMP. The measurement point of both ECE andSX is ρ∼0.84. ne ~ 1.5 * 1019 m-3, Te ~1.5keV.

3.Experimental Results
The MHD fluctuation of low-order mode
which is seem to be m/n = 1/1 are observed by SX,
MP and ECE. There are the fluctuation power peaksof ECE and SX at ρ∼0.85, which corresponds to thearea of ι= 1. Figure 1 a) is the power spectrum ofelectron temperature (Te) fluctuation measured byECE. We can see the existence of broad peak from
2-2.8kHz, and the peak frequency gradually
decreases with the increase of toroidal currents. Fig.
1 b) and c) show the coherence among thefluctuation data of ECE, SX and MP. We can see
the significant coherence of 0.7-2.8kHz. It isconsidered that these fluctuations are oriented froma same instability.
Fig. 2 a) shows the coherence profile
calculated by Te fluctuation. The basis channel tocalculate the coherence is a channel at ρ∼0.84. We
can see the high coherence at ρ=0.75-0.9. In thearea, Te oscillates in the same phase, as shown Fig.
2 b).
We compare the results of the coherence
profile calculated from ECE and SX fluctuation
data. The coherence shown in Fig. 3 a) is average ofcoherence of f = 1.9 - 2.7 kHz, to take into accountthe broad power peak. The fluctuation exited
around ρ = 0.85 seem to reach ρ = 0.6. The
coherence values calculated from the SX data havea tendency to be larger than those of ECE. To
clarify the mode structure, we calculated thedisplacement amplitude ξ. The displacementamplitude is defined as following equation.

ξ=− δfδf δρ

comparison of the displacement amplitude profile
using the ECE and SX fluctuation data. When SX
signal cannot show the structure clearly, we can see
the mode structure using ECE data, which is local
measurement tool.
4.Summary

The MHD mode structure excited at ι~1 isanalyzed using the SX and ECE. It is confirmedthat the ECE is useful to identify the mode structurewith SX measurements.
Acknowledgments
This work was supported by the NIFS budget(NIFS10ULPP008,NIFS10ULPP020,NIFS11ULPP00
8, NIFS11ULPP020).
References[1] S. Masamune,
23rd IAEA Fusion Energy Conference, Daejon,
Republic of Korea, EXS/P5-11[2] Hayato TSUCHIYA, Yoshio NAGAYAMA, Kazuo
KAWAHATA, Shigeru INAGAKI, Shin KUBO andLHD Experiment Group: Plasma Fusion Res. 6(2011) 2402114.
[3] O. Motojima et al., Nucl. Fusion 40 (2000) 599.
[4] S. Ohdachi, K. Toi, M. Takechi, and S. Yamamoto,Rev. Sci. Instrum. 72, (2001) 727.
[5] S. Sakakibara, H. Yamada, K. Yamazaki, K.Y.
Watanabe et al., Fusion Eng. Des., 34-35(1997)707.

(1)

Here, f is the measured physical value. Fig.2 b) is a

Fig. 2: a) Radial coherence profile of f =2.75kHz. b) Spectrum Phase. The basis channelis ρ∼0.84.

Fig. 3 Comparison of the coherence
profile (a) and ξ profile (b) between ECEand SX.