K.Y." Watanabe

M. "Okamoto

National Institute for Fusion Science (NIFS-Repository)

A high-speed tangentially viewing VUV imagingtelescope system has been developed in LHD from20081, 2). However, the sampling rate was restrained dueto the low signal/noise ratio in the experiment. In or-der to improve the signal/noise level, upgrade was madeduring the experiment campaign in 2013. In this up-grade, the modifications are: effective diameter of thefirst mirror is enlarged from 23.2mm to 34mm, the fo-cal length is reduced from 7 m to 5.2 m and the imagemagnification factor is increased from 60 to 80. The dis-tance from the detector to LHD is reduced about 0.9m.And the observable diameter in the plasma is enlargedfrom 0.6m to 1.0m. Hence, the maximum framing rateis improved from 2000 frame per second (fps) in the oldsystem to 6000 fps with a similar spatial resolution. Thepicture and viewing field of the upgraded imaging sys-tem are shown in Fig. 1 (a) and (b), respectively. Thelong dashed lines indicate the viewing field of the up-graded system, while the short ones denote that of theold system.Fig. 1: (a)Picture of the installation, (b)horizontallyand vertically viewing field of the upgraded (long dashedline) and old (short dashed) VUV imaging systems.In the experiment campaign in 2013, the upgradedimaging system has been used to investigate the two-dimensional (2D) C VI emission structure in the ex-periment. As an example, Fig.2 shows Topos, Chronosand corresponding singular values of the first two keyfluctuating components of the VUV imaging data. Inthis discharge, the magnetic configurations are: Bt =0.9T,Rax = 3.75m. The m/n = 2/1 MHD instabilitywith a frequency about 0.6 kHz is excited. And this fluc-tuation is also detected by the upgraded VUV imagingsystem. The 2D mode structure is estimated by compar-ing the experimental images (U1 and U2) and syntheticimages produced with plausible 2D emission profiles, asshown in Fig.3. This mode is localized at the ρ = 0.45and mode width is about 0.15.U(1)−0.04−0.020.000.020.04W=l94.4566U(2)−0.06−0.04−0.020.000.020.04W=l76.79954.020 4.022 4.024 4.026 4.028 4.030Timel(s)Topos Chronos SigularlvalueV(1)V(2)Fig. 2: Topos, Chronos and corresponding singular val-ues of the first two key fluctuating components.Fig. 3: Synthetic line-integrated images and correspond-ing plausible mode structures for U1(Top) and U2 (bot-tom), respectively1) M. Takeuchi et al, ”Development of a High-SpeedVUV Camera System for 2-Dimensional Imaging ofEdge Turbulent Structure in the LHD”, Plasma Fu-sion Res. 5 (2010) S10372) T.F. Ming et al, ”High speed vacuum ultraviolet tele-scope system for edge fluctuation measurement inthe large helical device”, Review of Scientific Instru-ments, Vol.83, Issue 10 (2012) 10E513.  The magnetohydrodynamic (MHD) stability behavior of plasmas in the toroidal magnetic confinement devices is one of the important issues for the achievement of the nuclear fusion power plant. In the Large Helical Device (LHD), MHD activities have been reported under various conditions1, 2). For example, the interchange mode is appeared in the weak magnetic shear configuration, which is formed by the plasma current generated by unbalanced Neutral Beam (NB) injection. Furthermore, when the plasma current of the opposite direction to the toroidal magnetic field is induced by NB injection, the electron temperature fluctuations are appeared in the core region by injection of supersonic gas puƥng (SSGP) and the tearing-like mode structure is measured by electron cyclotron emission (ECE) diagnostics3). It is suggested that the fluctuation is caused by a rotating m/n= 2/1 magnetic island. On the other hand, in the calculation for stability analysis, it is found that the tearing mode becomes unstable if the plasma current is higher than above experiment. In this study, it aims that the appearance mechanism of tearing-like mode, which is triggered by SSGP under the condition of relatively low plasma current, is clarified. In order to analyze the MHD stability, the rotational transform profile before and after SSGP injection is measured by motional Stark effect (MSE) diagnostic. Figure 1 shows the temporal evolution of the electorn density and temperature measured by FIR interferometer and ECE diagnostics with SSGP injection. The experimental condition is as follow: the magnetic axis Rax = 3.6 m, Bt = −2.75 T, which is the toroidal eld at Rax. SSGP is injected at t = 4.8 sec and the plasma current is about 43 kA. From Fig. 1, the electron density is rapidly increased and the electron temperature is decreased just after injection of SSGP. Unfortunately, electron temperature fluctuations triggered by SSGP is not appeared in this experiment.  Figure 2 shows the rotational transform profile just before and after SSGP injection. It is found that the rotational transform profile is hardly changed by SSGP and the current density profile is not changed, too. Therefore, it is considered that m/n= 2/1 tearing mode remain stable and electron temperature fluctuation can’t be observed. Figure 3 shows perturbation profiles of the electrostatic potential and poloidal magnetic flux estimated by the calculation for stability analysis. The circular cylinder model is used in the calculation. From Fig. 3, it is found that the magnetic confinement structure becomes the interchange mode structure and this calculation result is consist with the experimental result, which isn’t appear the m/n = 2/1 magnetic island. In this study, but the tearing-like mode structure can’t be observed, it is important that the current density profile with the mode structure is clarified in order to understand more about MHD instability of helical devices.                                       Fig. 1. Temporal evolution of the line averaged electron density and temperature before and after SSGP injection.                       Fig. 2. The rotational transform profile just before and after SSGP injection.                Fig. 3. Perturbation profiles of electrostatic potential I and poloidal magnetic flux \ estimated by calculation for stability analysis just after SSGP injection.   1) Sakakibara, S. et al.: Plasma Phys. Control Fusion 44 (2002) A217. 2) Sakakibara, S. et al.: Fusion Sci. Technol. 50 (2006) 177. 3) Murakami, A. et al.: Plasma Fusion Res. 6 (2011) 1402135. 0.30.50.70.91.11.30 0.2 0.4 0.6 0.8 1t=4.7sect=5secL2Sreff/a990.3 0.4 0.5 0.6 0.7I\U47§14. Study of MHD Fluctuation with  Tearing-Like Mode Structure in LHDOkamoto, M. (Ishikawa National College of Technology),  Watanabe, K.Y.

Study of MHD Fluctuation with Tearing-Like Mode Structure in LHD

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Study of MHD Fluctuation with Tearing-Like Mode Structure in LHD

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