K.Y. Watanabe

S. Sakakibara

T." Yamaguchi

Y. "Narushima

National Institute for Fusion Science (NIFS-Repository)

§26. Measurement of Magnetic Field Accompanied by Varying Magnetic Island in LHD Narushima, Y., Sakakibara, S., Watanabe, K.Y., Yamaguchi, T. Magnetic islands sometimes play important roles in toroidal plasma confinement from a viewpoint of MHD stability. For example, a seed island triggers a NTM in Tokamaks. On the other hand, it is possible that the island flattens the pressure profile at the resonant sur-face, contributing to the stabilization of the pressure-driven resonant MHD mode .. In the LHD experiment, a seed island width changes larger or smaller. during the plasma discharge[1,2J, which is accompanied by the change of the perturbed magnetic field b" Because the magnetic diagnostics measuring the profile of the field (b,) originating from the island motion is an ef-fective method to obtain its structure, a toroidal ar-ray of magnetic flux loops is set at the outer ports in LHD as shown in Fig.l. Each flux loop has to-tal cross-sections of about NS=12m2, which leads to enough electromotive force voltage to detect the slow (few lOOms) and small (few Gauss) change of the mag-netic field. The shapes of the flux loops at the toroidal angles ¢=-162, -54, 54, 90, and 162 are planar (solid line in Fig.1) that we use here. The detected magnetic fluxes IJ>Rs are normalized by the total cross section N S to b1 whose component is in the major radial di-rection. The 4-pairs of perturbation coils placed at the top and bottom of LHD around ¢=±90[degJ (bold line in Fig.I) produce a seed island with the structure of m/n=1/1 mode. The typical discharge with a seed is-land (11)~c""150[1ll1llJ) is shown ill Fig2 in which the island width 11) grows from 150 to 200[mmJ as shown in Fig.2(d). The magnetic field b, varies with time (Fig.2 (e)). The bl profiles are shown in Fig.3. At t=0.48[sJ, (w=wvoc ) the flux loops detect zero field b" At t=1.73[sJ, (w=200[mmJ) the finite field h, appears. The fitting :urve b, (¢) = bl~l cos(¢ - ¢n~,) well ex-presses the b, profile which has the dominant toroidal mode n=L Here, br=l and 4>n=l are the maximum am-plitude and its toroidal angle respectively. We carry out the magnetic diagnostics of non-rotational mag-netic island in LHD. It is confirmed that the finite mag-netic field appears with a change of the magnetic island width. References 1) K.Narihara et aI., Phys. Rev. Lett.87,135002 (2001) 2) N.Ohyabu et aI., Phys. Rev. Lett. 88,055005 (2002) 30 Fig. 1: -. .;.~--90 M ~ U LI 8 I' I' ~'l Fig. 2: Time evolution of (a) averaged beta </3>di. (b)electron temperature at centre T, (c) averaged elec-tron density n, (d) island width w (e) perturbed field h, 1.0 .-.-.-.-.... ~.-.-.- .. -.-.-.!- .... -.-.-.-.-;- .... -.. -- .. 0.5 (a) F(jI.48[s] o .····;··0·-·-··0··6··· -0.5 ---------------j -::~ P= ... ~ .."' ..~ ... ~.*:~:~:::"'.:~.:~ ... =i.,~ ..~ .."' ... "' .. ~ ..~ ... := ...~ ..~_ .. "' .. ~_.=l. '"" (b) 1.73[<] 0.5 -------.. --- .... --- "'--r b;~1 -: 0············;·····_,··_··· : ~n=1 ·.·_······ __ ····r· ........... , .. . . . -0.5 _1.0...L ....... L ............... -.--1 L80~---,90~~-0'-~-:9'::-0~-,J180 - <p [deg] Fig. 3: Toroidal profile of bl at (a)t=0.48[s], (b)l. 73[sJ 

Measurement of Magnetic Field Accompanied by Varying Magnetic Island in LHD

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