K.Y. Watanabe

S. Sakakibara

T." Yamaguchi

Y. "Narushima

National Institute for Fusion Science (NIFS-Repository)

§27. Estimetion of Island Width by Magnetic Diagnostics in LHD Narushima, Y., Sakakibara, S., Watanabe, K.Y., Yamaguchi, T. It is worthwhile to investigate the behaviour and characteristics of the magngnetic island because the magnetic island plays an important role in MHD sta-bility and plasma confinement. In LHD, generally, the width of magnetic island, w, is indicated by the flat-tening of the electron temperature profile measured by Thomson scattering, which only can be obtained at one toroidal position and therefore gives liInited knowledge of the structure of the island. The width w is related to the perturbed field[l] as w2=Cho + h, B, (1 ) Here C, bO, bI and B t are a constant value, perturbed field during the plasma discharge, perturbed field pro-ducing the vacUUIIl island and the toroidal field, respec-tively. For h, =0 and /)0 ,,"0, the width ow is equal to that of the vacuum island (w=Wvac). The magnetic diagnos-tics measuring the profile of bI is an effective method to find the structure of the magnetic island. The ar-ray of the flux loops detects the profile of the field (h,) originating from the island motioIl. The component of b, is in the major radial direction. The toroidal pro-file of h, with the toroidal mode n=1 indicates that the change of the magnetic island width during a dis-charge also has the n=1 mode component. The ampli-tude h,~' estimates the width w by using Eq (I) that can be rewritten as W= (2) Here b1=1 is the maximum amplitude as shown in Fig.I. The calibration between w2 - w~ac and b1=1 / Bt pro-vides the coefficient a for the island enlargement cases (w;:: w~) as shown in Fig.2. As a result, the mag-netic diagnostics can estimate the island width wmag . Figure3 shows the relationship between wand wmag . These results show that the magnetic diagnostics can estimate the structure of an magnetic island. References 1) A. H. Boozer, Phys. Fluids 27, 2055 (1984) #43579 1.0 · . .--.t= 1. ~3 [s r----·~-·--····--·-· · . 0.5 ~ '" '" :::s 0 '" 0 ~1..£ -0.5 · . · . ::r--:_-=::. b;~r_:i __ ':: ______ -: : '" I : ! 't'IF : · . . · . . -.. --------------,.- ------------. ----- -- ----· . . · . . · . . · . . · . . · . -1.0 · . - - - - - - - ---- --< - - - - - - - ------.. - - - - - - - - - - - - -. - - - - - - - -- ---. . -180 -90 <I> [aeg] 90 180 Fig. I: Profile of b, for magnetic island grows. 4xlO' ~ 3xlO' N ~ 2xlO' Q N ~ " if! , IxlO' N~ O~~ __ -L-.~ __ ~.-~~~~ __ ~ ° 2.OxIO" 4.OxIO" 6.OxIO" 8.OxIO" "'IFI bl IBt[au] Fig. 2: Calibration between w 2 - w;ac and b1=1 / Bt 200 160 -u-} 120 oj '" 80 " f 40 0 ° 40 80 120 160 200 w[nnn) (Thomson) Fig. 3: Relationship between wmag and W 31 

Estimetion of Island Width by Magnetic Diagnostics in LHD

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