K. Nishimura

K. Yamazaki

K.Y. Watanabe

S. Sakakibara

W.A." Cooper

Y. "Narushima

National Institute for Fusion Science (NIFS-Repository)

§31. Study of External Mode of Limiter Plasma on LHD Narushima, Y, Watanabe, K.Y, Sakakibara, S., Nishimura, K., Yamazaki, K., Cooper, W.A. (CRPP Lausanne, Switzerland) Low-n ideal MHD analysis of limiter plasma on LHD is carried out using TERPSICHORE[l]. We focus on the effect of the profile of the rotational transform, es-pecially the value at edge ~(1)/27r. To obtain the equi-librium in various ~(1)/27r, the position of the plasma boundary is changed in the calculation with VMEC[2]. One way to produce such plasma configuration exper-imentally is usage of a limiter[3]. The plasma pres-sure P of the equilibrium analyzed here is expressed as P = Po(1 _ p2)O.5(1_p8). Here, Po and p are the cen-tral plasma pressure and normalized minor radius in flux coordinate, respectively. The equilibrium with this pressure profile is solved as current free plasma. The low-n unstable region under the free boundary condition is shown in Fig. I. The points of open and the other symbol mean low-n stable and unstable, respec-tively. In the unstable symbols, the circles mean that the mode structure has the largest amplitude of dis-placement vector spectrum of 'S for m/n=2/1 modes and stars for 1/1 mode. The solid and dashed lines in-dicate the contours of Mercier parameter with DI =0, 0.2, at ~/27r=0.5 resonance surface, respectively, and dotted line indicates the contour of Dr=O at ~/27r=I.O resonance surface. The contour of Dr=0.2 for ~/27r=I.O does not exist in this equilibrium region. The half-filled symbols mean low-n unstable even in the fixed bound-ary condition and these modes are destabilized in the Mercier unstable region with Dr(@~/27r=0.5»0.2 re-gardless of the value of ~(1)/27r. The mode structures are shown in Fig.1(a)rv(d). Un-der the fixed boundary condition, there does not ex-ist the mode structure in which the m/n=1/1 appears independently. The mode structure shows the inter-nal mode and the main component is m/n=2/1 mode (Fig1.(a)). Under the free boundary condition, the low-n unsta-ble area is expanded to Mercier stable and ~(1)/27r> 1.0 region more widely than the case under the fixed bound-ary condition. In case of ~(1) /27r> 1.0, the low-n un-stable area stays in Mercier unstable region and the mode structure is similar to an internal mode (Fig. 1 (b)). In case of ~(1)/27r<1.0, on the other hand, the unstable region exists even in Mercier stable region and the mode structure clearly shows the character-istics of an external mode (Fig.1 (c)). In this case, the amplitude of m/n=l/l becomes larger than that of m/n=2/1. At the region with Mercier unstable and ~(1)/27r<1.0, the mode structure has the comparable amplitude of,s ofm/n=2/1 and 1/1 (Fig.1 (d)). These show that the structure of an external mode, which has the meaningful m/n=l/l mode, appears in the region of ~(1)/27r<1.0 regardless of the value of Dr under the free boundary condition. The region where external mode appears lies under the condition of the free boundary and ~(1)/27r<1.0, even in Mercier stable and/or low-,B region. If the con-dition with no rational surface of ~(1)/27r=1.0 inside the plasma can be produced, the external mode could be studied experimentally. O.O~f-(i)_Flxcd __ ....., __ , ___ 7=..J2L. -MI ( ~.~ VI 0.06(1))- ~ 0.04 211 0.02 O ______ ~ ·_·t. ·_· _· · ___ ~ :::::~ d402 m · 0(')- .<::: :: _·_-~L-::_ .. _ -0.04 -O.ffi 0.5 1.0 1.5 2.0 2.5 3.0 <Pdi.> Fig. 1: Unstable domain with respect to low-n mode(n=l mode family). Fourier vector ampritude un-der the conditon[Boundary, Dr, ~(1)/27r] of (a) [Fixed, >0.2, >1.0], (b)[Free, >0.2, >1.0], (c)[Free, <0.2, <1.0] and (d) [Free, >0.2, <1.0], respectivery. References 1) W. A. Cooper, Plasma phys. and Controlled Fusion 34, (1992) 1011 2) S. P. Hirshman, W. I. Van Rij, and P. Merkel, Com-put. Rhys. Commun. 43, (1986) 143 3) K. Nishimura, K. Ida, K. Kawahata, and K. Kozo et al., Journal of Plasma and Fusion Research SERIES, 4, (2001) 106 35 

Study of External Mode of Limiter Plasma on LHD

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