(TH/P8-1) Effects of "Sharpness" of the Plasma Cross-Section on the Stability of Peeling-Ballooning Modes in Tokamaks

N. Aiba1), S. Tokuda1), T. Takizuka1), G. Kurita1), T. Ozeki1)
1) Japan Atomic Energy Agency, Naka, Ibaraki-ken, Japan

Abstract.  An ideal magnetohydrodynamic (MHD) mode destabilizing near the plasma surface is one of the causes of edge-localized modes (ELMs), which constrain the maximum pressure gradient in the pedestal at the tokamak edge region. The stability of ideal MHD modes, called a peeling, a ballooning and a peeling-ballooning modes, depends on the shape of the equilibrium, the ellipticity, the triangularity, the squareness and so on. In this paper, the effect of the new shaping parameter ‘sharpness’ on the stability of the peeling-ballooning mode is investigated numerically with the linear ideal magnetohydrodynamic (MHD) stability code MARG2D, where the sharpness is defined in terms of the curvature at the top and bottom of the outermost flux surface. Though the increase of the sharpness has little impact on the stability of current driven (peeling and kink) modes but the stable pressure gradient limit restricted by the stability of the peeling-ballooning mode significantly improves as the sharpness increases. The increase of the sharpness also makes broader the second stable region against an ideal ballooning mode on s-α diagram, where s is the magnetic shear and α is the normalized pressure gradient. This expansion of the second stability region is considered to be important to enhance the stable pressure gradient limit restricted by the peeling-ballooning mode, and the sharpness is an important parameter for high performance H-mode operations with high pedestal pressure.

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