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(THP2/31) Improvement of MHD Stability in Negative/Weak Shear Configurations for a Steady State Tokamak

   
T. Ozeki , Y. Ishii , S. R. Hudson , S. Tokuda , K. Hamamatsu , M. Yamagiwa , G. Kurita , A. Hirose ¹, D. Monticello ², T. Tuda , T. Hirayama 
 
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken, Japan
¹ University of Saskatchewan, Canada
² Princeton Plasma Physics Laboratory, USA

Abstract
Comprehensive stability analyses have been carried out to clarify characteristics of MHD stability  of both strongly negative and weak shear configurations for improvement of plasma performance. Hard beta limits due to ideal MHD stability can be improved by reducing the pressure gradient in the weak shear region and by increasing the pressure gradient near the plasma surface in both configurations. In the negative shear  configuration, the double tearing mode may emerge just under $q_{min} = 2$. However, small current modulation, to reduce the shear, at the outer resonance surface stabilizes the double tearing mode. In the weak shear configuration, the neoclassical tearing mode, which is induced by the helical bootstrap current  along the island, reduces the beta limit  below the ideal kink limit. However, a localized additional current at the O-point of the island significantly stabilizes the mode. Stabilization of the KBM requires a large shear, which may be either positive or negative. A narrow stable window appears near null shear and expands with the pressure gradient.

 

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