IMPROVEMENT OF MHD STABILITY IN NEGATIVE/WEAK SHEAR CONFIGURATIONS FOR A STEADY STATE TOKAMAK

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 qmin=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. Two promising configurations can be considered for a steady state tokamak with a large bootstrap current. One is a strongly negative magnetic shear configuration exploiting a large bootstrap current, and the other is a weak shear configuration which utilizes a moderate bootstrap current and external current drive at the core. Whether a given configuration can achieve and sustain enhanced plasma performance critically depends on its MHD stability. In the negative shear configuration, the low-n kink mode can be stabilized by strongly negative magnetic shear. However, the ideal/resistive interchange modes become destabilized, and the double- tearing mode may be induced due to adjacent rational surfaces. Furthermore, when the intrinsic ion magnetic drift resonance is taken into account, a kinetic ballooning mode (KBM) emerges even in negative shear. On the other hand, in weak magnetic shear configuration, the neoclassical tearing mode persists an in positive shear, imposing a lower beta limit than that of the kink mode. So far, these MHD modes have been analyzed individually. In this work, an attempt is made to seek stable domain in high beta negative and weak shear tokamak discharges through comprehensive stability analyses.