Resistive wall stabilization by toroidal rotation effects of partial wall configurations and aspect ratio

Toroidal rotation of the plasma coupled with resistive walls can fully stabilize pressure-driven kink modes and allow the beta value to be extended beyond the Troyon limit. It is well understood that for a rotating resistive wall mode (RWM), the mode is more stable when the wall is moved farther away, as long as the wall is close enough to stabilize the ideal plasma mode. By introducing gaps in the resistive wall - an effect similar to moving the wall away - the RWM can be stabilized at a lower rotation frequency. This effect is greatly enhanced by gaps near the outboard midplane, where the pressure-driven kink couples most strongly to the wall. This improvement in stabilization comes at the cost of requiring a closer wall to stabilize the ideal plasma mode, but this trade-off can be quite desirable: it will be shown that an advanced tokamak equilibrium can be stabilized by a pair of close-fitting plates at a rotation frequency reduced by a factor of 3.6 compared with a fully surrounding wall while maintaining ideal stability. Increasing the number of rational surfaces residing in the plasma is also seen to lower the rotation frequency needed to stabilize the RWM. By lowering the aspect ratio, and thereby increasing the toroidal coupling and the number of rational surfaces inside the plasma, the necessary rotation frequency can be greatly reduced. An optimized low-aspect-ratio equilibrium with 55% beta can be fully stabilized against the n = 1 pressure-driven kink with ωr/ωA = 0.005. (author). 8 refs, 4 figs, 1 tab