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Toroidal rotation of the plasma coupled with resistive walls can fully stabilize pressure-driven kink modes and allow the beta value to be extended. 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 the RWM can be stabilized at a lower rotation frequency. This effect is greatly enhanced by gaps near the outboard midplane, albeit at the cost of requiring a closer wall to stabilize the ideal plasma mode. But this trade-off can be quite desirable: in this paper it is 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 to a fully surrounding wall while maintaining ideal stability. Increasing the number of rational surfaces residing in the plasma is also seen to lower the necessary 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.

Additional information

Authors: WARD D J, École Polytechnique Fédérale de Lausanne, Centre de Recherches en Physique des Plasmas, Association EURATOM-Confédération Suisse (CH)
Bibliographic Reference: Article: Proceedings of the 16th IAEA Fusion Energy Conference, Montréal (CA), October 7-11, 1996
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