Internal kink stability of large aspect ratio high-beta tokamaks

Theoretically, high-beta (beta similar to 1) tokamaks offer a large fusion power efficiency advantage over low-beta devices. However, if high-beta tok amaks are inherently unstable then such devices will never be realized. In particular, kink modes are thought to be the most serious obstacle to high- beta operations. High-beta tokamaks are characterized by a very large Shafr anov shift with a thin "boundary layer" on the outboard side of the device and a large "core" region of vertical flux surfaces comprising most of the central volume. In this paper, the energy principle is used to compute the magnetohydrodynamic internal kink stability of such devices in the large as pect ratio limit with a low toroidal mode number. A class of internal kink mode similar to the usual low-beta kink is present; the stability against t hese modes is computed. A set of parameters describing a kink stable high-b eta equilibrium is given. Stability is shown to be dependent on the shape o f the plasma boundary. (C) 2000 American Institute of Physics. [S1070-664X( 00)00510-3].