Spectrum of ballooning instabilities in a stellarator

The recent revival of interest in the application of the ''ballooning formaiism'' to low-frequency plasma instabilities has prompted a comparison of the Wentzel-Kramers-Brillouin (WKB) ballooning approximation with an (in principle) exact normal mode calculation for a three-dimensional plasma equilibrium. Semiclassical quantization, using the ideal magnetohydrodynamic (MHD) ballooning eigenvalue to provide a local dispersion relation, is applied to a ten-field period stellarator test case. Excellent qualitative agreement, and good quantitative agreement is found with predictions from the TERPSICHORE code [D. V.Anderson et al., Int. J. Supercomput. Appl. 4, 34 (1990)] for toroidal mode numbers from 1 to 14 and radial mode numbers from 0 to 2. The continuum bands predicted from three-dimensional WKB theory are too narrow to resolve. (C) 1996 American Institute of Physics.