The recent revival of interest in the application of the `ballooning formalism` to low-frequency plasma instabilities has prompted a comparison of the Wentzel-Brillouin-Kramers (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 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. (author) 3 figs., 24 refs.
Cooper, W A;  Singleton, D B;  Dewar, R L 
- Ecole Polytechnique Federale, Lausanne (Switzerland). Centre de Recherche en Physique des Plasma (CRPP)
- Australian National Univ., ANU Supercomputing Facility, Canberra (Australia)
- Australian National Univ., Canberra, ACT (Australia). Research School of Physical Sciences