Strong toroidal effects on tokamak tearing mode stability in the hybrid and conventional scenarios

The hybrid scenario is thought to be an important mode of operation for the ITER tokamak. Analytic and numerical calculations demonstrate that toroidal effects at finite beta have a strong influence on tearing mode stability of hybrid modes. Indeed, they persist in the large aspect ratio limit, R/a -> infinity. A similar strong coupling effect is found between the m = 1, n = 1 harmonic and the m = 2, n = 1 harmonic if the minimum safety factor is less than unity. In both cases the tearing stability index, Delta' increases rapidly as beta approaches ideal marginal stability, providing a potential explanation for the onset of linearly unstable tearing modes. The numerical calculations have used an improved version of the T7 code (Fitzpatrick et al 1993 Nucl. Fusion 33 1533), and complete agreement is obtained with the analytic theory for this demanding test of the code.