STABILITY ANALYSIS OF ELECTROMAGNETIC INTERCHANGE MODES IN TOROIDAL GEOMETRY

An investigation is presented of how the stability of collisionless electromagnetic interchange modes depends on epsilon(n) (the ratio of the magnetic drift frequency to the diamagnetic drift frequency), the ion temperature gradient and the electron temperature gradient. A linear two fluid model in toroidal geometry is used. The eigenvalue problem is solved analytically and then the complex frequency is solved numerically from the dispersion relation. Comparison is made with Mercier's criterion, in the magnetohydrodynamic limit. The most important observed effects are: 1. When epsilon(n) increases Mercier's criterion becomes increasingly incorrect. The toroidal system becomes more stable than Mercier's criterion predicts. epsilon(n) is large in regions where we have hat density profiles, L(n) >> L(B) (the characteristic scale length of density and magnetic field inhomogeneities). 2. Finite eta(i) (L(n)/L(Ti)) may cause instability below the critical pressure gradient in the Mercier criterion.