Stabilization of trapped-electron shear-Alfvén instabilities by temperature gradient

Localized shear-Alfvén modes with large m numbers are shown, numerically, to be strongly damped by the collisionless electron response in the presence of a temperature gradient. The trapped-electron drift-tearing instability is stabilized by this effect in a tokamak unless the inverse aspect ratio exceeds a critical value, typically between 0.1 and 0.2. Analytical models demonstrate the scaling of these results with plasma parameters.