Unified approach to ballooning and trapped electron modes in tokamaks

A unified dispersion relation for the curvature drift driven MHD ballooning instabilities and the trapped electron drift instabilities is derived from the drift kinetic equation in toroidal geometry. The two branches of the dispersion relation, the shear Alfven (MHD) and the electron drift (trapped electrons), are coupled by curvature drifts as well as by finite ion gyroradius effects. Kinetic effects on the MHD ballooning modes, such as finite gyroradius corrections and trapped electron corrections, are found to weakly affect the growth rate and the critical beta for onset of instability, but greatly modify the frequency and radial localization properties. Curvature drift effects on the trapped electron instability are found to be weakly destabilizing. 9 references, 1 figure