Collisionless and resistive ballooning stability

It has been suggested [Kleva and Guzdar, Phys. Plasmas 6, 116 (1999)] that reconnecting ballooning modes in which electron inertia replaces resistivit y in a nonideal magnetohydrodynamic Ohm's law can have substantial growth r ates in the low collisionality regime. Numerical calculation, albeit necess arily at unrealistically large values of the collisionless skin depth, show ed that strongly growing ballooning modes exist at beta values which are be low the ideal beta limit. In order to investigate stability at more realist ic values of the skin depth we exploit an analytic approach. As in the case of resistive ballooning modes, we find that inertial ballooning modes are stabilized by favorable average curvature effects at moderate values of Del ta(B)('), the stability index for resistive ballooning. Instability only be comes possible close to the ideal stability boundary (Delta(B)(')-->infinit y) or at unrealistically large values of the toroidal mode number n (e.g., n greater than or similar to 10(2)). Another ballooning mode, the collision less analogue of the Carreras-Diamond mode [Carreras, Diamond, Murakami, Du nlap , Phys. Rev. Lett. 50, 503 (1983)] can also be excited at larger value s of the collisionless skin depth, but this mode is not valid for realistic parameters in a hot plasma. (C) 1999 American Institute of Physics. [S1070 -664X(99)01211-2].