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It is shown that an ion temperature gradient (ηi) induces a collisionless ballooning mode in the MHD (magnetohydrodynamic) second stability regime. Both two-fluid and fully kinetic analyses predict the instability, in qualitative agreement, indicating that the ion magnetic drift resonance plays a key role in destabilization. The instability is characterized by broad eigenfunctions in the ballooning space and at a small magnetic shear has a growth rate comparable with that of the MHD mode. Trapped electrons, the ion transit effect, and magnetosonic perturbation have stabilizing influences, but are unable to suppress the mode.

Stability analysis of tokamaks based on a fully kinetic, electromagnetic integral equation code indicates the existence of a temperature gradient driven instability in the short wavelength regime (k⊥ρi)2>1. The mode propagates in the ion diamagnetic direction (ion mode with ωr<0) and requires that both ηi and ηe exceed thresholds. Circulating (untrapped) electrons are not adiabatic and parallel electron dynamics provides destabilization. Trapped electrons are not essential for the mode. Toroidicity has a stabilizing influence and the growth rate does not vanish in the slab limit. The growth rate is approximately proportional to |s| where s is the magnetic shear parameter. The mode is subject to finite α (ballooning parameter) stabilization as the conventional ηi mode in the long wavelength regime (k⊥ρi)2<1.

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The nature of the kinetic ballooning mode in the magnetohydrodynamic (MHD) second stability regime is clarified through the behaviour of the eigenfunction. It is found to be a continuation of the MHD ballooning mode (not of the second mode with a smaller growth rate which coexists with the MHD mode). The kinetic shooting code is also applied to the parameters of TFTR in which the kinetic ballooning mode has recently been observed. The toroidal mode number and the frequency for the fastest growing mode are found to be consistent with the experimental observations. Finite-beta stabilization of the electrostatic ion temperature gradient (ITG) mode and destabilization of the ITG-driven ballooning mode are demonstrated. Also studied is the mode stability in the negative shear region, where the MHD ballooning mode is known to be stable. The kinetic ballooning mode persists for s < 0 with a narrow stable window near null shear.