Parallel ion compressibility effects on kinetic ballooning mode for different magnetic shears

Various gyrokinetic simulations suggest that the kinetic ballooning mode (KBM) instability is sensitive to the numerical implementation of equilibrium magnetic configuration in tokamaks. In this work, the gyrokinetic code GTC is employed to investigate the KBM's sensitivity to equilibrium plasma profiles. An outward radial shift of the radial mode is found for the normal magnetic shear case, but there is no shift if the shear is negative. The simulation results are explained by a linear eigenmode theory. It is found that the observed phenomenon is an effect of the parallel ion compressibility.