Effect of precession drift motion of trapped thermal ions on ballooning modes in helical plasmas

The linear stability analysis of resistive ballooning modes in Large Helical Device plasmas has been carried out by numerical simulation based on the kinetic magnetohydrodynamic (MHD) model where thermal ions are simulated with the drift-kinetic model. Due to the curvature drift and the gradient B drift, the orbit of trapped ions not only deviates from the magnetic surface but also has the precession motion in the poloidal and toroidal directions. Even when the trapped ions remain well inside the radial profile of the MHD mode, the response of the trapped ions is weakened when the precession drift frequency of the trapped ions with respect to the mode phase is close to or larger than the linear growth rate of the instability. This results in the suppression of the ion perpendicular pressure perturbation to the magnetic field leading to the reduction of the linear growth rate.