MHD-kinetic hybrid modeling of low-frequency bursting modes in DIII-D high-beta high-q_min scenario

Low frequency bursting modes with frequency chirping are often observed in high-beta, beam heated DIII-D plasmas, with plasma pressure being close or even slightly exceeding the no-wall Troyon limit for the onset of n=1 ideal external kink instability. Despite the high central safety factor (qmin~1.5), bursting modes appear with many features similar to classical (1,1) fishbones. The difference is that the bursting mode often peaks near the q=2 surface. These off-axis fishbone-like modes and the tearing modes induced by them often prevent sustainment of the high-performance phase. Frequency chirping and a sudden drop in neutron rate suggest a strong interaction between the mode and fast ions. MHD-kinetic hybrid codes MARS-K and MEGA are utilized to investigate the mode stability and associated fast-ion transport. Initial analysis suggests that the bursting mode is an energetic-particle driven branch of the external ideal kink/resistive wall mode. The mode is excited by the presence of a sufficiently large fraction of trapped fast ions, with the mode frequency linked to the precessional drift frequency. The effect of the resistive wall, plasma toroidal rotation, anisotropy of the fast-ion distribution, and drift kinetic damping associated with thermal particles will be discussed.