Linear gyrokinetic simulation of kinetic infernal mode

Kinetic infernal mode (KIM) is an electromagnetic instability driven by thermal ions in the weak magnetic shear region with a frequency similar to the kinetic ballooning mode (KBM). We present herein the global gyrokinetic simulations of KIM by using the Gyrokinetic Toroidal Code (GTC). It is found that the electromagnetic instability shows a smooth transition from KBM to KIM in both frequency and growth rate when magnetic shear varies from strong to weak, suggesting that KIM and KBM may belong to the same physical mode but exhibit different mode structures due to the influence of magnetic shear. The variation in the spacing of adjacent mode rational surfaces, as revealed by mode structure analysis, is identified as the key mechanism governing the mode transition. Moreover, the magnetic shear and driving source effects are investigated in detail. The simulation results show that KIM prefers to grow on the mode rational surface nearest to the minimum magnetic shear, i.e. where the shear stabilizing effect is weakest, rather than at the location of maximum density gradient or temperature gradient. However, the magnitude of the growth rate is determined by both the magnetic shear and pressure gradient.