LAUSR

High poloidal beta scenarios with favorable plasma performance (β_p~2.0,H_98~1.3) have be demonstrated on Experimental Advanced Superconducting Tokamak (EAST) since the 2018 IAEA. Linear electromagnetic gyrokinetic simulations using GTC code are performed based on the experimental data of a typical high β_p discharge on EAST. In the core region of plasma, an electromagnetic instability with similar β_e dependence and mode structure with kinetic ballooning mode (KBM) but exhibits positive or nearly zero frequency is observed. This instability is referred as unconventional KBM (UKBM). Simulation results demonstrate that UKBM occurs in low ion-electron temperature ratio conditions and UKBM will transit to KBM when ion-electron temperature is high. Further investigation shows the driven mechanism of UKBM differs at different electron beta region. When the electron beta exceeds the threshold of electrostatic instability to UKBM, UKBM is driven by electron pressure gradient. When electron beta is close to the beta threshold of UKBM, the main driven mechanism is ηe. Reversed magnetic shear is found to have a significant stabilize effect on UKBM.