Mitigation of electrostatic micro-instabilities by maximum-J effects in Chinese First Quasi-axisymmetric Stellarator (CFQS)

CFQS is the Chinese First Quasi-axisymmetric Stellarator which is characterized with very low neoclassical transport, good fast-ion confinement and ideal magnetohydrodynamic (MHD) stability. In this work, we investigated systematically the suppression of electrostatic trapped electron modes (TEMs) via external controlling the maximum-J (second adiabatic invariant) condition in CFQS for the first time. It is found that the controllability of the maximum-J region can be realized by altering normalized plasma pressure β and adjusting the currents in poloidal field (PF) coils. The simulation results reveal that: (a) As β increases, the maximum-J region in the core expands, leading to a significant reduction in the growth rate of TEMs. Meanwhile, ion-temperature-gradient (ITG) modes do not show significant changes. (b) When the PF-coil current (I_PF) is applied in the opposite direction to the magnetic field, the core maximum-J region also extends, resulting in considerable suppression of TEM instabilities, and the behaviour of the ITG modes becomes slightly weak. This work provides insight into the mitigation of micro-instabilities in quasi-axisymmetric stellarators and provides two feasible methods to decrease micro-instabilities in future CFQS experiments.