Global kinetic-MHD simulations of downward-sweeping reversed shear Alfvén eigenmodes in tokamak plasmas

Abstract

Hybrid simulations are performed to investigate the dependence of downward-sweeping reversed shear Alfvén eigenmodes (RSAEs) on key parameters in tokamaks. The investigation mainly focuses on downsweeping RSAEs with toroidal number n = 3. It is found that the occurrence of downward-sweeping RSAEs strongly depends on the bulk plasma profiles, including the pressure profile and the safety factor profile. The influence of bulk plasma pressure value and pressure gradient at q min on downsweeping RSAEs is studied, respectively. It is shown that the excitation of downsweeping RSAEs is more associated with the plasma pressure gradient at q min than with the pressure value. The enhancement of the pressure gradient at q min improves the likelihood of the destabilization of downsweeping RSAEs. Moreover, the downsweeping RSAEs only occur in a weak magnetic shear configuration in our simulations. By increasing the magnetic shear strength of central plasmas, it is numerically verified that the downsweeping RSAE activities are suppressed and eventually replaced by toroidal Alfvén eigenmodes (TAEs). Following this process, the transition region from TAEs to RSAEs is shifted to a region with lower q min values. In this work, the downsweeping RSAE, for which it has been confirmed that the mode is in the nonperturbative regime, has a twisted mode structure in the poloidal plane, while the upsweeping RSAE, which is weakly affected by the energetic ion effects, has a nearly up–down symmetry mode structure. Further, downward-sweeping RSAEs, showing a relatively narrow mode spatial profile, are generally more stable than the upward-sweeping ones, which can be a reason why the downsweeping RSAEs are rare in experiments. Finally, it is found that the kinetic effect of the energetic ion central pitch angle does not affect the excitation of downward-sweeping RSAEs, but affects the mode stability.