Hybrid simulations of fishbone instabilities and Alfvén eigenmodes in DIII-D tokamak.

In DIII-D hybrid discharges, the intense Alfvén eigenmode (AE) activity driven by Neutral Beam Injection that is typically observed can be suppressed and replaced by fishbone modes when Electron Cyclotron Current Drive (ECCD) is centrally applied. Simulations have been carried out with the kinetic-magnetohydrodynamic hybrid code M3D-K based on DIII-D discharges #161401 without ECCD and #161403 with ECCD, respectively. In both cases, unstable modes are found—the mode frequency and the mode structure indicate that the instability excited in #161403 is of fishbone type, while that in #161401 is identified as the beta-induced Alfvén eigenmode-like mode. Moreover, we find that the calculated mode frequencies of these two shots are consistent with experimental observations. A systematic scan has been performed to study the instability region of n = 1 , 2 , 3 modes in (q 0 , β h o t) parameter space, where n is the toroidal mode number, q 0 is the safety factor value at the magnetic axis, and β h o t is the energetic particle beta. It is found that the transition between AEs and fishbone modes can occur when q 0 is changed. In addition, the modes of n = 1 , 2 , 3 are stable or weakly unstable in the region of P h o t / P total ≤ 0.5 and 1.2 < q 0 < 1.3 , where P h o t is the central energetic particle pressure and P total is the central total pressure. These results provide useful guidance for future experiments for minimizing energetic particle-driven instabilities and associated transport. [ABSTRACT FROM AUTHOR]