Minimum-q induced alternation between infernal modes and EP-driven modes in advanced tokamak configurations

Abstract

For an advanced tokamak configuration in the presence of energetic particles (EPs), the dominant instability is found to alternate between infernal modes and Alfvén eigenmodes with the variation of the minimum safety factor $q_{\min}$. For relatively high $q_{\min}$, the mode is identified as a reversed-shear Alfvén eigenmode (RSAE), characterized by its finite Alfvénic frequency and radial localization near the minimum of safety-factor profile. As $q_{\min}$ is further reduced, the dominant branch sequentially transitions through a low-frequency infernal-mode interval, then an energetic-particle-mode (EPM) regime, and finally another low-frequency infernal-mode interval. Increasing the EP beta fraction $β_h$ tends to destabilize the RSAE and EPM branches but to stabilize the infernal modes. Phase-space diagnostics further indicate that the destabilizing effects of EPs on the RSAE and EPM branches are mainly associated with trapped-particle drive, whereas the stabilizing effects of EPs on the infernal modes is dominated by passing particles.