Destabilization of geodesic acoustic-like modes in the presence of poloidally asymmetric sources and energetic particles with finite orbit width in tokamak plasmas

This study demonstrates two destabilization mechanisms of geodesic acoustic-like modes. First, the effects of a poloidally inhomogeneous heat source are investigated. A new gyrokinetic dispersion relation is derived, assuming equilibrium heat flows are generated to remove the injected poloidally nonuniform heat source. The dispersion relation is numerically solved for both outboard and inboard sources. Inboard injection excites both Stringer spin-up (SSU) and geodesic acoustic mode (GAM). Conversely, outboard injection leads to the emergence of a new GAM-like mode, named the Q-GAM (with Q representing heat source), whose frequency range lies around half that of the standard GAM. Second, energetic particle-driven GAMs (EGAMs) are studied, considering finite orbit width (FOW) effects. A gyrokinetic dispersion relation is derived for a double-shifted Maxwellian distribution of energetic particles and compared with simulation results. Enhanced damping of EGAM is observed, matching well with simulation results when FOW is moderate. However, with increased FOW, another new unstable EGAM branch, named δEGAM (with δ representing FOW), appears. The δEGAM is found to be destabilized by higher-order transit resonance arising from FOW effects and has a higher frequency than the standard GAM.