Speaker
Description
We develop a full-orbit delta-f energetic-particle model in the hybrid kinetic-MHD code MEGA and apply it to nonlinear simulations of a beam-driven, sub-cyclotron global Alfven eigenmode (GAE) in a low-field DIII-D plasma. The equilibrium and beam parameters are based on the NSTX-similarity DIII-D discharge reported by Heidbrink et al. (Nucl. Fusion 46, 324, 2006), for which the observed sub-cyclotron modes were later identified as counter-propagating GAEs by linearized HYM simulations (Belova et.al. Nucl.Fusion 62,106016, 2022). By resolving the full gyromotion and particle trajectories, the present model retains finite-Larmor-radius effects and captures the Doppler-shifted cyclotron resonance beyond the guiding-center approximation.
The most unstable counter-propagating mode occurs at n = -16 with a frequency of 0.60 times the on-axis deuteron cyclotron frequency, in close agreement with the HYM result. A beam-density scan reproduces an instability threshold of a few percent. The mode exhibits shear-Alfven polarization in the core, low poloidal harmonics, and core localization, and its frequency lies just below the shear-Alfven continuum accumulation point, identifying it as a GAE. The mixed polarization observed toward the plasma edge is consistent with the compressional signature reported in the original DIII-D experiment. Wave-particle energy-transfer analysis identifies co-passing energetic ions as the driving population through the fundamental Doppler-shifted cyclotron resonance.
Nonlinear evolution reveals that resonant energetic particles are redistributed along resonance characteristics, analogous to the characteristic-constrained transport observed for conventional Alfven eigenmode-induced fast-ion redistribution. The redistribution relaxes the pitch-angle anisotropy responsible for the instability, resulting in local flattening of the resonant energetic-particle distribution at saturation. These results demonstrate that the full-orbit delta-f framework enables self-consistent investigation of cyclotron-resonant wave-particle interaction and nonlinear saturation of sub-cyclotron Alfvenic instabilities.