Space structure of the EGAM modes in TCV, experimental observations and modeling

The complex spatial structure of EGAMs has been recently observed experimentally in the TCV tokamak [M.B. Dreval et al., 2025 Nucl. Fusion 65 016037]. In the present work, we model the spatial structure of EGAM modes in the TCV equilibrium using a realistic fast ion distribution function, implemented in the gyrokinetic particle-in-cell code ORB5. For a peaked radial profile of fast ions, the EGAM spatial structure coincides with that of the conventional GAM, characterized by an n/m=0/0 plasma potential perturbation and an m=1 standing density perturbation, where the amplitude of the density oscillations is proportional to the sine of the poloidal angle. In contrast, for a hollow fast-ion radial profile and a marginally stable mode, our simulations reproduce a complex EGAM spatial structure similar to that observed experimentally in TCV, with two coexisting density fluctuation frequencies at different radial locations.