Direct and indirect cross-scale couplings in burning plasmas mediated by energetic particles

Energetic particle (EP) related physics, including fusion alpha particles, are expected to play crucial roles in burning plasma of future reactors. In particular, EPs, and the collective oscillations driven by EPs, act as mediators of cross-scale couplings among the rich spatiotemporal scales characterizing burning plasmas. In this work, taking the well-known toroidal Alfvén eigenmode (TAE) as example, it is shown that meso-scale TAE can interact with micro-scale drift wave turbulence (DW) via both direct and in-direct channels. Three examples of cross-scale couplings among TAEs and DWs are given, including, (1) TAE instabilities can be effectively reduced or even suppressed by ambient stationary DW due to stimulated absorption via direct-scatterings into short-wavelength electron Landau damped kinetic Alfvén waves (KAWs); (2) in the “reverse” process of DW scattering by ambient stationary TAEs with typical amplitude expected in tokamak experiment, direct nonlinear scatterings to short-wavelength KAWs have a negligible damping effect on DW due to the cancellation of stimulated absorption via the upper-sideband KAW and spontaneous emission via the lower-sideband KAW; and (3) TAE can indirectly regulate the DW intensity via the nonlinearly excited zonal structures, which, in the local limit, yield an unexpected weak destabilizing effect, contrary to usual speculations. These findings illuminate the richness and complexity of nonlinear plasma physics that underlying these processes, required for extrapolating to future reactor burning plasmas. Future works along this line are also addressed.