Regulation of Alfven eigenmodes by microturbulence in fusion plasmas

Meso-scale Alfven eigenmodes (AE) and microturbulence in tokamak plasmas can strongly couple despite the scale separation. Energetic particle (EP) scattering by the microturbulence can affect phase space dynamics in nonlinear AE-EP interactions. Zonal flows can be generated/damped by, and in turn, suppress both the AE and microturbulence. AE and driftwave can have direct mode-mode coupling. Understanding these cross-scale interactions requires global integrated simulations incorporating multiple physical processes. In this work, reversed shear Alfven eigenmodes (RSAE) in the DIII-D tokamak is found to saturate through zonal flows in global GTC [1] simulations using gyrokinetic thermal and fast ions and drift kinetic electrons. The RSAE amplitude and EP transport are much higher than experimental levels at the nonlinear saturation, but quickly diminish to very low levels after the saturation if background microturbulence is artificially suppressed. In contrast, in simulations coupling micro-meso scales, the RSAE amplitude and EP transport decrease drastically at the initial saturation but later increases to the experimental levels in a quasi-steady state due to regulation by the thermal ion temperature gradient (ITG) microturbulence. The quasi-steady state EP transport is larger for a stronger microturbulence. The EP radial scattering by the microturbulence is much more effective than Coulomb collisions in destroying EP coherent structures to maintain the quasi-steady state. The RSAE amplitude from gyrokinetic simulations agree, for the first time, very well with the DIII-D ECE and BES measurements [2]. GTC simulations of AE and microturbulence interactions in ITER operational scenarios will also be presented.

Work supported by DOE SciDAC ISEP and INCITE.

[1] Z. Lin et al, Science 281, 1835 (1998).

[2] P. Liu et al, Phys. Rev. Lett. 128, 185001 (2022).