Microturbulence suppression by Alfvén eigenmodes via increased Reynolds stress force and shear flow in DIII-D plasmas

Experimental evidence demonstrates the suppression of low-k turbulence during the nonlinear evolution of toroidal Alfvén eigenmodes (TAE). Low-k density fluctuations, peaking at ~0.8 cm^-1, along with flow dynamics are measured with 2D Beam Emission Spectroscopy (BES). Turbulence is largely reduced in the presence of a single TAE, and in some cases fully suppressed when TAEs are strongly excited. This suppression is accompanied by a remarkable improvement of plasma confinement, evidenced by a >50% increase in electron temperature. Velocimetry measurements from BES show an increased Reynolds stress force during turbulence suppression, which drives an acceleration of turbulence poloidal flow in the electron diamagnetic drift direction. This flow is localized in a narrow radial region of ρ~0.65 – 0.72. This results in an increased flow shear with the shearing rate (reaching up to 160 kHz), exceeding the turbulence decorrelation rate (100 kHz – 140 kHz). BES imaging also shows more tilted eddy structures during suppression, consistent with enhanced shear flow. Theory predicts that zonal flows are generated when the polarization of TAE shifts from pure shear Alfvén waves to acoustic polarization, a process that aligns with these experimental observations. These results demonstrate that AE-driven zonal flows play a significant role in reducing and/or suppressing low-k turbulence.



Work supported by US DOE under DE-FG02-08ER54999, DOE DE-SC0020337, DE-FC02-04ER54698, DE-SC0019352, and DE-SC0020287