Alfvén eigenmodes suppress microturbulence in DIII-D plasmas

Compelling evidence that microturbulence decreases in response to an increase in Alfvén eigenmode (AE) activity has been obtained in neutral-beam heated DIII-D discharges, raising the possibility that fast-ion driven AE activity can improve performance (as well as degrade it). The effect is observed in many discharges, with up to~50% reductions in turbulence for a single toroidal AE and nearly complete suppression in plasmas with multiple AEs that have identical toroidal mode numbers. Reductions in turbulence with increased AE activity occur on both short (~1 ms) and long (~100 ms) timescales. When the turbulence is locally suppressed, the electron and ion temperature increase ~30%. Beam emission spectroscopy (BES) and Doppler backscattering (DBS) diagnostics simultaneously measure the AEs and turbulence. In both instruments, increases in shear flow during AE activity correlate with reductions in density fluctuations n ̃. A pair of DBS systems confirms that shear flows with an even toroidal mode number and with radial mode numbers that are intermediate between the ion gyroradius and the density scale length correlate with reductions in n ̃. BES velocimetry shows that turbulent eddies are tilted and sheared during the turbulence mitigation phase. It is also found that the enhanced flow shear is correlated with the increase of local Reynolds stress force in the electron diamagnetic direction. The exact mechanism of AE-generated shear flows is still under investigation but increases in AE mode polarization suggest that an increased electrostatic potential may play a critical role in breaking the pure Alfvénic state that occurs in the ideal MHD framework. Because they ultimately determine alpha and thermal transport, understanding the interactions that determine AE and turbulence amplitudes is essential for accurate predictions of reactor performance.