2D properties of core turbulence on DIII-D and comparison to gyrokinetic simulations

Abstract

Quantitative 2D characteristics of localized density fluctuations are presented over the range of 0.3<r/a<0.9 in L-mode plasmas on DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. Broadband density fluctuations increase in amplitude from ñ/n<0.5% in the deep core to ñ/n∼2.5% near the outer region. The observed Doppler-shift due to the E×B velocity matches well with the measured turbulence group and phase velocities (in toroidally rotating neutral beam heated plasmas). Turbulence decorrelation rates are found to be ∼200 kHz at the edge and to decrease toward the core (0.45<r/a<0.9) where they approach the E×B shearing rate (∼50 kHz). Radial and poloidal correlation lengths are found to scale with the ion gyroradius and exhibit an asymmetric poloidally elongated eddy structure. The ensemble-averaged turbulent eddy structure changes its tilt with respect to the radial-poloidal coordinates in the core, consistent with an E×B shear mechanism. The 2D spatial correlation and wavenumber spectra [S(kr,kθ)] are presented and compared to nonlinear flux-tube GYRO simulations at two radii, r/a=0.5 and r/a=0.75, showing reasonable overall agreement, but the GYRO spectrum exhibits a peak at finite kr for r/a=0.75 that is not observed experimentally; E×B shear may cause this discrepancy.