Radial Correlation and Propagation of Turbulence on DIII-D

Turbulent density fluctuations on the DIII-D tokamak measured by Doppler back-scattering (DBS) [1] show strong correlation (>0.7) across radially separated channels. Previous studies of DBS signals on other machines have shown that measurements made at nearby locations can lead to calculations of turbulent eddy tilt angles based on cross correlation techniques [2]. This compares the ExB velocity at the measurement locations to the time lag between data channels. The cross correlations of fluctuation amplitude data from DIII-D, however, show time lags between channels that do not always correspond to eddy tilts: showing both high correlation where the time lags would lead to eddies with strong shearing angles (>45 degrees) as well as strong correlation with time lags across large radial extents (>10 cm). These correlated signals suggest that there is some radial propagation of turbulent structures inside the plasma similar to those seen in scrape off layer filaments. These correlations are detected in a variety of L- and H-mode plasma scenarios, and at multiple probed wavenumbers (0.15<k_θ ρ_s<5) of the plasma turbulence. These turbulence amplitude measurements also show some level of intermittency via skewness and kurtosis, which may be a result of radial propagation.

[1] W A Peebles et al 2010 Rev Sci Instrum 81, 10D902

[2] J R Pinzon et al Plasma Phys. Control. Fusion 61, 105009