Radial Propagation of Turbulence Signals in DIII-D

Turbulent density fluctuations measured via Doppler back-scattering (DBS) on the DIII-D tokamak show strong signs of radial propagation of RMS signals across the plasma. The DBS system on DIII-D [1] uses injected microwave beams at 8 frequencies (55 - 75 GHz) to simultaneously probe wavenumber-resolved fluctuations in plasma density near the cutoff layers for each frequency. Measurements in the plasma edge can see overlapping signals, leading to strong correlation between channels for turbulent modes with radial extent; however, core measurements are radially separated by several centimeters, and correlations between channels have a time lag, suggesting the turbulent modes are propagating. Poloidal propagation of sheared eddies can lead to radial propagation of RMS signals. Comparisons of RMS signals over a range of perpendicular wavenumbers in L-mode plasmas are made in both quiescent plasmas and those with MHD activity. Vertically displaced plasmas allow for comparing radial drift of turbulent modes with the DBS diagnostic beam aimed at various degrees of tangency to the plasma flux surfaces. A comparison to data from H-mode discharges is also included.

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