Density turbulence measurements by using fast sweep reflectometry in DIII-D

The fast RF frequency sweep reflectometry, with the state-of-art capability of electron density radial profile measurement, is able to determine the density turbulence radial profile by the analysis of reflectometer phase fluctuations¹. In DIII-D, the ordinary mode launch/receive phase fluctuations have been used to derive the radial profile of n_e fluctuations analytically. The calculated radial wavenumber range of the turbulence is from ~1 cm^-1 to ~10 cm^-1, which is limited by the Bragg scattering condition. Initial turbulence measurement results by this technique in various DIII-D plasma conditions, such as L-H mode transition and the wide pedestal QH mode to standard QH-mode transition, are obtained. In a typical L-mode plasma the relative fluctuation level (δn/n) varies from ~1% in the core (rho < 0.6) to more than 10% in the edge. A significant reduction in δn/n is observed when H-mode occurs. The comparison with other turbulence diagnostics, e.g., DBS and BES, has shown reasonable qualitative agreement (e.g., with BES showing ~4% and reflectometry showing ~5% at rho=0.9 for a QH mode plasma). The analysis of turbulence fluctuations via extraordinary mode reflectometer phase fluctuations is comparatively more challenging and will also be presented.

[1] Clairet F., et al, Nucl. Fusion 56 (2016) 12619