Toroidal matching of DBS for higher wavenumber measurement and signal enhancement during turbulence measurement in DIII-D tokamak plasmas*

We use experiments to demonstrate the dependence of signal strength from a Doppler backscattering (DBS) probe on its toroidal launch angle alignment, and thus optimize it for higher wavenumber (k) density turbulence measurement. With fluctuation k matching by toroidal steering of a DBS launch mirror, a spectrum of higher signal intensity is obtained. This increases the probe’s sensitivity to high-k (∼20 cm^-1) fluctuations. For unmatched k, the measured DBS SNR is small, making it difficult to measure high-k turbulence. Since DBS has been used mainly for low to medium wavenumber density fluctuation and flow measurement in magnetically confined fusion plasmas, this extra toroidal matching of fluctuation k values will be critical to understanding high-k turbulent transport in fusion relevant research at DIII-D. The density fluctuation power is derived from the measured scattered wave power, and the Doppler shift of the scattered wave is used to determine the propagation velocity of turbulent structures. For the k matching, the 3-D ray tracing code GENRAY is used, which takes as input, parameters from other diagnostics such as profile reflectometry, Thomson scattering, and the EFIT plasma equilibrium code.