Measurement of high-k density fluctuation wavenumber spectrum in MAST and Doppler backscattering for spherical tokamaks

摘要

The high-k ($7 \lesssim k_{\bot} \rho_i \lesssim 11$) wavenumber spectrum ofdensity fluctuations has been measured for the first time in MAST [B. Lloyd etal, Nucl. Fusion 43, 1665 (2003)]. This was accomplished with the firstimplementation of Doppler backscattering (DBS) for core measurements in aspherical tokamak. DBS has become a well-established and versatile diagnostictechnique for the measurement of intermediate-k ($k_{\bot} \rho_i \sim 1$, andhigher) density fluctuations and flows in magnetically confined fusionexperiments. A novel implementation with 2D steering was necessary to enableDBS measurements in MAST, where the large magnetic field pitch angle presents achallenge. We report on the scattering considerations and ray tracingcalculations used to optimize the design and present data demonstratingmeasurement capabilities. Initial results confirm the applicability of thedesign and implementation approaches, showing the strong dependence ofscattering alignment on toroidal launch angle. We also present comparisons ofDBS plasma velocity measurements with charge exchange recombination and beamemission spectroscopy measurements, which show reasonable agreement over mostof the minor radius, but imply large poloidal flows approaching the magneticaxis in a discharge with an internal transport barrier. The 2D steering isshown to enable high-k measurements with DBS, at $k_{\bot}>20\\mathrm{cm}^{-1}$ ($k_{\bot} \rho_i>10$) for launch frequencies less than 75GHz; this capability is used to measure the wavenumber spectrum of turbulenceand we find $|n(k_{\bot})|^2 \propto k_{\bot}^{- 4.7 \pm 0.2}$ for $k_{\bot}\rho_i \approx 7-11$, which is similar to the expectation for the turbulentkinetic cascade of $|n(k_{\bot})|^2 \propto k_{\bot}^{- 13/3}$.