Evaluation of a new DIII-D Doppler backscattering system for higher wavenumber measurement and signal enhancement

Abstract

The high density fluctuation poloidal wavenumber, kθ ( kθ > 8 cm−1, kθρs > 5, ρs is the ion gyro radius using the ion sound velocity), measurement capability of a new Doppler backscattering (DBS) system at the DIII-D tokamak has been experimentally evaluated. In DBS, wavenumber ( k) matching becomes more important at higher wavenumbers, owing to the exponential dependence of the measured signal loss factor on wave vector mismatch. Wave vector matching allows for the Bragg scattering condition to be satisfied, which minimizes the signal loss at higher k’s. In the previous DBS system, without toroidal wave vector matching, the measured DBS signal-to-noise ratio at higher kθ (>8 cm−1) is substantially reduced, making it difficult to measure higher kθ turbulence. The new DBS system has been optimized to access higher wavenumber, kθ [Formula: see text] 20 cm−1, density turbulence measurement. The optimization hardware addresses fluctuation wave vector matching using toroidal steering of the launch mirror to produce a backscattered signal with improved intensity. The probe’s sensitivity to high- k density fluctuations has been increased by approximately an order of magnitude compared to the old system that has been in use at DIII-D. Note that typical measurement locations are above or below the tokamak midplane on the low field side with normalized radial ranges of 0.5–1.0. The new DBS probe system with the toroidal matching of fluctuation wave vectors is thought to be critical to understanding high- k turbulent transport in fusion-relevant research at DIII-D.

Beam model of Doppler backscattering

“…where the dependence of dτ /dτ on our choice of H is given in equation (45). We use Cardano's solution in equations (B.6)-(B.8) in post-processing to determine g ant and g in equation (33) consistent with our definition of H.…”

“…It has been reported [30,33] that for high K, the backscattered signal's amplitude is especially sensitive to the toroidal steering angle. This can be explained by mismatch.…”

“…The misalignment can be empirically optimised with 2D beam steering [30,33]. However, such empirical optimisation is ungainly and expensive, requiring several repeated shots for every measurement.…”

Density wavenumber spectrum measurements, synthetic diagnostic development, and tests of quasilinear turbulence modeling in the core of electron-heated DIII-D H-mode plasmas

“…Furthermore, the beam-model includes diagnostic effects such as wavevector 'mismatch' where poor alignment with the binormal direction attenuates the scattered signal. This effect was studied on DIII-D and validated against the beam model in [13,14].…”

Impact of the turbulence wavenumber spectrum and probing beam geometry on Doppler reflectometry perpendicular velocity measurements

“…Due to the high complexity of the DR diagnostic, the measurements can be subject to systematic diagnostic effects, e.g. toroidal mismatch angle attenuation [15,16]. Previous studies addressing the diagnostic impacts on the perpendicular velocity measurements with 2D full-wave analysis are shown in the references [17][18][19].…”