Abstract
Two synthetic diagnostics are implemented for the high-k scattering system in NSTX (Smith et al 2008 Rev. Sci. Instrum. 79 123501) allowing direct comparisons between the synthetic and experimentally detected frequency and wavenumber spectra of electron-scale turbulence fluctuations. Synthetic diagnostics are formulated in real-space and in wavenumber space, and are deployed in realistic electron-scale simulations carried out with the GYRO code (Candy and Waltz 2003 J. Comput. Phys. 186 545). A highly unstable electron temperature gradient (ETG) mode regime in a modest-β NSTX NBI-heated H-mode discharge is chosen for the analysis. Mapping the measured wavenumbers to field aligned coordinates shows that the high-k system is sensitive to fluctuations that are closer to the spectral peak in the density fluctuation wavenumber spectrum (streamers) than originally predicted. The analyses of synthetic spectra show that the frequency response of the detected fluctuations is dominated by Doppler shift and is insensitive to the turbulence drive. The shape of the high-k density fluctuation wavenumber spectrum is sensitive to the ETG turbulence drive conditions, and can be reproduced in a sensitivity scan of the most pertinent turbulent drive terms in the simulation.
Highlights
We have presented a formulation of two synthetic diagnostics for coherent scattering of microwaves and applied it to the particular case of the high-k scattering diagnostic in NSTX [26]
This work has built on previous synthetic diagnostic efforts of high-k scattering [42] and Doppler backscattering (DBS) [45], which were based on the standard interpretation of scattering in k-space
We have shown the equivalence of the formulation in k-space to a formulation in real space
Summary
In this article we make direct comparisons of density fluctuation spectra between experimental turbulence measurements by high-k scattering and non-linear gyrokinetic simulations, which are part of an extensive validation study of electron thermal transport in NSTX [7]. The main outcome of this work is the successful validation of electron-scale gyrokinetic simulations in the core-gradient region of a modest-β NSTX H-mode plasma via direct comparison with measured high-k density fluctuation spectra. Coherent scattering from turbulence fluctuations inherently takes place in a confined region known as the scattering volume Vs, which is generally delimited by the size of the electromagnetic wave beam input in the plasma and by the magnetic field geometry This leads one to interpret the scattering process as the integration of fluctuations in real space within the scattering volume. Full information about the detected turbulence wavenumber ⃗k+ and the spectral width ∆⃗k3 is preserved in the computation of δnu(⃗k+, t) according to both formulations, and motivates their implementation for realistic tokamak scattering experiments
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