Quantitative comparisons of electron-scale turbulence measurements in NSTX via synthetic diagnostics for high- k scattering

Abstract

© 2020 The Author(s). Published by IOP Publishing Ltd. 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.