Validation of novel hybrid scale ETG simulations in NSTX via comparisons of simulated turbulence with a new high-k scattering synthetic diagnostic

Despite much research since the discovery of ETG (Electron Temperature Gradient) turbulence, open questions remain regarding the impact of ETG on the electron thermal transport observed in spherical tokamak plasmas. A rigorous, first-of-its-kind, validation study of ETG turbulence in NSTX H-modes has been performed, combining hybrid-scale nonlinear gyrokinetic simulations with a new synthetic diagnostic for high-k scattering. Novel hybrid-scale simulations (GYRO) capture both ion and electron scale modes, with minimum kt*r_s = 0.3 and maxima beyond the peak of the linear ETG growth rate. In conditions of low ETG drive, hybrid scale simulations can match the experimental heat flux (Qe) within uncertainty, but e-scale simulations underpredict Qe. In the strongly driven ETG case, ion scale modes are shown to be stabilized by ExB shear, and both hybrid and e-scale simulations underpredict Qe. To quantitatively compare the measured frequency and wavenumber spectra of high-k fluctuations, a new real-space implementation of a synthetic diagnostic for high-k scattering has been developed. Comparisons with measured turbulence reveal that when Qe is underpredicted in simulation, it is still possible to reproduce the frequency spectral shape (mean frequency and width). But the changes in the fluctuation levels between the two cases and the shape of the high-k wavenumber spectra disagree with experiment, suggesting for the first time that the measured frequency spectra is not a critical constrain on simulation. The comparison with heat fluxes also indicates that ion scale dynamics, and possibly cross-scale coupling, might be needed to account for electron thermal transport in these NSTX H-modes.