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

A rigorous validation study of ETG turbulence in an NSTX H-mode using nonlinear gyrokinetic simulation and a new high-k scattering synthetic diagnostic is presented, suggesting that electron scale turbulence (e- scale) can account for experimental heat fluxes (Qe) in highly unstable ETG regimes, but cross-scale interactions might be necessary to match Qe in marginally stable turbulence regimes. At high ETG drive, ion scale modes are stabilized by ExB shear and e- scale simulations (k_θρ_s = [1.5, >40]) can marginally match Qe. Hybrid scale simulations capturing both ion and e- scale modes (k_θρ_s = [0.3, >40]) underpredict Qe. At low ETG drive, hybrid scale simulations can match the experimental Qe within uncertainty (dominant transport is from e- scales with non-zero transport from ion scales), but e- scale simulations underpredict Qe. Quantitative comparisons of the experimental high-k fluctuation spectra with a new high-k scattering synthetic diagnostic reveal that the measured frequency spectrum is not a critical constraint on simulation. The measured wavenumber spectral shape can be matched at low ETG drive (consistent with the agreement in Qe), but not at high ETG drive.