Global Gyrokinetic Simulation of Electron Temperature Gradient Turbulence and Transport in NSTX Plasmas

Global, nonlinear gyrokinetic simulations of electron temperature gradient (ETG) driven turbulence were carried out with the GTS code using actual experimental parameters of NSTX discharges. Our simulations reveal remarkable new features with regard to nonlinear spectral dynamics in 2D perpendicular wavenumber space. Specifically, there exists direct, strong energy coupling between high-k ETG modes and electron geodesic acoustic modes (e-GAMs with high frequency and poloidal mode number m = 1). At the same time, zonal flows are generated and continuously grow with a fine radial scale. This direct energy coupling may represent a new insight into the underlying mechanism for nonlinear ETG saturation. It also implies that the collisional damping of zonal flows and e-GAMs may have considerable impact on the formation of the steady state spectrum and saturation level. Further, the ETG fluctuation spectra are characterized by strong anisotropy with kr ≪ kθ . The k⊥ spectrum of density fluctuations is in general agreement with the experimental measurement using coherent scattering of electromagnetic waves. Within experimental uncertainties in plasma profiles, ETG turbulence is shown to drive experimentally relevant transport for electron heat in NSTX.