Reduced Electron Thermal Transport in Low Collisionality H-mode Plasmas in DIII-D and the Importance of Small-Scale Turbulence

Full text: Understanding electron thermal transport in tokamaks is of crucial importance in next generation burning plasma experiments where α-particles produced by fusion reactions primarily heat the electrons. The first systematic investigation of core electron thermal transport and the role of local ITG/TEM/ETG-scale core turbulence is performed in high temperature, low collisionality DIII-D H-mode plasmas. Core ITG/TEM-scale turbulence is substantially reduced/suppressed by E x B shear promptly after the L-H transition. As a result, a substantial reduction of the electron heat diffusivity across the entire minor radius within 10 ms of the L-H transition is found from time-dependent transport analysis. Initial nonlinear gyrokinetic (GYRO) simulations indicate that a significant portion (> 50%) of the remaining H-mode electron heat flux results directly from residual short-scale TEM/ETG turbulence. The studies are performed at ITER-relevant collisionality (νe* ∼ 0.05, r/a ≤ 0.6) and are important since the ITER plasmas will be electron heat-dominated. Core turbulence wavenumber spectra, obtained via Doppler backscattering, indicate an exponential dependence of fluctuation levels on the normalized poloidal wavenumber kθρs in L-mode. Substantially reduced ITG/TEM fluctuation amplitudes are found in H-mode within the wavenumber range (0.4 ≤ kθρs ≤ 3) where shear stabilization is expected from a comparison of linear instability growth rates and the flux-surface-averaged shearing rate. Taking advantage of the unique set of DIII-D turbulence and profile diagnostics, experimentally determined H-mode core turbulence spectra and transport fluxes are directly compared for the first time with nonlinear gyrokinetic simulation results. Initial GYRO calculations indicate flattened H-mode fluctuation spectra in the ITG/TEM spectral range consistent with measured wavenumber spectra. Multi-scale GYRO simulations with improved low-k resolution are underway to allow quantitative comparisons. The results presented provide evidence that ITG-scale density/electron temperature fluctuations as well as intermediate-scale turbulence is significantly reduced in the core of high-performance H-mode plasmas, and that smaller-scale modes can play a substantial role in electron transport in these plasmas. Work supported by USDOE DE-FG03-08ER54984. (author)