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

The first systematic investigation of core electron thermal transport and the role of local ion temperature gradient/trapped electron mode/electron temperature gradient (ITG/TEM/ETG)-scale core turbulence is performed in high temperature, low collisionality DIII-D H-mode plasmas. Taking advantage of the unique set of DIII-D turbulence and profile diagnostics, experimentally determined L-mode and H-mode turbulence wavenumber spectra are directly contrasted for the first time with nonlinear gyrokinetic simulation results. Core ITG/TEM-scale turbulence is substantially reduced/suppressed by E×B shear promptly after the LH transition, resulting in reduced electron thermal transport across the entire minor radius. For small kθρs, both experiment and nonlinear gyrokinetic (GYRO) simulations show density fluctuation levels increasing with kθρs in H-mode (r/a=0.6), in contrast to ITG/TEM-dominated L-mode plasmas. GYRO simulations also indicate that a significant portion of the remaining H-mode electron heat flux results directly from residual short-scale TEM/ETG turbulence. These studies are performed at ITER-relevant collisonality ( € νe * ~ 0.05, r/a≤0.6) and address transport in electron heat-dominated regimes, thought to be important in ITER due to α-particle heating.