Variation of Turbulence and Transport with the $T_e/T_i$ Ratio in H-mode Plasmas*

Confinement and transport vary strongly with $T_e/T_i$. Recent experiments on DIII-D have sought to examine the physical mechanisms behind this dependence by systematically varying $T_e/T_i$ in L- and H-mode plasmas, while $T_i$, rotation, density and gradient scale lengths are held roughly constant. $T_e/T_i$ is increased by 25\% (achieving $T_e/T_i\leq 1$) in non-sawtoothing, long-pulse hybrid H-mode plasmas using 3.3~MW of ECH power, reducing $\tau_E$ by 30\%. The magnitude of low-k density fluctuations, measured with BES, is found to correspondingly increase by 30\%-50\% over the radial range $0.35 < r/a < 0.8$. This turbulence behavior contrasts with that observed in L-mode experiments, in which confinement is reduced at higher $T_e/T_i$, but little change is observed in the magnitude of low-k density turbulence. $S(k_r,k_\theta)$ spectra and related turbulence properties for L- and H-mode plasmas will be compared. Calculations of growth rates and predicted turbulence levels will be performed.