Reduction of TEM/ETG-scale Density Fluctuations in the Core and Edge of H-mode DIII-D Plasmas

Improved confinement during H-mode has been linked to $E\times B$ shear suppression of large-scale ($k_\theta \rho_s \leq 0.3$) turbulence within an edge transport barrier. While larger scale eddies are preferentially suppressed by increased shear flow in this paradigm, the effects on smaller scale (TEM/ETG-scale) turbulence are less certain. Recent results from DIII-D provide the first experimental evidence that intermediate-scale turbulence ($1 < k_\theta \rho_s \leq 3$) together with larger-scale electron temperature fluctuations [1] are also reduced promptly at the L-H transition. These reductions are not confined to the edge region. Intermediate-scale density fluctuations obtained via Doppler backscattering, are significantly reduced (30\%-50\%) over a range of normalized radii ($0.5 \leq r/a \leq 0.85$) within a few ms of the L-H transition. A larger reduction ($\geq$75\%) is observed at the top of the pedestal ($r/a \sim 0.9$) within 0.2~ms. In addition, low-k electron temperature fluctuations ($k_\theta\rho_s \leq 0.3$, from correlation ECE) are strongly reduced ($>$75\%) at the L-H mode transition and during QH-mode ($r/a \sim 0.7$). Gyrokinetic simulation results [2] predict that $\tilde T_e$ fluctuations contribute significantly to L-mode electron heat transport, hence, the observed reduction is likely an important factor in the observed improved H-mode electron heat confinement ($\chi_e^{QH}/\chi_3^L < 0.25$). Doppler backscattering is also utilized to probe time-dependent shear flows (i.e. zonal flows). The results clearly indicate that zonal flow levels are anti-correlated with the amplitude of intermediate-scale density turbulence in L-mode, suggesting that zonal flows play an important role in turbulence/transport regulation.\par \vskip3pt \noindent [1] L.~Schmitz et al., Phys.\ Rev.\ Lett.\ {\bf 100}, 035002 (2008).\hfil\break [2] A.E.\ White et al., Phys.\ Plasmas {\bf 15}, 056116 (2008).\par