Observation of reduced core electron temperature fluctuations and intermediate wavenumber density fluctuations in H-mode plasmas

In this paper, we report observations of reduced core electron temperature and intermediate-scale density fluctuations in H-mode. Electron temperature fluctuation levels are observed to decrease from L-mode levels ( for kθρs < 0.5 as measured by correlation electron cyclotron emission radiometry) by at least a factor of four in H- and quiescent H-mode regimes in the DIII-D tokamak (r/a = 0.7). Linear stability calculations (using the trapped gyro-Landau fluid (TGLF) code) indicate that the observed temperature fluctuations are associated with L-mode ITG turbulence which is shear-stabilized at the L- to H-mode transition. Recent results from DIII-D provide the first experimental evidence that, in addition, intermediate-scale turbulence (0.5 < kθρs ⩽ 3) is reduced at the L–H transition. A 30–40% prompt reduction (r/a ⩾ 0.7) has been found at the L–H transition in co-injected medium density plasmas, with a larger decrease (⩾75%) observed near the pedestal top. Experimental results and TGLF calculations indicate that intermediate/small scale turbulence persists in H-mode at a reduced amplitude (0.6 ⩽ r/a ⩽ 1) and may substantially contribute to the residual anomalous H-mode electron heat transport.