Long-radial-range turbulent transport events in high collisionality H-mode plasmas on DIII-D tokamak

We report on the observation of spatially asymmetric long-radial-range transport events in the plasma core when the mean shear layer is reduced, which may explain confinement degradation in high-collisionality H-mode plasmas. In this study, a dimensionless collisionality scan experiment was performed on the DIII-D tokamak, and the turbulent transport events are observed by Doppler backscattering. These events develop from sub-ion-gyroradius turbulence into radially elongated, streamer-like structures, and the intensity spans a wide radial scale in the mid-radius region. The underlying turbulence of these events shows clear intermittency, e.g., large skewness and kurtosis. The underlying turbulence also features a Hurst exponent between 0.7 and 0.8, indicating a long-term memory effect. The wavenumber power spectrum obeys a scale-invariant 1/k power law, which resembles avalanches in self-organized criticality. The amplitude and the radial scale of these transport events increase substantially when the shearing rate of the mean flow is reduced below the turbulent scattering rate. These findings constitute the first experimental observation of long-radial-range turbulent transport events in high-collisionality H-mode fusion plasmas and demonstrate the role of mean shear flows in the formation and propagation of turbulence with long-radial-range correlation. Linear CGYRO simulations suggest that the underlying turbulence is likely driven by the electron temperature gradient. The power balance analysis shows that the core thermal diffusivity increases substantially as collisionality is raised. Such core turbulent transport events may serve as a candidate explanation for the degradation of normalized energy confinement time at high collisionality. These findings are of significance to future fusion power plants which would operate with high density and weak flow shear.