Abstract:
A dimensionless collisionality scan has been performed with detailed
measurements of intermediate-to-high wavenumber turbulence using Doppler
backscattering systems on the DIII-D tokamak. It is found that the shorter
wavelength turbulence develops into spatially asymmetric turbulent
structures with a long-radial-range correlation (LRRC), i.e., streamers, in
the mid-radius region of high-collisionality discharges. The LRRC transport
events are highly intermittent and show a power spectrum of 1/k, which is
usually associated with self-organized criticality. The magnitude and the
radial scale of those turbulent structures increase significantly when the
ExB mean flow shearing rate decreases below the turbulent scattering rate.
The enhanced LRRC transport events appear to be correlated with the
degraded energy confinement time. The emergence of such LRRC transport
events may serve as a candidate explanation for the degrading nature of
H-mode core plasma confinement at high collisionality.