UC San Diego

We report the observation of a net inward, up-gradient turbulent particle flux which occurs when a collisional drift waves generate a sufficiently strong radially sheared azimuthal zonal flow in a cylindrical magnetized plasma. At low magnetic fields (B≤1.0 kG), particle transport is outward at all radii. As the magnetic field is further increased to 1200G, an up-gradient inward particle flux develops between the peak of the velocity shear and the maximum density gradient. The mean density gradient is also observed to steepen in response to this inward flux. Time-domain and bispectral Fourier domain analysis shows that at the peak of the velocity shear, where the particle flux is outward, the turbulent Reynolds stress acts to reinforce the shear flow. In contrast, in the region of the inward particle flux, the zonal flow drives the fluctuations, and a transient increase in the shearing rate is occurs prior to an increase in the magnitude of the inward flux. The results suggest a hypothesis in which the shear flow is responsible for the up-gradient particle flux and the corresponding steepening in the mean density gradient. However, a linear instability analyses using experimentally measured density and E×B flow profiles in a linear, modified Hasegawa-Wakatani theory model with the coupled potential and density fluctuations failed to reproduce the essential elements of our experimental observations, suggesting some other mechanism is responsible for the inward flux. We summarize recent new experimental results which point towards the possible role of finite ion temperature gradient effects, possibly combined with parallel flow shear, in driving up-gradient particle flux