Turbulent electrostatic fluctuations play a crucial role for the confinement quality of fusion plasmas, since they cause cross-field transport out of the confinement region. Turbulent particle transport relies on density perturbations advected in the velocity field created by potential perturbations. The amount of transport is sensitive to the spatial phase relation between density and potential fluctuations, which, in particular, is indicative for the turbulence driving instability. Strong ExB shear flows can act on turbulent transport by, e.g., reducing the radial size of turbulent structures through the shear decorrelation mechanism or by modifying the cross-phase.
Poloidal ExB shear flows can be excited spontaneously or driven externally. In TJ-K, plasma biasing is applied in order to investigate the influence of shear flows on turbulence in a controlled way. With strong flow shear, cross-phase modifications are found, which relate a high-frequency mode to local inward transport.
Due to the curvature of the magnetic field lines in toroidal geometry, perturbations should be stabilised on the high- and destabilised on the the low-field side. Measurements in TJ-K indeed show a maximum in the turbulent transport on the low-field side (see figure).
Head of Plasma Dynamics and Diagnostics / Library officer