Observation of large-scale coherent structures under strong e × B shear in the torsatron TJ-K

The influence of strongly sheared E × B flows on the spatio-temporal structure of turbulence has been investigated in a toroidally confined plasma. The experiments have been carried out on the low-temperature plasma of the TJ-K torsatron, where the flow shear was induced by core plasma biasing. The spatio-temporal structure of the turbulence has been measured in great detail with multi-probe arrays. It turned out that because of strong shear, the plasma undergoes a transition, where broadband density fluctuations are suppressed and the particle confinement is improved, but large-scale coherent structures become dominant. They have increased correlation lengths and a reduced lifetime. The large-scale structures could be associated with inward transport due to a modification in the cross-phase between density and potential fluctuations.