Radial electric fields and improved confinement regimes in the TJ-II stellarator

The influence of plasma density and edge gradients in the development of perpendicular sheared flows and radial electric fields, has been investigated in the plasma edge region of the TJ-II stellarator. Experimental results show that the development of the “naturally occurring” velocity shear layer requires a minimum plasma density (or gradient). Near this critical density, the level of edge turbulent transport and the turbulent kinetic energy significantly increase in the plasma edge; once sheared flows are fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. Furthermore, the resulting shearing rate of the spontaneous sheared flows turns out to be close to the one needed to trigger a transition to an improved confinement regimen (H-mode like regimes), suggesting that spontaneous sheared flows and fluctuations keep themselves near marginal stability. These findings provide the first experimental evidence of coupling between the development of sheared flows and increasing in level of edge turbulence. Experimental results are consistent with the expectations of second-order transition models of turbulence driven sheared flows.