The influence of plasma density and edge gradients on the development of
perpendicular sheared flow has been investigated in the plasma edge region
of the TJ-II stellarator. The development of the naturally occurring velocity
shear layer requires a minimum plasma density. Experimental findings have
shown that there is a coupling between the onset of sheared flow development
and an increase in the level of plasma edge turbulence; once sheared flow
is fully developed the level of fluctuations and turbulent transport slightly
decreases whereas edge gradients and plasma density increases. Electron
density profiles show a broadening evolution as density increases above the critical value where sheared flow is developed, while the temperature profile
remains similar, reflecting the strong impact of plasma density in the global
confinement scaling. Furthermore, the shearing rate of the spontaneous
sheared flow turns out to be close to the one needed to trigger a transition
to improved confinement regimes. Density ramp experiments show, within
the experimental uncertainty, no evidence of hysteresis during the spontaneous
shear development. Power modulation, in the proximity of the critical plasma
density, allows the characterization of plasma potential and electric field
relaxation during the transition. The present results have a direct impact on the
understanding of the physics mechanisms underlying the generation of critical
sheared flow, pointing to the important role of turbulent driven flow. |