Spontaneous edge E × B sheared flow development studies in the TJ-II stellarator

Abstract

The influence of plasma density and edge gradients in the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. It has been experimentally observed that the generation of spontaneous perpendicular sheared flow (i.e. the naturally occurring shear layer) requires a minimum plasma density or gradient. It has been found 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 increase. The resulting shearing rate is close to the one required to trigger a transition to improved confinement regimes with reduction of edge turbulence, showing that spontaneous sheared flows and fluctuations keep themselves near marginal stability. : 52.55.Fi, 52.55.Gj, 52.35.Ra, 52.55.Hc PACS

Parametric Dependence of the Perpendicular Velocity Shear Layer Formation in TJ-II Plasmas

“…The contribution from each parameter to the critical line-density is obtained subtracting the contribution from the other parameters. These results partially confirm preliminary results obtained using Langmuir probes and reported in [13,14]. The opposite exponential dependences as compared with the energy confinement time may reflect the influence of the radial electric field profile on the confinement.…”

Velocity shear layer measurements by reflectometry in TJ-II plasmas

“…In plasmas with densities below the critical value, the plasma potential increases from the plasma edge to the centre, and therefore the radial electric field is positive in the entire plasma column. At higher densities the radial electric field is negative at the periphery and remains positive at the plasma core [20]. These HIBP measurements indicate that the inversion in the perpendicular rotation velocity that we measure using the reflectometer is dominated by the radial electric field.…”

Long‐Range Correlations During Plasma Transitions in the TJ‐II Stellarator

“…Transition to an improved confinement regime has been observed in TJ-II for some ECRH plasma conditions [19], [20]. In agreement with this result, it has been shown that the development of sheared flows at the plasma edge of the TJ-II requires a critical value of plasma density or density gradient that depends on global plasma parameters [9], [10], [21], [22]. Radial profiles of measured plasma edge parameters are strongly modified as plasma density increases: the gradient of the ion saturation current increases and the floating potential becomes more negative at the plasma edge.…”