Role of Turbulence on Edge Momentum Redistribution in the TJ-II Stellarator

Radial profiles of the parallel-radial Reynolds stress component, proportional to the cross correlation between parallel and radial fluctuating velocities, have been measured in the plasma boundary region of the TJ-II stellarator. Experimental results show the existence of significant parallel turbulent forces at plasma densities above the threshold value to trigger perpendicular $E\times B$ sheared flows. This finding provides the first experimental evidence of the role of parallel turbulence forces on edge momentum redistribution in fusion devices.

Figure 1

Radial profiles of ion saturation current, floating potential, parallel flows, and the level of edge fluctuations versus plasma density in the plasma boundary of the TJ-II stellarator. As density increases above a threshold value ($\approx 0.55{\mathrm{m}}^{-3}$), the edge floating potential becomes negative.

Figure 2

Time evolution of ion saturation current, floating potential, Mach number, the rms of the poloidal electric field, and the quadratic term $⟨{\tilde{\nu }}_r{\tilde{M}}_{\parallel }⟩$ at a fixed position ($\rho \sim 0.85$) during a density scan.

Figure 3

Radial profiles of the cross correlation between parallel and radial fluctuating velocities in TJ-II plasmas near the LCFS at different plasma densities (for the time series shown in Fig. 1).

Figure 4

Radial profile of production term ($P$) at different plasma densities in TJ-II (for the time series shown in Fig. 1).

Figure 5

(a) Density ramp experiments show transients in floating potential in the proximity of the TJ-II threshold density (shadow region in the figure) to trigger edge sheared flows. (b) Time evolution of floating potential at a fixed position ($\rho \sim 0.9$) during a density scan. Fast transients are observed in the proximity of the threshold density to trigger edge sheared flows.