Experimental evidence of coupling between sheared-flow development and an increase in the level of turbulence in the $\mathrm{TJ}\text{−}\mathrm{II}$ stellarator

The link between the development of sheared flows and the structure of turbulence has been investigated in the plasma boundary region of the $\mathrm{TJ}\text{−}\mathrm{II}$ stellarator. The development of the naturally occurring velocity shear layer requires a minimum plasma density. Near this critical density, the level of edge turbulent transport and the turbulent kinetic energy significantly increases in the plasma edge. The resulting shearing rate in the phase velocity of fluctuations is comparable to the one required to trigger a transition to improved confinement regimes with reduction of edge turbulence, suggesting that spontaneous sheared flows and fluctuations keep themselves near marginal stability. These findings provide the experimental evidence of coupling between sheared flows development and increasing in the level of edge turbulence. The experimental results are consistent with the expectations of second-order transition models of turbulence-driven sheared flows.

Figure 1

(Color online) Profiles of (a) ion saturation current ($I_s\approx n{T}_e^{-1∕2}$, $n$ and $T_e$ being the plasma density and electron temperature, respectively), (b) floating potential, (c) poloidal wave number and (d) phase velocity measured at different plasma densities (No. 9748, $0.35\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9749, $0.45\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9751, $0.55\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9752, $0.65\times {10}^{19}{\mathrm{m}}^{-3}$) in a $\mathrm{TJ}\text{−}\mathrm{II}$ plasma configuration with $\bar{\iota }\left(a\right)\approx 1.7$. (d) also shows (thick line) the $E_r∕B$ velocity estimated from floating potential profiles (b).

Figure 2

(Color online) Profiles of (a) ion saturation current, (b) floating potential, and (c) phase velocity measured at different plasma densities (No. 9768, $0.5\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9769, $0.7\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9767, $0.8\times {10}^{19}{\mathrm{m}}^{-3}$) in a $\mathrm{TJ}\text{−}\mathrm{II}$ plasma configuration with $\bar{\iota }\left(a\right)\approx 1.8$.

Figure 3

(Color online) Profiles of (a) turbulent transport and (b) radial velocity fluctuations at different plasma densities (No. 9748, $0.35\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9749, $0.4\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9751, $0.55\times {10}^{19}{\mathrm{m}}^{-3}$, No. 9752, $0.65\times {10}^{19}{\mathrm{m}}^{-3}$). $\mathrm{TJ}\text{−}\mathrm{II}$ plasma configuration with $\bar{\iota }\left(a\right)\approx 1.7$ (see Fig. 1).

Figure 4

(Color online) Gradients in phase velocity, radial velocity fluctuations, and $E\times B$ turbulent transport versus gradients in the ion saturation current (computed in the proximity of $\rho \approx 0.95–1$) for plasma configurations with $\bar{\iota }\left(a\right)\approx 1.7$ (open symbols), 1.8 (solid symbols).

Figure 5

(Color online) Time evolution of the plasma density, $H_{\alpha }$ (a), turbulent transport (b), and radial gradient in the poloidal phase velocity of fluctuations (c) during limiter biasing experiments. The time evolution of $d{v}_{\theta }∕dr$ was computed from simultaneous measurements of the poloidal phase velocity at two different radial locations $\left(\Delta r\approx 0.5\text{cm}\right)$. Biasing is turned on at $1150\mathrm{ms}$; however, turbulent transport reduction takes place $10\mathrm{ms}$ latter when gradients in the phase velocity of fluctuations are close to ${10}^5{\mathrm{s}}^{-1}$. When biasing is turned off, $d{v}_{\theta }∕dr$ decreases with a concomitant drastic increase in the turbulent transport.