Generation of a Sheared Plasma Rotation by Emission, Propagation, and Absorption of Drift Wave Packets

Collisional electron drift wave turbulence generates drift wave packet structures with density and vorticity fluctuations in the central plasma pressure gradient region of a linear plasma device. Tracking these packets reveals that they follow an outward directed spiral-shaped trajectory in the $(r,\theta )$ plane, are azimuthally stretched, and develop anisotropy as they approach an axisymmetric, radially sheared azimuthal flow located at the plasma boundary. Nonlinear energy transfer measurements and time-delay analysis confirm that structure absorption amplifies the sheared flow. Similar mechanisms likely operate at the edge of confined toroidal plasmas and should lead to the amplification of sheared flows at the boundary of these devices as well.

Figure 1

Time-averaged radial profiles of the Controlled Shear Decorrelation Experiment plasma. (a) Time-averaged density $n_0$. (b) Gradient of the time-averaged density $-\partial n_0/\partial r$. (c) Time-averaged azimuthal velocity $⟨V_{\theta }⟩$. (d) Residual stress ${\pi }_{r\theta }^{\mathrm{res}}$ (solid red line) and the diffusive momentum flux ${\pi }^{\mathrm{diff}}\equiv -⟨{\tilde{v}}_r^2⟩{\tau }_c\frac{\partial ⟨V_{\theta }⟩}{\partial r}$ (solid black line). (e) Skewness of density (solid black line) and vorticity (solid red line) fluctuations. (f) Time-averaged radial flux of vorticity $⟨{\tilde{v}}_r\tilde{\omega }⟩$ (solid black line) and Reynolds force $F_{\theta }^R$ due to turbulent vorticity (dashed red line).

Figure 2

Sequential visible light emissions images showing the birth, evolution, and death of vortexlike structures. Radii of $r=3.0\mathrm{cm}$ and $r=4.0\mathrm{cm}$ are denoted by the two dashed circles.

Figure 3

(a) Nonlinear kinetic energy transfer vs effective azimuthal mode number at $r=2.4\mathrm{cm}$ (dashed black line) corresponding to the vortex structures generation region and $r=3.2\mathrm{cm}$ (solid red line) corresponding to the location of the shear layer. A positive (negative) $T_u$ value indicates a gain (loss) of turbulent kinetic energy through nonlinear processes. The statistical noise level was estimated as $\sim 1/\sqrt{N}<3\%$ ($N$ is the number of realizations) and thus is negligibly small. (b) Time-resolved density (blue) and vorticity (red) fluctuations. (c) Correlation between density and vorticity fluctuations, which shows that density and vorticity fluctuations have a significant correlation with zero phase shift. (d)–(g) Cross correlation between vorticity flux at different radial positions [(d)–(g) correspond to $r=2.1$, 2.6, 3.1, and 3.6 cm, respectively] and turbulent azimuthal velocity in the shear layer at $r=3.6\mathrm{cm}$. An increase in central plasma vorticity flux leads to a subsequent increase in the boundary plasma azimuthal velocity $\sim 110\mu \mathrm{s}$ later. Solid blue lines are the calculated correlations. Up-down symmetric envelopes (solid purple line) were computed from the correlations; the center of each envelope is indicated by the dotted red vertical lines. The dotted green line corresponds to zero time delay.