Effect of Drift Waves on Plasma Blob Dynamics

Most of the work to date on plasma blobs found in the edge region of magnetic confinement devices is limited to 2D theory and simulations which ignore the variation of blob parameters along the magnetic field line. However, if the 2D convective rate of blobs is on the order of the growth rate of unstable drift waves, then drift wave turbulence can drastically alter the dynamics of blobs from that predicted by 2D theory. The density gradients in the drift plane that characterize the blob are mostly depleted during the nonlinear stage of drift waves resulting in a much more diffuse blob with a greatly reduced radial velocity. Sheath connected plasma blobs driven by effective gravity forces are considered in this Letter and it is found that the effects of resistive drift waves occur at earlier stages in the 2D motion for smaller blobs and in systems with a smaller effective gravity force. These conclusions are supported numerically by a direct comparison of 2D and 3D seeded blob simulations.

Figure 1

Basic physics of plasma blobs driven by charge polarizing forces.

Figure 2

Time slices of density contours from 2D simulation (top) and 3D simulation (bottom) for $\Delta =1$, $\rho =2.87\times {10}^{-4}$, $\sigma =32.3$, and $\alpha =3.68\times {10}^{-3}$.

Figure 3

2D density contours from 3D simulation taken at different slices along the magnetic field line corresponding to the averaged density contour shown at $t=5.5$ in Fig. 2.

Figure 4

Time slices of density contours from 2D simulation (top) and 3D simulation (bottom) for $\Delta =0.3$, $\rho =2.87\times {10}^{-4}$, $\sigma =32.3$, and $\alpha =3.68\times {10}^{-3}$.

Figure 5

Time slices of density contours from 2D simulation (top) and 3D simulation (bottom) for $\Delta =0.3$, $\rho =6.4E-5$, $\sigma =11.4$, and $\alpha =1.1\times {10}^{-3}$.