Characterization of Quasi-Keplerian, Differentially Rotating, Free-Boundary Laboratory Plasmas

We present results from pulsed-power driven differentially rotating plasma experiments designed to simulate physics relevant to astrophysical disks and jets. In these experiments, angular momentum is injected by the ram pressure of the ablation flows from a wire array Z pinch. In contrast to previous liquid metal and plasma experiments, rotation is not driven by boundary forces. Axial pressure gradients launch a rotating plasma jet upward, which is confined by a combination of ram, thermal, and magnetic pressure of a surrounding plasma halo. The jet has subsonic rotation, with a maximum rotation velocity $23\pm 3\mathrm{km}/\mathrm{s}$. The rotational velocity profile is quasi-Keplerian with a positive Rayleigh discriminant ${\kappa }^2\propto r^{-2.8\pm 0.8}{\mathrm{rad}}^2/{\mathrm{s}}^2$. The plasma completes 0.5–2 full rotations in the experimental time frame ($\sim 150\mathrm{ns}$).

Figure 1

Schematic diagrams of experimental setup and self-emission images. (a) 3D schematic of inertially driven rotating plasmas. (b) 3D schematic of experimental hardware. Axial jets are not shown. (c) End-on optical self-emission image. (d) Extreme ultraviolet (XUV) image of rotating plasma.

Figure 2

(a) Line integrated electron density map. TS volumes are overlaid in white circles. Additional arrows indicate the scattering geometry. (b) Schematic TS setup and vector diagram. (c) TS spectrum from outside the jet (plasma halo). Best fit is shown in red. The spectrometer response is shown in gray. Fitting parameters shown in gray box. (d) TS spectrum from inside the jet. The characteristic double peak of the ion-acoustic wave feature is observed.

Figure 3

Plasma parameters. (a) Line-integrated electron density lineout at the same height as the scattering volumes. Uncertainty was estimated to be 20%. (b) Abel inverted electron density. Left- and right-hand-side inversions (black lines) and averaged inversion (dashed blue line). (c) Rotation velocity. (d) Radial velocity. (e) Ion temperature. (d) Electron temperature. Downward pointing arrows indicate the value is an upper constraint.

Figure 4

Rotation velocity distribution depending on TS beam impact parameter $b$. (a) Diagram of the velocity map (black arrows) of each TS volume (black circles). Example impact parameter $b=0.3\mathrm{mm}$ is shown. (b) Angular frequency distribution. (c) Angular momentum distribution.