Centrifugal Separation and Equilibration Dynamics in an Electron-Antiproton Plasma

Charges in cold, multiple-species, non-neutral plasmas separate radially by mass, forming centrifugally separated states. Here, we report the first detailed measurements of such states in an electron-antiproton plasma, and the first observations of the separation dynamics in any centrifugally separated system. While the observed equilibrium states are expected and in agreement with theory, the equilibration time is approximately constant over a wide range of parameters, a surprising and as yet unexplained result. Electron-antiproton plasmas play a crucial role in antihydrogen trapping experiments.

Figure 1

Images of centrifugally separated plasmas trapped in (a) a 1 T and (b) a 3 T solenoidal field. In both cases the $\overline{p}$ plasma is the ring on the left, and the $e^{-}$ plasma is the disk on the right. While the $\overline{p}$ and $e^{-}$ plasmas are concentric in the trap, they do not image to the same center. The faint halos visible near the $\overline{p}$ plasmas are thought to be due to optical reflections. In these and subsequent figures, the color bar is linear in density (red highest); each image is individually adjusted to saturate the color bar. The 1 mm length bars give the true plasma dimensions in the trap.

Figure 2

Centrifugal separation as a function of plasma size. In all cases the solid line is the $e^{-}$ radial profile and the dotted line is the $\overline{p}$ radial profile. Apertures clip images D and E.

Figure 3

Comparison of the measured $e^{-}$ and $\overline{p}$ radial profiles to the computed radial profiles at bracketing temperatures of 60 and 160 K. The experimental conditions are identical to profile Fig. 2. The $e^{-}$ radial profiles (solid lines) are nearly indistinguishable.

Figure 4

Temperature and hollowness $H$ as a function of time in a cooling plasma for a plasma similar to that in Fig. 1. The predicted temperature is fit through the data at 0.25 s.

Figure 5

Hollowness as a function of time after $e$-kicking leaves the stated percentages of electrons in the plasma. The state before $e$-kicking is similar to that shown in Fig. 1. Images (a) correspond to the 0.5% series. The $e^{-}$ plasma is the faint disk visible in the lower right of the images (But remember that there are still more $e^{-}$’s than $\overline{p}$’s.) The apparent smallness of the $e^{-}$ plasma is an artifact of the faintness of the image. Images (b) correspond to the 0% series.

Figure 6

Radius and hollowness as a function of time after injection of 35 000 $\overline{p}$’s (circles) and 8800 $\overline{p}$’s (triangles). The top set of images corresponds to the 35 000 $\overline{p}$’s series.