Centrifugal Separation of Antiprotons and Electrons

Centrifugal separation of antiprotons and electrons is observed, the first such demonstration with particles that cannot be laser cooled or optically imaged. The spatial separation takes place during the electron cooling of trapped antiprotons, the only method available to produce cryogenic antiprotons for precision tests of fundamental symmetries and for cold antihydrogen studies. The centrifugal separation suggests a new approach for isolating low energy antiprotons and for producing a controlled mixture of antiprotons and electrons.

Figure 1

(a) A $\rho =5\mathrm{mm}$ spheroidal $e^{-}$ plasma within axially symmetric trap electrodes. The centrifugally separated $\overline{p}$ have the indicated maximum radius, but their distribution along the $\widehat{\mathbf{z}}$ axis is not known. (b) 100 V applied to electrode LTE3 produces wells that are nearly harmonic on axis (dot-dashed curve) and deepen off axis (solid curves). Potentials including the $e^{-}$ space charge are shown with dashed lines.

Figure 2

Resonances that reveal the axial frequency ${\omega }_z$ of the center-of-mass (a) and the quadrupole frequency ${\omega }_2$ of the oscillation of the length (b) of an electron plasma of ${10}^8$ electrons in the 80 eV potential well of Fig. 1.

Figure 3

(a) Number of $\overline{p}$ and $e^{-}$ escaping the trap each time the applied potential is reduced by nonadiabatic, 2-V steps. (b) Fractions of $\overline{p}$ and $e^{-}$ remaining after the well depth is ramped adiabatically to the indicated value (with curves to aid the eye). The initial $e^{-}$ plasma has a radius of 5 mm in an 80 eV well.

Figure 4

Outer $\overline{p}$ plasma radius at 3.7 T as a function of the $e^{-}$ plasma radius, deduced by ramping $B$ from 3.7 to 1 T and observing the value at which $\overline{p}$ annihilations begin (for ${10}^6$ $\overline{p}$ and ${10}^8$ $e^{-}$ in a 80 eV well). No observed annihilations for the four measurements on the left establish that the outer $\overline{p}$ extent is smaller than 9.4 mm. The $e^{-}$ plasma radius (dotted line) is included for comparison.