Sympathetically cooled and compressed positron plasma

Abstract

We report sympathetic cooling and compression of a few thousand positrons by laser-cooled ${}^{9}{\mathrm{Be}}^{+}$ ions in a Penning ion trap. The observed centrifugal separation of the two species implies approximate rigid rotation of the positrons and ${}^{9}{\mathrm{Be}}^{+}$ ions, and a positron density comparable to the ${}^{9}{\mathrm{Be}}^{+}$ ion density of $\ensuremath{\gtrsim}4\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3}.$ We use the sharpness of the separation to place a 5-K upper limit on the positron temperature of motion parallel to the magnetic field. The positron lifetime is greater than two weeks in our room-temperature Penning trap.