Sympathetically cooled and compressed positron plasma

We report sympathetic cooling and compression of a few thousand positrons by laser-cooled ⁹Be⁺ ions in a Penning ion trap. The observed centrifugal separation of the two species implies approximate rigid rotation of the positrons and ⁹Be⁺ ions, and a positron density comparable to the ⁹Be⁺ ion density of ≳4×10⁹ cm^-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.