Sympathetic cooling of trapped ions: A laser-cooled two-species nonneutral ion plasma

Sympathetic cooling of trapped ions has been demonstrated in an experiment where ¹⁹⁸Hg⁺ ions were confined in a Penning ion trap with laser-cooled ⁹Be⁺ ions. ¹⁹⁸Hg⁺ temperatures below 1 K were achieved. Ion plasma sizes, shapes, and rotation frequencies were measured. Dramatic changes in the ⁹Be⁺ plasma were observed when the ¹⁹⁸Hg⁺ ions were introduced into the trap. These observations are consistent with the prediction of centrifugal separation for ions of different charge-to-mass ratios.