Laser cooling of a stored ion beam to 1 mK

The interaction of laser-induced and intrabeam forces has been studied in a dense stored beam of 100-keV ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}$ ions. A fraction of the ions (∼${10}^{\mathrm{-}4}$) exist in the metastable 1s2s ${}_{}{}^3{}_{}{}^{}$S state. Using this state as a laser spectroscopic probe, we observe fast longitudinal heating in the injected, nonequilibrium distribution due to Coulomb scattering. Laser cooling of the metastable ions is ineffective during this heating period. The metastable fraction of the equilibrated beam is subsequently laser cooled to a longitudinal temperature of ∼1 mK, the lowest temperature ever reported in a stored ion beam.