LARGE ION CRYSTALS IN A LINEAR PAUL TRAP

Plasmas of Mg+ ions, containing more than 10(5) ions, have been observ ed to reach well-ordered (crystalline) states by applying laser coolin g. The crystals are highly elongated with up to ten concentric cylindr ical shells surrounding a central string. Such large structures have n ot previously been observed in a Paul trap. The amplitude of the micro motion of the ions can be larger than the shell spacings. As the diame ter changes along the crystals, sharp transitions are observed when ne w shells form, in good agreement with molecular dynamics simulations. The predictions from simulations of how ordering develops with decreas ing temperature are also confirmed.