Plasmas of ${\mathrm{Mg}}^{+}$ ions, containing more than ${10}^5$ ions, have been observed to reach well-ordered (crystalline) states by applying laser cooling. The crystals are highly elongated with up to ten concentric cylindrical shells surrounding a central string. Such large structures have not previously been observed in a Paul trap. The amplitude of the micromotion of the ions can be larger than the shell spacings. As the diameter changes along the crystals, sharp transitions are observed when new shells form, in good agreement with molecular dynamics simulations. The predictions from simulations of how ordering develops with decreasing temperature are also confirmed.

• Received 5 December 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.2878