Crystalline ion structures in a Paul trap

We have observed crystalline structures formed by laser cooled Ca+ ions in a three-dimensional confining potential. The potential has been realized us ing a linear Paul trap with different ratios of potential strength in the a xial and radial directions. For radial confining potentials stronger than t he axial potential, we find linear structures with a continuous transition from strings to helices with decreasing potential asymmetry. When a quasi-t wo-dimensional potential is formed we observe ring structures with a given maximum ion number per ring which we followed up to 19. The observations ar e essentially in agreement with molecular dynamics calculations in static t wo-dimensional potentials.