A structural transition for laser cooled ion Coulomb crystals in a linear Paul trap just above the stability limit of parametrically resonant excitation of bulk plasma modes has been observed. In contrast to the usual spheroidal shell structures present below the stability limit, the ions arrange in a "string-of-disks" configuration. The spheroidal envelopes of the string-of-disks structures are in agreement with results from cold fluid theory usually valid for ion Coulomb crystals if the ion systems are assumed to be rotating collectively.
Kjærgaard, N., & Drewsen, M. (2003). Observation of a structural transition for coulomb crystals in a linear Paul trap. Physical Review Letters, 91(9), 095002. https://doi.org/10.1103/PhysRevLett.91.095002