Normal modes of cold confined one-component plasmas

It is possible to trap a collection of ions and cool them to cryogenic temperatures where strong correlation effects, such as transitions to liquid and crystalline states, are observed. This paper examines the effect of correlations on the linear normal modes of oscillation in these strongly coupled plasmas. Recently a theory has been developed to treat such modes in the cold-fluid limit, where correlations are neglected. Here the fluid theory is compared to molecular-dynamics simulations of the modes. Simulated mode frequencies are observed to shift slightly compared to the cold-fluid predictions, and the modes are also observed to damp in time. Unmagnetized simulations also reveal a set of torsional oscillations which have no counterpart in the fluid theory. These correlation effects are also compared to a recently developed model that treats the trapped plasma as a viscoelastic spheroid. The model allows us to extract from the simulations the values of high-frequency bulk and shear moduli of a strongly correlated plasma.