Ellipsoidal Coulomb crystals in a linear radio-frequency trap

Abstract

A static quadrupole potential breaks the cylindrical symmetry of the effective potential of a linear rf trap. For a one-component liquid plasma at low temperature, the resulting equilibrium charge distribution is predicted to be an ellipsoid. Laser-cooled Be+9 ellipsoidal ion crystals were produced and good agreement was found between their shapes and the cold fluid prediction. In two-species mixtures, containing Be+9 and sympathetically cooled ions of lower mass, a sufficiently strong static quadrupole potential produces a complete spatial separation of the species.