Crystalline order in laser-cooled, non-neutral ion plasmas

Laser-cooled trapped ions can be strongly coupled and form crystalline states. In this paper we review experimental studies that measure the spatial correlations of Be+ ion crystals formed in Penning traps. Both Bragg scattering of the cooling-laser light and spatial imaging of the laser-induced ion fluorescence are used to measure these correlations. In spherical plasmas with more than 2×105 ions, body-centered-cubic (bcc) crystals, the predicted bulk structure, are the only type of crystals observed. The orientation of the ion crystals can be phase locked to a rotating electric-field perturbation. With this “rotating wall” technique and stroboscopic detection, images of individual ions in a Penning trap are obtained. The rotating wall technique also provides a precise control of the time-dilation shift due to the plasma rotation, which is important for Penning trap frequency standards.