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

Laser-cooled trapped ions can be strongly coupled and form crystalline stat es. In this paper we review experimental studies that measure the spatial c orrelations of Be+ ion crystals formed in Penning traps. Both Bragg scatter ing 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 X 10(5) ions, body-centered-cubic (bcc) crystals, the pred icted bulk structure, are the only type of crystals observed. The orientati on of the ion crystals can be phase locked to a rotating electric-field per turbation. With this "rotating wall'' technique and stroboscopic detection, images of individual ions in a Penning trap are obtained. The rotating wal l technique also provides a precise control of the time-dilation shift due to the plasma rotation, which is important for Penning trap frequency stand ards. [S1070-664X(00)01501-9].