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We report the observation of shell structure in ^9Be^+ nonneutral plasmas (ion clouds) confined in a Penning trap. Clouds containing up to 15000 ions (density 10^8 cm^-3) were laser cooled to temperatures of about 10 mK. Under these conditions, the ions are strongly coupled and exhibit liquidlike and solidlike behavior through the formation of concentric shells. The shells were observed by direct imaging of the laser-induced ion fluorescence for values of the Coulomb coupling constant ranging from about 20 to 200. The shell structure is compared with theoretical predictions.
Gilbert et al. (Mon,) studied this question.