  The electric field in mm-sized clouds of cold Rb^+-ions is measured using the
Stark effect of Rydberg atoms embedded in the ion clouds. The ion clouds are
produced by UV photoionization of laser-cooled rubidium atoms in a
magneto-optic trap. In very dense ion clouds, the Rydberg atom excitation
spectra reflect the presence of a neutral plasma core and a positively charged
ion shell.
