Anomalous behaviour of laser cooled fast ion beams

We have observed a sudden disappearance of intrabeam scattering in a laser cooled stored 9Be+ beam at the Heidelberg Test Storage Ring. The transition takes place at about 106 ions corresponding to a mean ion distance of ≃50c μm. The disappearance of IBS is accompanied by a decrease of the longitudinal temperature by more than two orders of magnitude. At the same time, the transverse width of the ion beam shows an increase which is limited to the diameter of the laser beam. Experimental signatures of this anomalous beam behaviour are described, and possible interpretations are discussed.