Extremely cooled ion beams in the ESR with evidence of ordering

Experiments with electron-cooled highly charged ions in the storage ring ESR have evidenced an unusual behavior for small particles numbers. The momentum spread drops suddenly by, a factor of three to ten, depending on the ion charge, when the ion number decreases below a few thousands. The transition particle number increases with the intensity of the electron beam which determines the cooling rate. Destructive measurements of the beam radius suggest a similar reduction of the transverse emittance near the transition point of the momentum spread. The assumption that the momentum spread reduction is connected with a suppression of intrabeam scattering could be confirmed by direct observation of the longitudinal heating of a cooled beam above and below the transition point. A possible explanation is, that the ions are circulating in a longitudinally ordered structure, where the ions are confined by their neighbors to a fixed longitudinal position relative to each other