Heavy ion beam accumulation, cooling, and experiments at the ESR

The Experimental Storage Ring ESR-part of the heavy ion accelerator facility at GST-started operation early in 1990. This paper informs about the progress achieved in accumulation, electron cooling and internal experiments with fully stripped heavy ions from O⁸⁺ to U⁹²⁺ at energies between 90 MeV/u and 300 MeV/u. RF-stacking combined with compression in phase space by electron cooling turned out to be a very effective accumulation technique. Maximum numbers of stored ions for the heaviest ions are determined by the equilibrium between accumulation rate and beam loss rate due to radiative electron capture of cooler electrons. Beam currents for lighter ions up to Ar¹⁸⁺ are limited by coherent instabilities. First excellent results of internal experiments with cooled, circulating beams of either primary ions or of secondary nuclear fragments confirm that the combination of the heavy ion facility Unilac-SIS-FRS with the storage ring ESR offers unique possibilities for atomic, nuclear and accelerator physics