Experimental investigation of electron cooling and stacking of lead ions in a low energy accumulation ring

This report gives the results of a programme of experimental investigations, which were carried out to test stacking of lead ions in a storage ring (the former Low Energy Antiproton Ring, LEAR) at 4.2 MeV per nucleon. The motivation was to demonstrate the feasibility of gaining the large factor in the phase-space density required for injection into the LHC. In the first part of the report, the layout of the experiments is described, the choice of the parameters of the electron cooling system used for stacking is reported and the multi-turn injection using horizontal- and longitudinal- (and in the final project also vertical-) phase space is discussed. In the second part the experimental results are presented. Factors of vital importance are the stacking efficiency, the beam life-time and the cooling time of the ions. The beam decay owing to charge exchange with the residual gas and to recombination by the capture of cooling electrons was intensively studied. Beam instabilities and space-charge effects in the ion beam turned out to be additional, although less serious, limitations of the accumulation rate. The cooling speed as a function of cooler and storage-ring properties was investigated over a wide range of parameters. Among the 'surprises' encountered are an anomalously fast recombination rate for certain ion charge states (Pb53+), a strong dependence of the cooling time on the dispersion function of the storage ring, and an intensity-dependent outgassing of equipment in the vacuum chamber. After a careful choice of parameters and antidotes, an overall factor of 120 in intensity could be gained, by multi-turn injection and stacking for 4 s. The intensity obtained ($6 x 10^8$ ions with a length corresponding to four LHC bunches) is only a factor of two short of the LHC requirement, and the stacking time (4 s instead of 2 s foreseen for filling each LHC ring in 8 min) is another factor of two off.