This paper reviews the recent performance of the AAC and LEAR. Activities on the AAC include the successful exploitation of a magnetic horn as an antiproton collector lens and an energy-saving mode of operation, which has been possible since 1992, when LEAR became the only client of the AAC. LEAR worked in its full momentum range between 100 MeV/c and 2 GeV/c, with performance (intensities, ejection modes and spill length) exceeding the design specifications. Improvements are described, which contributed to the quality of the beam delivered to experiments. The reliability and availability of the antiproton machines are also discussed.

Because of their high density together with extremely small spreads in betatron frequency and momentum, cooled beams are very vulnerable to incoherent and coherent spacecharge effects and instabilities. Moreover, the cooling system itself, i.e. the electron beam in the case of e-cooling, presents large linear and non-linear 'impedances' to the circulating ion beam, in addition to the usual beam-environment coupling impedances of the storage ring. Beam blow-up and losses, attributed to such effects, have been observed in virtually all the existing electron cooling rings. The adverse effects seem to be more pronounced in those rings, like CELSIUS, that are equipped with a cooler capable of reaching the presently highest energy (100 to 300 keV electrons corresponding to 180 to 560 MeV protons). The stability conditions will be revisited with emphasis on the experience gained at LEAR. It will be argued that for all present coolers, three conditions are necessary (although probably not sufficient) for the stability of intense cold beams:(i) operation below transition energy, (ii) active damping to counteract coherent instability, and (iii) careful control of the e-beam neutralisation. An extrapolation to the future 'medium energy coolers', planned to work for (anti)protons of several GeV, will also be attempted.

New methods to create hollow distributions in longitudinal phase space based on manipulations with a double harmonic RF system at high energy are presented with application to the PS Booster synchrotron (PSB). Whereas the first tentative to create hollow bunches at the PSB aimed to improve the performance of the PSB itself, these new methods are expected to reduce the limitations due to direct space charge forces in the receiving PS (where no double harmonic RF system is available) after transfer. One method aims to introduce empty phase space in the centre of the phase space by recombination of the beam in one bucket with another empty bucket. The second method is based on redistribution of phase space surfaces during the transfer of the beam from one second harmonic sub-bucket to another. During that process, phase space surfaces are exchanged and low density from the periphery ends up in the centre, whereas the high density surfaces from the centre are transferred to the periphery. Both methods have been simulated by particle tracking. The second method has been applied in practice at the PSB. The set-up turned out to be simple and fast, and to yield hollow distributions with good reproducibility. CERN PS, CH-1211 Genève 23, SwitzerlandAbstract. New methods to create hollow distributions in longitudinal phase space based on manipulations with a double harmonic RF system at high energy are presented with application to the PS Booster synchrotron (PSB). Whereas the first tentative to create hollow bunches at the PSB aimed to improve the performance of the PSB itself, these new methods are expected to reduce the limitations due to direct space charge forces in the receiving PS (where no double harmonic RF system is available) after transfer. One method aims to introduce empty phase space in the centre of the phase space by recombination of the beam in one bucket with another empty bucket. The second method is based on redistribution of phase space surfaces during the transfer of the beam from one second harmonic sub-bucket to another. During that process, phase space surfaces are exchanged and low density from the periphery ends up in the centre, whereas the high density surfaces from the centre are transferred to the periphery. Both methods have been simulated by particle tracking. The second method has been applied in practice at the PSB. The set-up turned out to be simple and fast, and to yield hollow distributions with good reproducibility.

The LEAR machine is foreseen to be used as a lead ion accumulator in the injector chain for the LHC. Therefore, to test the principle of Pb54+ ion accumulation in LEAR, at 4.2 MeV/u, a number of studies and experiments have been made aimed at determining the improvements and modifications necessary to fulfill the LHC requirements. Amongst subjects studied we have investigated: a) The ion production and the injection line matching, b) The linac energy ramping in view of accumulation through combined longitudinal-transverse multi-turn injection, c) The ion beam lifetime and recombination processes, and finally d) The electron cooling time, as a function of the lattice parameters at the cooler. Recent experiments and measurements will be reported and commented in this paper. Also mentioned are some particular diagnostic methods.

The fast-growing "Ring 4" instability occurring at intensities above 6.5×10 12 protons in the top one of the four rings of the CERN PS Booster (PSB) is finally explained by an asymmetry in the 40 vacuum pump manifolds common to all rings. Impedance measurements (by wire method) and numerical calculations show a sharp resonant peak (Q~2000) at 1100 MHz and shunt impedances two times higher for the Ring 4 ports as compared to the other rings. This factor is sufficient to explain that the threshold of the instability falls below the maximum intensity only in Ring 4. A final, but labor-intensive and expensive, cure consists of inserting short-circuiting sleeves into all 160 beam ports. Results of beam and impedance measurements and the planned cure will be presented and discussed.