M. Chanel

Measurement of the lifetime of Pb52+, Pb53+ and Pb54+ beams at 4.2 MeV per nucleon subject to electron cooling

Abstract By measuring the lifetime of stored beams, the recombination of the ions with cooling electrons was investigated. Rates found are larger than expected for radiative electron capture and significantly higher for Pb53+ than for Pb54+ and Pb52+. These results are important for the design of the lead ion injection system for the Large Hadron Collider and for recombination theories.

Stability of cooled beams

Abstract Because of their high density together with extremely small spreads in betatron frequency and momentum, cooled beams are very vulnerable to incoherent and coherent space-charge 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 (100–300xa0keV electrons corresponding to 180–560xa0MeV 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.

Further results and evaluation of electron cooling experiments at LEAR

Abstract First electron cooling experiments were performed with 10 7 to 2×10 9 stored antiprotons of 50, 21 and 6 MeV at the Low Energy Antiproton Ring (LEAR) at CERN. Most effort was put into the study of the longitudinal cooling. Schottky pick-up signals were used to measure the equilibrium momentum spread and the longitudinal cooling time. From the equilibrium between stochastic heating and electron cooling the longitudinal friction force in the low 10 3 m/s relative velocity range could be deduced. This method was used also to increase the cooling force by improving the alignment between the antiproton and the electron beam. Some of the experimental data are compared with results of a simulation program for electron cooling (SPEC).

Measurements of H− intra-beam stripping cross section by observing a stored beam in lear

Abstract The cross section for H − “intra-beam stripping”: H − + H − → H − + H 0 + e − was measured by analysing the decay of a stored H − beam. Results ( σ max = 3.6 × 10 −15 cm 2 ± 30%) agree with recently published classical trajectory Monte Carlo calculations but suggest a smaller cross section than obtained from earlier theoretical models. This sets more favourable conditions for the storage of H − beams than previously assumed.

Production of MeV antiprotons

In view of a future antihydrogen programme at CERN, the options for producing MeV antiprotons are revisited. The current limitations, operational performances and foreseen improvements are detailed. An alternative scheme using a dedicated machine for production and deceleration is also discussed.

Space-charge compensation options for the LHC injector complex

Space-charge effects have been identified as the most serious intensity limitation in the CERN PS and PS Booster, on the way towards ultimate LHC performance and beyond. We here explore the application of several previously proposed space-charge compensation methods to the two LHC pre-injector rings, and the challenges which need to be overcome. The methods considered include the reduction of tune shift and resonance strengths via octupoles, pole-face windings, electron lenses, or neutralisation.

RECENT RESULTS ON LEAD-ION ACCUMULATION IN LEAR FOR THE LHC

Abstract To prepare dense bunches of lead ions for the LHC it has been proposed to accumulate the 4.2 MeV/u linac beam in a storage ring with electron cooling. A series of experiments is being performed in the low-energy ring LEAR to test this technique. First results were already reported at the Beam Crystallisation Workshop in Erice in November 1995. Two more recent runs to complement these investigations were concerned with: further study of the beam lifetime; the dependence of the cooling time on optical settings of the storage ring and on neutralization of the electron beam; and tests in view of multiturn injection. New results obtained in these two runs in December 1995 and in April 1996 will be discussed in this contribution.

Cooled beam diagnostics

Abstract The specific diagnostics systems used in cooler rings will be reviewed. The hardware used and the ways in which the physical characteristics of the beam are accessed will be described. It will also be shown how to evaluate the forces that are exerted on the beam during cooling.

Operational aspects of electron cooling at the low energy antiproton ring (LEAR)

The major modifications made to the LEAR (Low Energy Antiproton Ring) electron cooler for its reliable and effective use in everyday operations are described. The transverse feedback system used to counteract the coherent instabilities observed with the dense beams obtained with electron cooling is also discussed. Experience gained while using the cooler in a semi-operational manner has made it possible to redesign the critical components. Supplemented by the active feedback system, electron cooling can now be used consistently between decelerating ramps to obtain beams of ultralow momenta with an appreciable gain in duty cycle and beam quality as compared to previous modes of operation.<<ETX>>

SPACE CHARGE MEASUREMENTS AT THE PSB

The PS Booster (PSB) is an accelerator working under very high space charge during the first part of its cycle. After the change of tune, it was necessary to revisit the space charge effects to try to understand and possibly improve the situation. In addition, the Linac2 will be replaced by the Linac4 providing an H - injection in the PSB at 160 MeV. This energy is chosen to reduce by a factor 2 the space charge effect with the same beam characteristics. It was then needed to make some measurements at this energy, not only to find the limits but also to benchmark some simulation with available programs [1].