R. Ley

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.

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.

The new collector for the electron cooling device at LEAR

Abstract An important aspect of the low-energy physics programme at LEAR is the use of electron cooling for phase-space compression of the circulating ion beam [1–3]. In order to improve the reliability of the cooling device, a new electron beam collector has been developed in a collaboration between CERN and CAPT (Centre of Applied Physics and Technology) Lipetsk. The collector is designed to recuperate 3.3 A of 35 keV electrons with an efficiency better than 99.99%. It was constructed at the INP (Institute of Nuclear Physics) Novosibirsk workshop in accordance with a design produced by CAPT and the electron cooling team at CERN. The vacuum and electrical test of the collector were performed at Lipetsk. The device is now installed at LEAR and has undergone first reception tests in the machine.

Overview of the recent operation of the AAC and LEAR for the low-energy antiproton physics programme

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.

The variable current gun: the parameter tests and the results of the first electron cooling experiments at LEAR

Abstract The electron cooling time (or equivalently the cooling force) is inversely proportional to the electron beam current Ib, aimed to cool hot low-energy ions [1–3]. On the other hand, highly cooled ions tend to become unstable. This implies that once the ion beam upper density limit is attained, the electron current intensity has to be reduced to a level which prevents the ion beam instability and maintains low emittances. An adiabatic-type gun [4–6] has been constructed, which provides low-temperature electrons of large, but variable densities. The electron current density is adjusted through the voltage control, Ug, of a so-called grid electrode. Its main drawback is the storage of secondary electrons when the grid potential is larger than the anode potential, thus inducing a reduction of the nominal electron current intensity. A detailed analysis of the storage process is presented, and the way to cure this drawback is explained. Finally, experimental results are given.

First Results of Electron Cooling Experiments at LEAR

The first results are presented of electron cooling experiments in the Low-Energy Antiproton Ring (LEAR) at CERN, performed with a proton beam of about 50 and 21 MeV. The number of stored protons ranged from 107 to 3 × 109. Cooling times of the order 1 s and proton drag rates of up to 0.7 MeV/s were obtained. The capture of cooling electrons by protons producing hydrogen atoms was used to derive an effective electron temperature (0.25 eV). From the angular profile of the neutral hydrogen beam an upper limit of 3π mm.mrad could be deduced for the horizontal equilibrium proton-beam emittance. The lowest equilibrium momentum spread was 2 × 105 (FWHM), as derived from the analysis of the longitudinal Schottky signal. This Schottky signal exhibited an unusual behaviour with beam intensity and under certain conditions showed a doublepeak structure which was associated with collective beam noise. For very cold beams transverse instabilities were observed, which resulted in a rapid spill-off of protons and a stabilization at lower intensities. The threshold of these instabilities was raised by heating the proton or the electron beam. The cooling of a bunched proton beam was investigated. The reduction of the proton momentum spread led to bunch lengths of about 2 m, containing 3 × 108 protons.

Active methods of instability suppression in a neutralized electron beam

Abstract A large space-charge is detrimental to the cooling process of a dense beam. Generation of a neutralised electron beam is restricted by beam-drift instability. To suppress this instability, active methods are used: a kicker with a transverse electric field (shaker) and longitudinal electrodes placed in the vacuum chamber and in the gun region expel secondary electrons. Results of experiments dedicated to the development of beam stabilisation on the JINR test bench and in the electron cooling system at LEAR are discussed.

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.