Search

A possibility to improve the luminosity in hadron colliders is to apply electron cooling. For the e–p and possibly in the future for e-ion a cooling scheme is proposed for the HERA collider which improves the brightness of the hadron beam in two steps. In a first step the hadron beam is cooled at an energy of 15–20 GeV in the pre-accelerator PETRA. In a second step the hadrons are cooled at top energy in HERA, mainly to maintain the beam quality. The first part of the cooler in PETRA consists of a thermionic gun which delivers electron bunches of 120 keV. These bunches are then compressed so that they fit into a 208 MHz travelling wave linac where they are accelerated to an energy of 10 MeV. Gun, compressor, and linac are surrounded by a solenoid so that emittance growth is avoided especially at the low energy end. In order to reduce the energy spread of the electron beam and to lengthen it, the electrons run through a decompressor before they are brought together with the hadron beam in a 40 m long cooling section which is again surrounded by a solenoid. In this paper a schematic layout of the system will be presented and simulation results of the first part of the cooler will be shown. The cooler in HERA consists of a small electron storage ring with two long straight sections. It is shown that all requirements of such a ring running at an energy between 180 and 450 MeV can be met to fight emittance growth in case of light hadron beams (protons) and to cool heavy ions sufficiently fast.

It is under discussion to operate HERA with heavy ions instead of protons. Because of the heavily increased intrabeam scattering of heavy ions compared to protons, the achievable luminosity is much smaller than desired by the experiments. A way to increase luminosity could be electron cooling of the heavy ion beam. Because of the high electron energy of 180 MeV necessary for cooling heavy ions in HERA the conventional approach using electrostatic DC sources is not possible. Instead an electron storage ring could be used. For this ring there are special requirements like small damping times, emittances and energy spread. The feasibility of such an electron storage ring has been studied, and the results with a possible solution are presented in this paper. Electron cooling of the HERA proton beam is also discussed briefly.

At the 3.5 GeV electron accelerator ELSA of Bonn University, polarized electrons have been accelerated up to 2.0 GeV, crossing several depolarizing resonances without significant depolarization. The polarized electrons were produced by photoemission from an AlGaAs–GaAs superlattice crystal using circularly polarized laser light. A harmonic correction scheme was successfully applied to the third imperfection resonance ( γa =3) and the polarization required for the planned experiments was conserved up to 2.0 GeV.

At the electron stretcher accelerator ELSA of Bonn University an external fixed target experiment using a polarized electron beam has been started. To provide a polarized beam with sufficient intensity a dedicated source for polarized electrons is used (with a GaAs-AlGaAs superlattice photocathode). To prevent depolarization during acceleration in the circular accelerators several depolarizing resonances have to be corrected for. One type of resonances (the intrinsic ones) is compensated using two pulsed betatron tune jump quadrupoles. This method is applied because adiabatic crossing techniques that became popular in proton accelerators in the last years cannot be used in an electron accelerator due to synchrotron radiation effects at higher energies.

The Electron Stretcher Accelerator (ELSA) of Bonn University is used to provide an external electron beam of high duty factor in the energy range from 0.5-3.5 GeV for fixed target experiments. In the near future polarized electron beams will be accelerated up to the highest energies. In order to avoid depolarization of the beam by imperfection and intrinsic resonances the control of the closed orbit and the symmetry of the ring lattice has to be improved. For calibration of the optics model of ELSA the orbit response matrix has been measured and fitted with help of the program CALIF. The strengths of the two quadrupole families and the relative scaling factors for the beam position monitors and corrector magnets have been determined this way. The results of the fits and a comparison with measurements of the optical functions of ELSA will be presented.

At the 3.5 GeV electron accelerator ELSA (Bonn University) an upgrade for polarized electrons is under way. A source of polarized electrons (GaAs crystal, electrons produced by photoeffect using circular polarized laser light) is operational. Recently, studies have been started to minimize the losses in polarization level due to crossing of depolarizing resonances, which necessarily exist in circular accelerators (storage rings). Calculations concerning different correction schemes for the depolarizing resonances in ELSA are presented, and first results of measurements, done by means of a Moller polarimeter in one of the external beamlines, are mentioned.

Polarized electrons have been accelerated in the electron stretcher accelerator ELSA for the first time. Up to 2.1 GeV the polarization of the electron beam supplied by the 120 keV polarized electron source has been measured with a Mo/ller polarimeter. Preliminary results of polarization measurements at high energies and the performance of the source are presented.

Angular distributions of the differential cross section for the reaction γ +p→ π 0 +p have been measured at the Bonn 2.5 GeV Synchrotron at c.m. angles between 0° and 65° in the energy range from 1.3 GeV to 2.2 GeV. The π 0 mesons were detected by their two decay photons. The data are compared with data of other laboratories and the Metcalf-Walker analysis.

Angular distributions of the target symmetry for the reaction γ + p → π0 + p have been measured at the Bonn 2.5 GeV Electron Synchrotron at pion c.m. angles between 13° and 63° and photon energies of 1.0 and 1.1 GeV. The π0 mesons were detected by their two decay photons with total absorption lead-glass Cerenkov counters. Butanol was used as target material in a continuous flow 3He cryostat operating at 0.5 K and 25 kG. The π0 counting rate from free protons in the butanol target was derived from the measurements of the differential cross section on hydrogen. The data are compared with data of other laboratories and the results of two recent partial-wave analyses.

Four angular distributions of the differential cross section of the reaction γ + p → π 0 + p have been measured at the Bonn 2.5 GeV Synchrotron at pion c.m. angles between 3° and 63° in the photon energy range from 0.9 GeV to 1.2 GeV. The π 0 mesons were detected by their two decay photons. The data are compared with data of other laboratories and predictions of two recent partial-wave analyses.

The polarization degree of accelerated longitudinally polarized electrons has been measured employing the spin dependence of the elastic electron-electron cross section. We observe strong depolarization as well as polarization flips in the neighbourhood of imperfection and intrinsic resonances. Measures to maintain the initial polarization degree up to the highest available energies are discussed.

ELSA (Electron Stretcher and Accelerator) operates in two different modes. Up to the maximum energy of the synchrotron it works at a constant magnetic field. The electrons from the synchrotron which runs at 50 Hz repetition rate are injected, stored, and ejected at a constant rate. In this operation mode the macroscopic duty cycle is 95 % at least. In the operation mode of post acceleration which is possible up to 3.5 GeV the duty cycle is reduced to 60 %. The intensity in this operation mode is 5 % of that in the stretcher mode. Higher intensities are available at lower duty cycle. ELSA mainly is dedicated to feed a tagged photon facility.

The differential cross section of the reaction γ + p → p + πo has been measured at the Deutsches Elektronen- Synchrotron, Hamburg, at mean phonon energies of 1.36, 2.0 and 3.0 GeV and pion center of mass angles between 0o and 70o. The results are compared with some theoretical calculations based on elementary and reggeized 1- meson exchange.

Polarized electrons have been accelerated in a synchrotron for the first time. The experiments clearly state that acceleration of a polarized beam in a synchrotron is possible without loosing polarization at least up to 2 GeV. Strong depolarization is observed only in narrow energy bands due to imperfection and intrinsic resonances. Careful measurements of a certain imperfection resonance allows to do a very precise absolute energy calibration of the synchrotron.

Polarized electrons have been accelerated for the first time in a synchrotron with the results that the polarization is maintained to a high degree up to 2.0 GeV. Strong depolarization only is observed in narrow energy bands due to imperfection and intrinsic resonances.

ELSA (Electron Stretcher and Accelerator) operates in two different modes. Up to the maximum energy of the synchrotron it works at a constant magnetic field. The electrons from the synchrotron which runs at 50 Hz repetition rate are injected, stored and ejected at a constant rate. In this operation mode the macroscopic duty cycle is 95% at least. In the case of post acceleration which is possible up to 3.5 GeV the duty cycle is reduced to 70%. The intensity in this operation mode is 6% of that in the stretcher mode. Higher intensities are available at lower duty cycle. ELSA mainly is designed to feed a tagged photon facility.