K. Cornelis

Measurements of impedance distributions and instability thresholds in LEP

The distribution of reactive and resistive components of the transverse impedance around the circumference of LEP has been obtained by exciting the beam and measuring the variations of phase advance and of closed orbit displacements with beam current. Tune shifts with current and instability thresholds were determined for two machine lattices with different beta functions and momentum compaction for various values of bunch length and synchrotron frequency. These measurements are particularly interesting for LEP2, which will require larger bunch currents to achieve the design luminosity.

Low emittance lattice for LEP

In order to obtain the largest luminosity with LEP2, it is attractive to make the beam emittance as small as possible because the beam-beam effect is not a limitation at the energy of E/spl ap/90 GeV for the obtained bunch currents. This can be achieved with a high tune lattice. Two possible candidates are lattices with a horizontal phase advance of 108/spl deg/ or 135/spl deg/ per cell. Both have a vertical phase advance of 60/spl deg/. These lattices were developed during 1994 and the results are presented. Tests to reach high intensity for the 108/spl deg/ lattice were performed and the bunch current achieved is compared with expectations. For this lattice the detuning vs. intensity and several optics parameters were measured as well.

The 1991 dynamic aperture experiment at the CERN SPS

This year’s dynamic aperture experiments at the SPS concentrated on the study of the diffusion enhancement by tune modulation in presence of strong nonlinear fields. The aim of this experiment is to specify an upper limit for the power supply ripple in view of the LHC, the proposed hadron collider in the LEP tunnel. Due to strong, unavoidable non‐linearities of the superconducting magnets we expect this accelerator to be very sensitive to power supply ripple. An additional tune modulation, large in comparison with the natural one, was therefore introduced in the SPS and a particle diffusion was meaured for several modulation amplitudes. The combined effect of two frequencies with equal ampltiude was also tested. In parallel long‐term tracking studies have been performed, which allow to find the dynamic aperture for a model of the SPS with the added non‐linearities and tune modulations.

Measurements of the SPS transverse impedance in 2000

We report on measurements of coherent tune shifts, head-tail growth rates, and current-dependent betatron phase advances at the CERN SPS in the year 2000. Comparing results obtained at two different energies shows that there is no notable contribution from space charge. Within the measurement resolution the impedance is the same as in 1999, consistent with the expected small effect from changes to only a small number of pumping ports. In 2000, data were taken over an expanded range of chromaticities, which increases the sensitivity to the impedance frequency distribution. Measurements of the current-dependent phase advance around the ring help localizing the most important impedance sources.

The LEP impedance model

The bunch current in LEP is limited by collective effects slightly below its design value. To understand this limitation, a set of experiments has been carried out to obtain a model of the impedance seen by the circulating beam. From measurements of the current dependance of the synchrotron and betatron frequencies information about the longitudinal and transverse reactive impedance has been obtained. The growth rate of the m=1 head-tail instability has been observed to gain some knowledge of the resistive transverse impedance. The dependence of the transverse mode-coupling threshold on bunch length has been measured. The energy loss per turn of a stable bunch is given by the longitudinal resistive impedance and has been measured by recording the change of the synchronous phase angle with current. All these quantities represent integrals over products of bunch mode spectra and impedances. By measuring them for different bunch lengths the frequency dependence of the impedances can be estimated. From these results a model of the LEP impedance has been obtained which can be used to estimate the current limitation under different operating conditions.<<ETX>>

Energy loss of proton and lead beams in the CERN-SPS

The energy loss of an unbunched beam circulating in the CERN-SPS has been obtained from the observed frequency change of a longitudinal Schottky signal. This experiment was carried out for protons at 14, 120 and 270 GeV/c and for lead ions Pb82/208 at Z/spl middot/270 GeV/c momentum. The dominant effects which determine the energy loss are synchrotron radiation, ionization of the residual gas and parasitic mode loss in the resistive longitudinal impedance. Since all the protons in a lead nucleus radiate coherently the synchrotron radiation is proportional to Z/sup 2/ like the other effects. The experimental results are analyzed and the contributions of the individual effects determined. Using an impedance of |.Z/n|/spl ap/12 /spl Omega/ gives the best fit through the experimental data.

Energy Calibration of the SPS with Proton and Lead Ion Beams

The momentum of the 450 GeV/c proton beam of the CERN Super Proton Synchrotron was determined by a high precision measurement of the revolution frequencies of proton and lead ion beams. To minimize systematic errors the magnetic cycle of the SPS had to be rigorously identical for both beams, and corrections due to Earth tides had to be taken into account. This paper presents how the beam momentum was determined from the RF frequency for which the beams are centered in the machine sextupoles. The measured beam momentum is 449.16 ± 0.14 GeV/c for a nominal momentum of 450 GeV/c, and the accuracy is limited by systematic errors.

Measurement of chromatic effects in LEP

The chromaticity produced by the insertions and the lattice of LEP is corrected in the FODO lattice of the arcs which has horizontal and vertical phase advances of 90/spl deg/ and 60/spl deg/ for the LEP optics presently used. This is done with two horizontal and three vertical sextupole families. To check this correction scheme the non-linear chromaticities have been measured by observing the betatron tunes as a function of RF-frequency, i.e. of momentum deviation. The differences between the results and the expected effects of the sextupoles are interpreted in terms of octupole and decapole components in the dipole magnets. To check the distribution around the ring of the chromaticities and their corrections the betatron phase advances have been measured as a function of momentum deviation. This was done by observing an excited betatron oscillation for 1024 turns in all beam position monitors for different RF-frequencies. The results clearly show the negative chromaticity produced in the straight sections and its correction in the arcs. The parasitic sextupole components are found to be spread uniformly in the arcs within the measurement errors. Under this condition one finds that they have a negligible effect on the dynamic aperture.

LEP operation in 1992 with a 90/spl deg/ optics

The optics for physics operation in LEP was changed from 60/spl deg/ to 90/spl deg/ at the start of 1992 with a view to improved Z/sup 0/ production, preparation for future operation at higher energies and the use of the same optics in machine developments. The developments included running LEP with twice the number of bunches and using resonant depolarisation for energy calibration, Perturbation to steady operation was felt at the start of the year but was soon overcome as the benefits of smaller emittances were realised. The peak luminosity increased to 1.15 10/sup 31/ and the luminosity lifetime improved. New operational software halved the time taken between dumping one coast and the start of data taking on the next. The 8+8 bunch operation was introduced as routine operation for the last month. Overall, there was an increase in integrated luminosity from 17.6 inverse picobarns per experiment in 1991 to 28.6 in 1992. Along with improvements in detector efficiency, almost 3 million hadronic Z/sup 0/s were recorded by the four experiments, an increase from 1.27 million in 1991.<<ETX>>

High luminosity performance of the SPS proton-antiproton collider

In September 1988, the CERN SPS (Super Proton Synchrotron) collider was brought into operation for the first physics production run with the improved antiproton production and accumulation complex (AAC). The increased antiproton flux immediately resulted in a considerable improvement in the machine performance with initial luminosities exceeding 2.4*10/sup 30//cm/sup 2/ s, and weakly integrated luminosities an order of magnitude greater than achieved in the past. A total integrated luminosity of 3.4 inverse pbarns was produced, a factor of 5 greater than the best achieved in previous years. Performance statistics and operational experience in this new regime are discussed.<<ETX>>