Malika Meddahi

IS LEP beam-beam limited at its highest energy?

The operation of LEP at 45.6 GeV was limited by beam-beam effects and the vertical beam-beam parameter /spl xi//sub y/ never exceeded 0.045. At the highest energy of 94.5 GeV, the increased damping allows higher beam-beam parameters /spl xi//sub y/. Values above 0.07 in the vertical plane averaged over four experiments have been obtained frequently with peak values up to 0.075 in a single experiment. Although the maximum intensity in LEP is presently limited by technical considerations, some observations indicate that the beam-beam limit is close and the question of the maximum possible values can be raised. These observations are shown in this paper and possible consequences are presented. The optimum operation of LEP in the neighbourhood of the beam-beam limit is discussed.

Experiments with bunch trains in LEP

During 1994 tests were made to evaluate the possibility of operating LEP with four equidistant bunch trains in each beam during 1995. A train will consist of up to four bunches with a bunch spacing of some 75 m. The bunch trains collide head-on. They are separated at the parasitic collision points on either side of the interaction points by vertical electrostatic separator bumps. The accumulation of bunch trains in a single beam was studied with magnetically simulated bumps. Following the installation of additional separators late in 1994, the parasitic encounters of the bunch trains near the interaction points were studied for one train of electron bunches colliding with one train of positron bunches in two diametrically opposite interaction points. The higher order mode losses in the superconducting RF cavities were measured for several arrangements of beams, trains and bunches. The synchrotron radiation background, enhanced by the off-centred beams in the quadrupoles inside the separator bumps, is reduced by super-imposing asymmetric magnetic bumps such that the offset of the incoming beams is reduced.

Design and performance of the cngs secondary beam line

An intense muon-neutrino beam (1017 Vmu/day) is generated at CERN and directed towards the Gran Sasso National Laboratory,LNGS, in Italy, 732 km from CERN. In the presently approved physics programme, it is foreseen to run the CNGS facility with 4.5 middot1019 protons per year for five years. During a nominal CNGS cycle, i.e. every 6s, two nominal SPS extractions of 2.4. 1013 protons each at 400GeV/c are sent down the proton beam line to the target. The CNGS secondary beam line, starting with the target, has to cope with this situation, which pushes the beam line equipment and instrumentation to the limits of radiation hardness and mechanical stresses during the CNGS operation. An overview of the CNGS secondary beam line is given. Emphasis is on the target, the magnetic focusing lenses (horn and reflector) and the muon monitors. The performance of the secondary beam line during beam commissioning and physics operation is discussed and measurements are compared with simulations.

Low emittance lattices for LEP

In 1997 LEP will enter its third phase and will be operated at energies well above 90 GeV. In order to reach the required luminosities at these higher energies, i.e., to reach the maximum beam-beam tune-shift parameter, an optics with a small horizontal emittance is desirable. Such a lattice must have a dynamic aperture sufficient to guarantee the beam life time. Several lattices with different phase advances per cell have been developed for this purpose and the reasons for these particular choices are explained. The relative merits of these different solutions as well as the experience gained both in dedicated experiments and in using these lattices in regular operation during 1996 are discussed.

Linac-based Proton Driver for a Neutrino Factory

A Neutrino Factory Proton Driver based on a superconducting proton linac has been designed in the CERN context. The 5 GeV/4 MW H - beam from the linac is accumulated using charge exchange injection in a fixed-energy synchrotron and afterwards transferred to a compressor ring, where bunch rotation takes place. The lattices of the accumulator and compressor are described, as well as magnet technology and RF manipulations. Critical issues related to charge-exchange injection, space-charge effects in the compressor and beam stability in the accumulator, are addressed. The analysis is focused on the baseline scenario, which provides 6 bunches on the target. Results of preliminary analysis of options with less bunches (three and one) are also presented.

Proton antiproton collisions at a finite crossing angle in the SPS

Very little experimental data is available on the effect of a crossing angle in hadron colliders with bunched beams. The electrostatic separators of the CERN SPS (Super Proton Synchrotron) used to separate the orbits in its normal operation as a proton-antiproton collider were used to establish a horizontal crossing angle in one of the interaction regions. The authors have studied the effect of the crossing angle on the beam dynamics to compare them with theory and to identify possible limitations for future high-luminosity colliders, in particular for the Large Hadron Collider currently under study at CERN.<<ETX>>

Analysis of the SPS Long Term Orbit Drifts

The Super Proton Synchrotron (SPS) is the last accelerator in the Large Hadron Collider (LHC) injector chain, and has to deliver the two high-intensity 450 GeV proton beams to the LHC. The transport from SPS to LHC is done through the two Transfer Lines (TL), TI2 and TI8, for Beam 1 (B1) and Beam 2 (B2) respectively. During the first LHC operation period Run 1, a long term drift of the SPS orbit was observed, causing changes in the LHC injection due to the resulting changes in the TL trajectories. This translated into longer LHC turnaround because of the necessity to periodically correct the TL trajectories in order to preserve the beam quality at injection into the LHC. Different sources for the SPS orbit drifts have been investigated: each of them can account only partially for the total orbit drift observed. In this paper, the possible sources of such drift are described, together with the simulated and measured effect they cause. Possible solutions and countermeasures are also discussed.

Overview of LEP operation in 1998

After the installation of 32 additional RF cavities in the 1997-1998 shutdown LEP was operated at a beam energy of 94.5 GeV. The total integrated luminosity for the year 1998 clearly surpassed its target and reached 198 pb/sup -1/. Vertical beam-beam tuneshifts of more than 0.07 were obtained. The performance did not seem to be beam-beam limited, but the total beam current was limited by power dissipation problems to around 6 mA. A high phase advance optics (102/spl deg/, 90/spl deg/), with a smaller natural emittance, was used for regular operation in 1998. This contributed to the excellent performance of LEP, together with the further reduction of both the horizontal and vertical beta function at the interaction points. No dynamic aperture problems were encountered.