C. Fischer

Wire scanners at LEP

Two sets of wire scanners, each for measuring the horizontal and vertical profile, are installed in LEP in a straight section where the dispersion in both planes is zero. The authors present the design and discuss some limitations of the instrument. A carbon fiber with a diameter of 36 mu m moves through the beam with a speed of about 0.5 m/s. The bremsstrahlung photons generated by the passage of the wire through the beam are detected in scintillators located 80-m downstream. During the first months of LEP operation, the fibers were destroyed occasionally. The various causes, tests and remedies are discussed. At injection energy a significant blowup of the beam results from the wire scan and has to be taken into account for the estimation of the genuine emittance. A model of this blowup is proposed, where the effect is renormalized on the actual measured data. This provides an effective data treatment to obtain the unperturbed beam size.<<ETX>>

Quartz wires versus carbon fibres for improved beam handling capacity of the LEP wire scanners

After the first investigations were performed in 1994, the study of thermal effects on carbon and quartz wires was pursued in 1995. Carbon wires of 8 μm have been studied. Light emission resulting from the two heating mechanisms, electromagnetic fields and collision losses with the beam, were observed. Quartz wires of 10 and 30 μm were investigated and light emission due to the heating by collision with the beam was observed. The heat pattern differs completely from that of carbon fibres. The quartz wires withstood circulating currents of at least 8 mA at 20 GeV, the 1995 operational level in LEP. Quantitative evaluations and the influence of various dissipation processes are presented with the aim of evaluating a beam current limit.

Design and Tests of a New Rest Gas Ionization Profile Monitor Installed in the SPS as a Prototype for the LHC

Based on the encouraging results obtained with a Rest Gas Ionization Profile Monitor of a first generation, a new monitor was designed and then installed in the SPS at the beginning of 2002. Its design fulfills all the requirements for a future installation in the LHC where four such monitors are foreseen. After the initial tests performed during the run of 2002, a few upgrading steps appeared necessary mainly in order to cope with the nominal LHC beam characteristics. They were implemented during the subsequent winter stop and the operation of the monitor was resumed in 2003 under various conditions of beam, ranging from an LHC pilot bunch up to beams having nominal distributions in bunch number, intensity and energy in the SPS for injection into the LHC. After a description of the monitor design, the measurements performed with the instrument during these last two years are discussed with the difficulties encountered and the corresponding implemented cures. Data acquired in 2003 on the whole spectrum of LHC beam characteristics are presented and the modifications made to prepare the 2004 campaign with a view to still improve the performance are also discussed.

Observation of thermal effects on the LEP wire scanners

A new wire scanner was installed in LEP for the 1994 run. It incorporates an improved mechanical design for the wire movement and can accept three mechanisms, two horizontal and one for vertical scans. Wires of different materials and of various diameters were fixed to new ceramic forks. Viewing ports were installed opposite each wire to allow the direct observation of the wires with a TV camera during a scan. Recordings of the wire resistance and of the light emitted by the wire were made during scans. These observations have provided the first experimental evidence of the various wire heating mechanisms by the beams. The heating due to coupling to the electro-magnetic field generated by the beam exceeds the heating due to particle collisions. An evaluation is made on the behaviour of carbon and quartz wires. Conclusions are drawn for defining a safe operating regime.

Performance of the CERN ISR at 31.4 GeV

Due to the recent improvements in phase displacement acceleration, operating techniques and beam diagnostics, considerable progress has been achieved in operating the ISR at the maximum energy of 31.4 GeV (this centre of mass energy corresponds to a 2 TeV fixed target machine). High intensity stacks are stored at 26.6 GeV before acceleration by phase displacement to 31.4 GeV. At this energy up to 34 Amps are obtained with corresponding initial luminosities of 2.1031cm-2s-1 per intersection and 4.3 1031cm-2s-1 in a low-ß intersection. Due to the long beam lifetime, physics data taking may be performed over periods of 60 hours. The principle of phase displacement and the associated beam phenomena are discussed together with the control of the magnetic machine and the mutual interaction of the two beams. The operational technique is described and it is shown that the present day performance of the ISR at 31.4 GeV is comparable to the previous performance (1977) at 26.6 GeV. Consequently, in 1978 about 90% of the time requested for physics data collection was at 31.4 GeV.

LHC beam instrumentation

Six years before the scheduled commissioning of the LHC at CERN, the basic list of beam instruments has been established. This early date is needed due to the impact of the mechanical design of some detectors (mainly the beam position detectors) on the cryogenic part of the machine as well as for other projects due to the long R&D period (emittance measurements, tune and chromaticity diagnostics and control). This paper gives a detailed overview of the basic requirements and specifications of all beam instruments foreseen for transfer lines and main rings.

Investigations on beam damping simulations and the associated model of CLIC

Controlling the beam stability in the CLIC main linac must be investigated numerically. Strong damping is required to cope with wakefield effects and machine imperfections require careful optics adjustments. The computation of the longitudinal and transverse wakefields has been reviewed and the Greens functions reevaluated for the most recent main cavity design. BNS damping and autophasing with magnetic and microwave focusing could then be simulated and the dependence of the results on parameters such as the RF phases and voltage gradients could be studied. Since the emittance is extremely small, beam break-up is sensitive to unavoidable misalignments and algorithms of trajectory corrections have to be investigated.<<ETX>>

The LEP injection monitors: design and first results with beam

The LEP (Large Electron Positron collider) injection monitors are comprised of split foil monitors, luminescent screens, and beam stoppers. The monitors are described with particular emphasis on their special features, including their low loss factors, their protection against synchrotron radiation, and the screen read-out with a CCD (charge coupled device) chip. The results obtained during the positron injection tests in the LEP in July 1988 are reported.<<ETX>>

Operation of the CERN-ISR for High Luminosity

The CERN Intersecting Storage Rings are routinely operated at 26 GeV/c for physics experiments with proton beam intensities greater than 25 Amps and luminosities greater than 2.1031cm-2sec-1. Six out of eight intersection regions are used concurrently for colliding beam physics experiments. Each experiment is sensitive to background caused by protons lost from the stacked beams and locally induced radiation. Operational techniques for each of the essential processes involved in establishing stable beam conditions are reviewed and methods of maintaining minimal beam loss rates and the control of background in individual intersections throughout the beam lifetime of about 40 hours are presented.

Beam scraping for lhc injection

Operation of the LHC will require injection of very high intensity beams from the SPS to the LHC. Fast scrapers have been installed and will be used in the SPS to detect and remove any existing halo before beams are extracted, to minimize the probability for quenching of superconducting magnets at injection in the LHC. We briefly review the functionality of the scraper system and report about measurements that have recently been performed in the SPS on halo scraping and re-population of tails.