J. Koopman

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.

First results of the beam gas ionization profile monitor (BGIP) tested in the SPS ring

The BGIP is a proposal for a new, non-destructive beam profile monitor for the future Large Hadron Collider (LHC). This device provides the rms beam size value by means of the analysis of the velocity spectrum of the rest gas ions created and accelerated by the beam itself. After a thorough computer simulation study of the related physics, a first prototype of the BGIP has been conceived, built up and installed in the SPS main ring during 1999. This paper contains a short presentation of the simulation work and a description of the test set-up. The first experimental results are presented and compared with theoretical computations.