R. Jung

The LEP synchrotron light monitors

Four monitors are installed in LEP (Large Electron-Positron colliding beam accelerator), two per particle type. Two of the monitors are at dispersion-free locations which permit measurement of the transverse emittance, estimation of the energy spread and detection of various instabilities. The light has its origin in the main bending magnets. The source is imaged onto the detectors by means of catadioptric optics. Two detectors are used, one a charge coupled device (CCD) matrix and the other made up of a fast optoelectronic shutter and wavelength shifter coupled to a CCD matrix. Observations can either be made in the normal TV mode or in digital modes where one or more profiles (burst mode) are stored on the CCD and then digitized. In the burst mode, up to 18 profiles separated by 1 to 256 turns can be acquired. The results are presented either as data mixed with the TV picture or as two or three dimensional plots and projections on a workstation. Results and present performance are presented.<<ETX>>

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.

Performance and operational experience of the LEP synchrotron light telescopes

The experience and performance over the past three and a half years of operation is reported. The dynamic range of the monitor has been increased by an improved cooling of the CCD chip with the help of Peltier cells. The contributions from the diffraction and the depth of field have been evaluated with precision in order to estimate the beam emittances. Comparisons and cross calibrations with the wire scanners have been started. A video frame grabber and fast projection calculation for real time beam size display has been implemented.<<ETX>>

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.

Design of a new generation of collimators for LEP 200

One hundred and twenty six movable collimator blocks have been installed for the first phase of LEP. They have proved indispensable for providing good conditions for data taking of the experiments and gave no problems for running LEP. Sixty four additional movable blocks are planned to be added for the second phase of LEP (LEP 200 project). The collimators will consist of copper blocks with tungsten inserts or of spherical tungsten blocks. Their design is adapted to maintain the original low RF loss factor of 0.06 V/pC at the nominal aperture with reduced variations over the entire useful stroke.<<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>>

High sensitivity beam intensity and profile monitors for the SPS extracted beams

Secondary emission monitors using caesium iodide coated thin aluminium foils have been installed in the SPS transfer channels to monitor the intensity of the extracted heavy ion beams. Tests have shown an increase by a factor twenty of their sensitivity with respect to bare aluminium foils. Luminescent screens viewed with TV cameras are used to monitor the position and the profiles of the extracted beams. Various luminescent screen materials have been tested. Results on chromium doped alumina, thallium doped caesium iodide and quartz are reported. A dynamic range of 10/sup 3/ in beam intensities can be achieved by using these three materials in turn in the usual three screen tanks. Intensifiers used together with CCD cameras and video frame grabbers with incorporated projection calculations are used in conjunction with these screens. Results with heavy ions in the transfer channels and with protons extracted from circulating beams in the SPS are given. Detection sensitivities down to a few tens of protons per video frame have been observed.<<ETX>>