Large Electron Positron Storage Ring Research Articles

Large high-vacuum systems for CERN accelerators

CERN operated over the more than 50 years of its existence particle accelerators and storage rings ranging from a few tens of metre to 27 km, the size of its latest project, the Large Hadron Collider (LHC) which is under construction and will be started in 2008. The challenges began with the Intersection Storage Rings (ISR) in the seventies. With a beam pipe length of 2 × 1 km, this accelerator required innovative solutions like bake-out and glow discharge to achieve the required static vacuum level, fight against beam-induced pressure increases and cancel beam neutralisation by trapped electrons. The vacuum system of the Large Electron Positron (LEP) storage ring (in operation between 1989 and 2001) of a total length of 27 km had to cope with very high levels of synchrotron power. The beam vacuum system of LHC (2 × 27 km) integrates some parts at 1.9 K and others at room temperature and will also have to cope with dynamic effects. In addition to the beam vacuum system, LHC requires insulation vacuum for the superconducting magnets and the helium distribution line. Whereas the required pressure is not very low, the leak detection and localisation is significantly more demanding for the insulation vacuum than for the beam vacuum because of the large volumes and the thermal insulation.When the size of an accelerator grows, the difficulties are not only to get a clean and leak tight vacuum system, but also to be able to measure reliably pressure or gas composition over long distances. Furthermore, in the case of LHC the integration of the beam vacuum system was particularly difficult because of the complexity induced by a superconducting magnet scheme and the reduced space available for the beam pipes. Planning and logistics aspects during installation, including the usage of mobile pumping and diagnostic means, were much more difficult to manage in LHC than in previous projects.

The ring imaging Cherenkov detector of DELPHI

The ring imaging Cherenkov detector system in the DELPHI experiment at the Large Electron-Positron storage ring at CERN, is designed to do particle identification over most of the solid angle in the momentum range from ∼2 GeV c to ∼40 sol GeV c . Two radiator media are used to cover the momentum range; (i) a 1 cm layer of liquid C 6F 14, and (ii) a volume filled with gaseous C 5F 12 or C 4F 10. Photosensitive time projection chambers record the conversion points of the ultraviolet photons produced in both radiator systems. The total active area is ∼30 m 2 in the barrel region and ∼8 m 2 in the two endcaps. The design of the detector systems is described in this paper. We will also report performance figures and compare them to simulation studies.

A drift tube detector for the DELPHI experiment

A wire chamber for an experiment at the CERN large electron-positron storage ring is described, which uses a mass conductive plastic as the cathode material for the construction of the detector. The development of the conductive plastic has been part of this project. This new material allows the information from the outer surface of the cathode to be picked up with metallic strips. The design of a precision table for positioning the anode wires was also included in this work. Construction and running experience is reported, with a description of various problems encountered and the efforts made to optimize this detector. Aging tests are also described.

First observation of transverse beam polarization in LEP

The results of beam polarization measurements performed in 1990 at the CERN Large Electron Positron storage ring (LEP) are reported. A significant asymmetry was observed in the Compton back-scattered photon distribution when illuminating the LEP electron beam with circularly polarized laser light. The corresponding polarization level is estimated to be 9.1% ± 0.3% (statistical) ± 1.8% (systematic). The validity of the polarization signal was assessed by varying the laser light polarization and by applying to the electron beam known depolarizing resonances. The measurement is consistent with the predicted polarization degree.

Measurement of electroweak parameters fromZ decays into fermion pairs

We report on the properties of theZ resonance from 62 500Z decays into fermion pairs collected with the ALEPH detector at LEP, the Large Electron-Positron storage ring at CERN. We findM Z=(91.193±0.016exp±0.030LEP) GeV, Γ Z =(2497±31) MeV, σ 0had =(41.86±0.66)nb, and for the partial widths Γinv=(489±24) MeV, Γhad(1754±27) MeV, Γ ee =(85.0±1.6)MeV, Γ μμ =(80.0±2.5) MeV, and Γττ=(81.3±2.5) MeV, all in good agreement with the Standard Model. Assuming lepton universality and using a lepton sample without distinction of the final state we measure Γu=(84.3±1.3) MeV. The forward-backward asymmetry in leptonic decays is used to determine the vector and axial-vector weak coupling constants of leptors,g 2v (M 2 Z )=(0.12±0.12)×10−2 andg 2A (M 2 Z )=0.2528±0.0040. The number of light neutrino species isN ν=2.91±0.13; the electroweak mixing angle is sin2θW(M 2Z )=0.2291±0.0040.

Chemical cleaning of aluminum alloy surfaces for use as vacuum material in synchrotron light sources

Photon and electron desorption from the vacuum chamber walls of electron storage rings such as the proposed advanced photon source (APS) are sometimes responsible for the production of large gas loads during operation, even in systems with very good static vacuum. The gas released by beam-induced desorption results in scattering of the beam electrons, and a consequent reduction in the beam lifetime. In extreme cases, the beam-induced outgassing may cause so much scattering that it is not possible to obtain the design goals with regard to obtainable beam current. Consequently, it is important that the surfaces that are exposed to the electron beam and photon fluxes contain as little trapped gas as possible, and that the gas burden during operation be kept as low as possible. The current study investigates the effectiveness of a chemical cleaning treatment developed at the Center for European Nuclear Research (CERN) for the large electron-positron (LEP) storage ring [A. G. Mathewson, LEP Vacuum Tech, Note Jan. 20, 1986; A.G. Mathewson, LEP Vacuum Tech. Note Jan. 15, 1986; A. G. Mathewson, et al., J. Vac. Sci. Technol. A 7, 77 (1989).] as applied to the 6063 alloy to be used in APS. Additionally, depth profiling of the chemically-cleaned samples and samples that were vapor-degreased only, provides new insight on the cleaning process as it applies to the alloy proposed for use in APS. To reduce the beam-induced outgassing of the APS vacuum chamber, a two-step cleaning process has been investigated, based on the treatment utilized for the 6060 Al used in the LEP vacuum chamber at CERN. The treatment is based on an understanding of the near-surface composition as follows: The surface consists of a superficial MgO layer, which is also contaminated with hydrocarbons, lying on top of another layer that is primarily Al2O3 (A.G. Mathewson, LEP Vacuum Tech. Note Jan. 20, 1986). In the first step of the cleaning process, MgO and C are removed by Almeco 18, an alkaline detergent manufactured by Henkel A. G. In the second step, Amklene, a modified KOH solution which is not capable of removing magnesium oxide, is believed to reduce the thickness of the remaining aluminum oxide layer. The current results suggest a modified interpretation of the mechanism by which these chemical cleaning agents alter the surface of the 6063 aluminum alloy. The surface contains both Mg (as MgO) and Al2O3, and the MgO concentration extends much deeper into the bulk than the aluminum oxide. Despite profiling reveals that the underlying aluminum is largely metallic, rather than Al2O3. The aluminum oxide layer observed after the Almeco 18 cleaning step is largely the native aluminum oxide layer that forms during room temperature exposure to air between cleaning operations. The Amklene step does little to remove the native aluminum oxide layer and at the same time acts to restore some of the original MgO layer. The present results do not show any obvious benefits arising from the use of Amklene after treatment in Almeco 18.

Industrial fabrication of uhv quadrupole chambers for LEP

CERN has constructed a Large Electron Positron Storage Ring (LEP) for colliding beam experiments. Particles storage requires an evacuated (uhv) beam pipe at p < 3 × 10 −7 Pa. The quadrupole vacuum chambers (VCQ) are made of aluminium without their own pumping system, hence they must have a very low outgassing rate. For the manufacture of 2635 m of VCQ, the Plasma arc welding process was chosen. After vacuum commissioning at 150°C, all VCQ had a vacuum limit corresponding to a specific outgassing rate lower than 2 × 10 −13 Pa m 3 s −1 cm −2.

Characteristics of a large system of pad readout wire proportional chambers for the HPC calorimeter

A large system of wire proportional chambers is being constructed for the readout of the High-Density Projection Chamber of the DELPHI experiment at the Large Electron-Positron (LEP) storage ring. The system consists of 144 chambers, each 0.3-m/sup 2/, and read out via cathode pads. located at the end of the HPC drift volume. The authors discuss the design of the readout chamber; the chamber construction, the mechanical tolerances and gain uniformity, and the performance. >

Proportional chambers for the barrel hadron calorimeter of the L3 experiment

The design and the mass production of the proportional wire chambers for the barrel part of the uranium-gas sampling hadron calorimeter of the L3 detector at the CERN Large Electron-Positron storage ring (LEP) are described. The chambers meet the specific requirements arising from the limited space available to the calorimeter, the radioactivity of uranium, and the physics criteria of good energy and position resolution for incident hadrons. The mass production techniques employed ensured that all of the 371664 chamber cells have uniform response (with 5% accuracy) to ionizing particles. Over 8000 chambers of the same design, in 53 different sizes, were manufactured and tested at the ITEP (Moscow), at the University of Michigan (Ann Arbor) and at the IHEP (Beijing).

Comparison of chemical cleaning methods of aluminum alloy vacuum chambers for electron storage rings

Three chemical precleaning methods suitable for aluminum alloy vacuum chambers for electron storage rings have been compared. The methods ranged from vapor degreasing with no chemical attack to light alkaline etching and strong alkaline etching in NaOH. The techniques used to evaluate and compare the three treatments included Auger analysis, thermal outgassing, electron and x-ray induced neutral gas desorption, argon glow discharge cleaning, and scanning electron microscope examination of the surface. In general, the NaOH treatment was best followed closely by the light alkaline etch and then the vapor degreasing. A variant of the light alkaline etch method was used to clean the 2000 Al vacuum chambers of the CERN large electron positron storage ring project. Two chambers cleaned by this method were exposed to synchrotron radiation in a dedicated beam line of a synchrotron radiation source. Compared to a reference chamber cleaned by NaOH these two chambers were initially worse, but after 1 A h of machine operation the difference practically vanished.

The large electron positron storage ring preinjector vacuum systems: Design and performance

The large electron positron storage ring (LEP) preinjector will provide the particles to be stored in the LEP. It is composed of a high-intensity linear accelerator (linac) providing 200-MeV electrons for the production of positrons, a low-intensity linac (LIL) for the acceleration of electrons and positrons to 600 MeV (LIL), and an electron positron accumulator (EPA) ring to collect dense bunches of electrons and positrons at 600 MeV (EPA). The LIL rf accelerating cavities and waveguides are typical of severely conductance limited vacuum systems with high rf induced gas loads, in which a low hydrocarbon free residual pressure has to be maintained (5×10−8 mbar). For the EPA ring, subjected to synchrotron radiation induced gas desorption, considerations of beam lifetime require a base pressure in the 10−9 mbar range, achievable without in situ bakeout in the shortest possible time. The design features with particular emphasis on conceptual choices, cleanliness and special treatments, new hardware, low outgassing achievements, as well as cost and initial performances of these vacuum systems are presented.

Controls for the vacuum system of LEP

The very large size (27-km circumference) of LEP (CERN Large Electron Positron storage ring) makes it mandatory to provide extensive remote control for installation, commissioning, and operation. The control facilities will be available both at fixed locations and in the form of mobile consoles which will follow the installation teams. All operations must be accessible to vacuum technicians, who may have little or no computing knowledge. Therefore, menu driven consoles, using a screen and light pen, will be made available to the operators. The large number of pumps, gauges, and valves makes it necessary to find solutions to reduce the cabling costs. An original way of powering the sputter ion pumps uses a single power supply and cable for up to eight pumps, yet allows individual current monitoring for each pump. An ionization gauge power supply for eight heads is economical and saves rack space by multiplexing the ion currents while powering and regulating continuously all filaments. All power supplies must be integrated in the general control system of the LEP machine. This is done by using so-called intelligent equipment controllers, which exchange messages with the main control room by a network of multidrop buses. A sophisticated data base management system is required in order to keep track of the numerous items connected to the vacuum system.

Comparison of the synchrotron radiation induced gas desorption in aluminum vacuum chambers after chemical and argon glow discharge cleaning

In a dedicated synchrotron radiation beam line at the DCI storage ring at Orsay, France a series of measurements were made to evaluate the effect of argon glow discharge cleaning either as a pretreatment in the laboratory, or in situ, following the installation and pumpdown of the vacuum system in the LEP (Large Electron Positron storage ring) machine. On exposure to synchrotron radiation the gases desorbed were H2, CH4, CO, and CO2 with Ar in the glow discharge cleaned chambers. With continued exposure the gas desorption decreased with time except that the Ar showed a definite reluctance to clean up. When applied to LEP conditions, chemical cleaning only gave the required beam-gas lifetime after the shortest exposure. The worst overall situation results from the in situ Ar discharge cleaning due to the low pumping speed for Ar in the LEP machine.

THE ALEPH EXPERIMENT AT LEP

The Large Electron-Positron storage ring(LEP) at CERN is expected to produce e+e− collisions with luminosities of the order of 1032 cm −2 s −1, at eight interaction points, initially at c.m. energies up to 100 GeV, later up to 200 GeV. Only the 4 even-numbered interaction regions will be equipped with experiments during the first phase of the LEP. ALEPH is one of these, and will be located in pit 4. Here we briefly report on the present design and construction of ALEPH, as well as on some test results obtained with prototypes, and Monte Carlo simulations.

A Superconducting 352 MHz Prototype Cavity for LEP

It is proposed to upgrade the large electron-positron storage ring LEP to energies above the initial 55 GeV by superconducting accelerating cavities operated at 352 MHz. A new cavity geometry has been developed which will allow to locate the main and higher order mode couplers at the beam tubes of the cavities. Experimental results obtained with monocell cavities fabricated from niobium material of different thermal conductivity and with a 4-cell prototype cavity are discussed.

Stanford linear collider scheduled to operate in 1986

Construction of the Stanford Linear Collider is well under way. The SLC is the crucial testing ground of a new accelerator technology, widely thought to be indispensible if we are to push electron–positron collision energies much beyond what LEP will make available. LEP, the Large Electron–Positron storage ring under construction at CERN, near Geneva, is scheduled to provide e+e− collision energies of 100 GeV by the beginning of 1989. But this gargantuan $400-million ring, 27 km in circumference, is regarded by many as the largest practicable e+e− collider one can base on the well-tried storage-ring technology. Significantly higher e+e− energies, it is believed, will require linear colliders.

The Chain of LEP Injectors

The CERN 400 Gev Proton synchrotron (SPS) will be used as 20 GeV positron/electron injector of the Large Electron-Positron storage ring (LEP) under construction at CERN. The SPS will be fed with 3.5 GeV positrons/electrons from the CERN 28 GeV Proton Synchrotron (PS) which in turn receives the particles from a new pre-injector under construction. The latter consists of a 600 MeV storage ring acting as a buffer between a 600 MeV S-band linac cycling at 100 Hz and the slow-cycling synchrotrons; the positron source is a converter target downstream of a 200 MeV linac.

Prototypes of LEP Bending Magnets with Steel-Concrete Cores

In the large European electron positron storage ring project (LEP), the guiding magnets have a field of 0.123 T at the maximum energy of 130 GeV obtainable with superconducting RF cavities. To take advantage of these low field levels, steel-concrete cores with a steel filling factor of 0.27 are proposed. The magnets are excited by means of aluminium bars running along the circumference (Fig. 1). The design features and the experience gained with prototypes are given.

I. Synchrotron radiation from the large electron-positron storage ring LEP

The properties of synchrotron radiation from LEP are investigated. This radiation from LEP are investigated. This radiation is assumed to be in a parasitic mode without changing any of the operating parameters. At 86 GeV the radiation from the normal bending magnet has a critical energy of 0.4 MeV and a power of ∼500 W/m, and is probably of limited interest. High photon energies (10–20 MeV) of high intensity can be obtained from normal and superconducting wiggler magnets. Undulators can give quasi-monochromatic radiation of high brightness with photon energies of up to 5 MeV. New magnet developments might increase this energy range. Quasi-monochromatic γ-rays of ∼100 MeV can be created with soft Compton back scattering without disturbing the electron bean. This relies on future free electron lasers in the submillimetre range. The natural collimation, the polarization and the time structure make all these photon beams unique tools for research in nuclear physics. The synchrotron rediation can be used to produced photoneutrons with intensities of up to 10 14 neutrons/s. It is foreseen that LEP will be equipped with superconducting cavities in later stage and that is energy will be increased to ∼130 GeV. This will approximately double the photon energies obtained from wigglers and undulators.

Design Study of a Large Electron-Positron Colliding Beam Machine - LEP

Early in 1976 a study group at CERN began to examine design concepts for a Large Electron-Positron storage ring, LEP. At that time, 50 km circumference and 100 GeV per beam - to be obtained with a conventional radio frequency system - were chosen. This first study was terminated with several problems still unsolved, including extreme sensitivity of orbit stability to closed-orbit tolerances, operation in collision mode with electrostatically separated beams and technical difficulties due to the low magnetic field at injection. In addition, the estimated cost was considered high. A fresh start was made in the second half of 1977. In order to explore the variation of difficulties and cost with machine size and in an attempt to arrive at a solid base for an entirely feasible machine, it was decided to reduce the nominal energy to 70 GeV while retaining the target for maximum luminosity at 1032cm-2s-1. The optimum radius for this design - later confirmed by the outcome of the study - is 3.5 km. This phase of the study ended in August 1978 with the issuing of a detailed Design Report including a cost estimate. The conclusions are that such a machine is not only feasible but that it can be developed to reach 100 GeV per beam when suitable superconducting cavities become available. In fact, the design is made such that this extension of energy requires no major change other than the substitution of cavities.