Alain Poncet

The Large Hadron Collider (LHC) in the LEP tunnel

After the remarkable start-up of LEP, the installation of a Large Hadron Collider, LHC, in the LEP tunnel will open a new era for the High Energy Physics. This report summarizes the main LHC parameters and subsytems and describes the more recent studies and developments.

Design of the Second Series of LHC Prototype Dipole Magnet Cryostats

An initial series of six LHC 10 m long prototype dipole magnets and cryostats have been manufactured by European Industry. The assembled cryo-magnets were tested singly and connected in series in a test string at CERN between March 1994 and December 1996. During the same period, an evolution in the requirements for LHC cryogenics distribution has lead project management to adopt a seChapaute cryo-distribution line running Chapaullel to the LHC machine.1 The former standard LHC half-cell, made up of a short straight section unit and four 10 m dipoles, has been discarded and replaced with one composed of a short straight section unit and three 15 m dipoles. The new 15 m LHC dipole magnet cryostats are described. These units house the dipole magnet cold mass, which stands on three low heat in-leak support columns, and is enclosed within an actively cooled radiation screen operating at 4.5–20 K and an actively cooled thermal shield operating at 50–75 K.

On the theory of coherent instabilities due to coupling between a dense cooled beam and charged particles from the residual gas

The authors discuss intensity limitations due to ions trapped in the p-beam and assess antidotes which have recently been applied in the Antiproton Accumulator (AA) at CERN. They reexamine the theory and analyze Landau damping of dipole and quadrupole modes, stabilization by the appropriate choice of the working point, and ion clearing by shaking of the p-beam.<<ETX>>

Mechanical performance of a twin-aperture 56 mm bore 1 m long dipole model made with SSC type cables

A twin-aperture dipole model made with standard SSC type cables was launched to initiate studies of lower field magnets for a 7 TeV collider machine. This model, which was entirely constructed at CERN, reached at 1.8 K a peak field of 9.7 T. Short mechanical models, made to check the assembly parameters, as well as the final model magnet were instrumented with mechanical force transducers based on strain gauges to monitor azimuthal stresses and axial forces in the coils during assembly, cooldown and excitation. Dynamic measurements of forces and temperatures during magnet quenching were also performed with a high sampling rate acquisition system. This paper reviews the magnet mechanical design principles, describes the design, fabrication and the calibration of the force transducers and presents the main results of the measurements.

Observation of transverse quadrupole mode instabilities in intense cooled antiproton beams in the AA

In the CERN Antiproton Accumulator (AA) a breathing mode type of instability has been identified as an intensity-limiting mechanism in cooled stacks. The more well-known dipole-mode instabilities are adequately controlled by the existing damper system. With the aid of a quadrupole pickup it was possible to observe transverse modes in the beam at frequencies (n-2Q). The instabilities occur only at certain emittance and intensity thresholds and are believed to be caused by uncleared pockets of ions trapped in the beam potential. A quadrupole kicker was added to the machine so that these modes could be excited, and beam transfer functions were measured for each of the possible modes. Feedback can be applied to actively damp the quadrupole modes. The best solution has been to rid the machine of the remaining ion pockets gradually by improving the clearing, by the careful choice of tunes, and even by shaking the beam.<<ETX>>

Final Design and Experimental Validation of the Thermal Performance of the LHC Lattice Cryostats

The recent commissioning and operation of the LHC String 2 have given a first experimental validation of the global thermal performance of the LHC lattice cryostat at nominal cryogenic conditions. The cryostat designed to minimize the heat inleak from ambient temperature, houses under vacuum and thermally protects the cold mass, which contains the LHC twin‐aperture superconducting magnets operating at 1.9 K in superfluid helium. Mechanical components linking the cold mass to the vacuum vessel, such as support posts and insulation vacuum barriers are designed with efficient thermalisations for heat interception to minimise heat conduction. Heat inleak by radiation is reduced by employing multilayer insulation (MLI) wrapped around the cold mass and around an aluminium thermal shield cooled to about 60 K.Measurements of the total helium vaporization rate in String 2 gives, after substraction of supplementary heat loads and end effects, an estimate of the total thermal load to a standard LHC cell (107 m) incl...

Transverse Instabilities Due to Beam-Trapped Ions and Charged Matter in the CERN Antiproton Accumulator

At stack intensities above 10 antiprotons with transverse emittances of between and 2 mm.mrad at 3.5 GeV/c, three distinct transverse heating mechanisms caused by positive matter trapped in the negative beam potential have been observed and identified. Two effects are incoherent and one is coherent. The incoherent effects are of two kinds distinguishable by the rate at which emittance growth occurs, and by the sensitivity to tune changes. The first is a slow growth at a rate which is about equal to, or up to ten times faster than the intrabeam scattering growth rate at small emittances: this is attributed to the excitation of 11th and 15th order non-linear resonances by residual ion pockets, an effect very similar to the beam-beam effect in colliders. The second kind of incoherent effect is an intermittent, violent emittance growth, often associated with a substantial stack loss rate. This effect is believed to be due to multiple Coulomb scattering by highly charged tiny dust particles trapped in the beam potential. Observations of the coherent instability fit the known antiproton-ion (similar to proton-electron) theory. It leads to growth rates faster than the transverse damper presently installed in the AA can handle.

Assembly and quality control of the lhc cryostats at CERN: Motivations, means, results and lessons learnt

In 2001, the project management decided to perform at CERN the final assembly of the LHC superconducting magnets with cryostat parts and cold masses produced by European Industry in large series. This industrial-like production has required a very significant investment in tooling, production facilities, engineering and quality control efforts, in contractual partnership with a consortium of firms. This unusual endeavour of a limited lifetime represented more than 850,000 working hours spanning over five years, the work being done on a result-oriented basis by the contractor. This paper presents the reasons for having conducted this project at CERN, summarizes the work breakdown structure, the production means and methods, the infrastructure specially developed, the tooling, logistics and quality control aspects of the work performed and the results achieved, in analytical form. Finally, the lessons learnt are outlined.

Mechanical design aspects of the LHC beam screen

Forty-four kilometers of the LHC beam vacuum system will be equipped with a perforated co-axial liner, the so-called beam screen. Operating between 5 K and 20 K, the beam screen reduces heat loads to the 1.9 K helium bath of the superconducting magnets and minimises dynamic vacuum effects. Constructed from low magnetic permeability stainless steel with a 50 /spl mu/m inner layer of high purity copper, the beam screen must provide a maximum aperture for the beam whilst resisting the induced forces due to eddy currents at magnet quench. The mechanical engineering challenges are numerous, and include stringent requirements on geometry, material selection, manufacturing techniques and cleanliness. The industrial fabrication of these 16 metre long UHV components is now in its prototyping phase. A description of the beam screen is given, together with details of the experimental programme aimed at validating the design choices, and results of the first industrial prototypes.

The Large Hadron Collider vacuum system

The two rings of the LHC beam vacuum system have a total length of about 54 km of which almost 48 km will be at 1.9 K, the temperature of the superconducting magnets. The total synchrotron radiation power emitted by the two beams is 0.41 Wm/sup -1/. A a so-called beam screen, maintained at a temperature between 5 K and 20 K by gaseous helium flow, is inserted in the magnet cold bore to intercept this power. We discuss the beam screen, magnetic permeability/vapour pressure aspects, beam screen vacuum behaviour, photon-induced gas desorption, intermagnet connection, pressure measurement, warm sections and the insulation vacuum.