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The Magnets for the Lhc Experiments View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH European Laboratory for Particle Physics Large Hadron Collider Project LHC Project Report 339 THE MAGNETS FOR THE LHC EXPERIMENTS T.M. Taylor Abstract The Large Hadron Collider (LHC) presently under construction at the European Laboratory for Particle Physics (CERN) will provide proton-proton collisions at the 14 TeV level. Each of the four approved detectors (ALICE, ATLAS, CMS, and LHCb) to be installed at the interaction points of this machine relies on a sophisticated magnet system for separation and momentum measurements of the charged particles. The magnets are being designed, manufactured, tested and installed under the technical and financial responsibility of the experiment collaborations, but must satisfy constraints imposed by the laboratory, regarding in particular the cryogenics, powering, controls, and safety. The delivery and assembly schedules are also highly constrained by the requirement to have the magnet systems fully installed before the projected commissioning of the accelerator in 2005. The report will compare the salient features of these magnet systems, and of their integration into the CERN environment. LHC Division Presented at the 16th International Conference on Magnetic Technology 26 September-2 October 1999 - Ponte Vedra Beach, USA Administrative Secretariat LHC Division CERN CH - 1211 Geneva 23 Switzerland Geneva, 1 December 1999 The Magnets for the LHC Experiments T.M. Taylor CERN, European Organization for Nuclear Research, LHC Division, CH-1211 Geneva 23, Switzerland Abstract-- The Large Hadron Collider (LHC) presently under performance, size and cost, the magnet is but one component construction at the European Laboratory for Particle Physics of the detector. As the full detector is subject to an overall (CERN) will provide proton-proton collisions at the 14 TeV level. budget ceiling its geometry and performance is chosen to Each of the four approved detectors (ALICE, ATLAS, CMS, and LHCb) to be installed at the interaction points of this machine match in with that of other components [1], and it is useful to relies on a sophisticated magnet system for separation and recall that momentum measurements of the charged particles. The magnets 1. the spectrum of particles produced by the interaction is are being designed, manufactured, tested and installed under the strongly skewed both towards the lower energies and in technical and financial responsibility of the experiment the direction of the colliding beams; collaborations, but must satisfy constraints imposed by the 2. the calorimeters, the resolution of which is inversely laboratory, regarding in particular the cryogenics, powering, proportional to the square root of the energy of the controls, and safety. The delivery and assembly schedules are also highly constrained by the requirement to have the magnet particle, complement effectively the magnet as concerns systems fully installed before the projected commissioning of the momentum measurement of high energy particles; accelerator in 2005. The report will compare the salient features 3. too strong a field at the interaction point causes low of these magnet systems, and of their integration into the CERN energy particles to spiral and may complicate the environment. analysis; 4. multiple scattering in the beam pipe and the track detectors sets a limit to the momentum resolution which I. INTRODUCTION can be attained. The LHC detectors have been carefully designed taking A large component of most particle physics experiments is these considerations into account, and arrived at very different the magnet used to identify and to provide for the configurations, in particular the two 4p detectors, one of measurement of the momentum of the particles emanating which is based on a solenoid the other (mainly) on toroids. from the reaction. The higher the energy of the particles the Before describing the actual magnets it is therefore interesting bigger these magnets need to be. Thus the magnets which will to compare the attributes of magnets for spectrometers in be used for the detectors being built to explore the interactions general terms. of the beams of the Large Hadron Collider (LHC), which will be the largest accelerator in the world, are correspondingly II. THE CHOICE OF MAGNETS FOR SPECTROMETERS large, both in terms of their volume and their stored magnetic energy. The major detectors are particularly interested in analyzing The LHC will provide collisions between counter-rotating particles having large transverse components of momentum. beams of 7 TeV protons. In nominal operation the beams 11 To be efficient the direction of the field of the spectrometer consist of bunches of about 10 protons spaced at 25 ns magnet should subtend a large angle to the associated intervals and focused to provide average luminosity of up to 34 -2 -1 trajectories, and be independent of the azimuthal angle. It 10 cm s . In a typical collision of bunches at this luminosity there will be about 25 events, each n of which follows that solenoids and toroids are the most appropriate gives off about 100 secondary particles, strongly peaked in the choices for the central region of the experiment. Although it does not provide analyzing coverage over the full azimuth, the forward directions. The big detectors which cover a large transverse field of dipoles (and possibly quadrupoles) finds fraction of the 4p srad solid angle (4p detectors) investigate up application in the far forward region because, in contrast with to 200 tracks per bunch crossing, of particles having energies the alternative toroidal geometry, the field volume is free of of up to several hundred GeV, and the magnetic field is used obstacles which can create confusing background. for particle identification. In addition, for charged particles the Regarding the technology, spectrometer magnets use momentum can be measured, either by measuring the sagitta superconducting coils when performance or economics rule of the trajectory in the magnetic field or by measuring the out the resistive option. It is generally admitted that this is the angle imparted by the field integral. As the LHC will be by far case if the power consumption of a suitable resistive magnet the most powerful accelerator in the world, it was to be would exceed some 2 to 5 MW, depending on the available expected that the spectrometer magnets should be more infrastructure, required duty cycle and coil geometry. As they powerful than those used in experiments on It is nevertheless do not need to be ramped rapidly, these large superconducting important to understand that, while impressive in coils are usually wound from aluminium stabilized conductor and rely on indirect cooling. Manuscript received September 27, 1999 1 A. Solenoids For an ideal toroid contained between current sheets at radii R and r and with field B at inside radius R the resolution The vast majority of recent 4p detectors have relied on obtained by combining a measurement of angle before the solenoidal type magnets, producing a cylindrically symmetric particle enters the toroidal field, and sagitta within the field, is field having the same axis as the colliding beams. The reason roughly proportional to BRln(r/R)/sinq. In this idealized case for this is easy to understand: the symmetric 2-D field the optimum resolution is obtained with R/r » 3.5, and is facilitates reconstruction of the events; there is no material reduced by a factor of about Ö2 when R/r is reduced to 2. The within the field volume to give rise to spurious secondary interest for small and intermediate q is evident. interactions, and the magnetic forces are relatively easy to So why have we seen so few toroids? It is not (only) contain. In addition, thanks to the large number of magnets because the toroid is a harder magnet to build (which it is). It that have been made and the experience with their operation, is mainly because of the difficulty of making in practice the associated technology is mature. anything resembling an ideal toroid, i.e. to make the inner The sagitta of the trajectory of a charged particle emanating at conductor sheet and its supporting structure sufficiently zenithal angle q (i.e. the angle between the trajectory and the transparent, or to divide it up so as to cover a sufficiently axis of the colliding beams) from an interaction on the axis of small proportion of the azimuth. This material creates a long solenoid of radius R and producing a magnetic field B, confusion in that some particles are absorbed or interact with is proportional to BR2/sinq. The change of angle is the structure, an effect that can outweigh the benefits of proportional to BR. The “analyzing power” depends on the conceptual elegance. Various studies made over the last 25 layout of the detector and is proportional to some combination years [1,2] have concluded that the best practical of measurements of sagitta and changes in angle; this is approximation for a high field (and therefore superconducting) reduced at small q because of the finite length L of the toroid, covering the central region of the detector, consists of solenoid. Within the limits of known technology and eight lumped coils. With care these can be made to cast transportable size, the cost of the solenoid is roughly shadows over as little as 30% of the azimuth, which can be proportional to LR2B2. It is therefore clear that, as concerns considered acceptable. It implies, however, that a significant resolution, it is preferable to invest in size than in central field. fraction of the central detector should lie within the inner The basic technology for building these large solenoids is radius of the toroid. mature, so it is relatively easy to design one for incorporation In point of fact the catalogue of advantages of the toroid into a given detector.
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