The Large Hadron Collider (LHC), the particle accelerator at CERN, Geneva, is the largest and probably the most complex scientific instrument ever built. Superconductivity plays a key role because the accelerator is based on the reliable operation of almost 10?000 superconducting magnets cooled by 130 tonnes of helium at 1.9 and 4.2?K and containing a total stored magnetic energy of about 15?000?MJ (including detector magnets). The characteristics of the 1200 tonnes of high quality Nb?Ti cables have met the severe requests in terms of critical currents, magnetization and inter-strand resistance; the magnets are built with an unprecedented uniformity, about 0.01% of variation in field quality among the 1232 main dipoles, which are 15?m in length and 30 tonnes in weight. The results of this 20-year-long enterprise will be discussed together with problems faced during construction and commissioning and their remedies. Particular reference is made to the severe incident which occurred nine days after the spectacular start-up of the machine on 10 September 2008. The status of repair and the plan for the physics programme in 2010 are also presented.

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Superconductivity: its role, its success and its setbacks in the Large Hadron Collider of CERN

A particle beam is a collection of a large number of charges and represents an electromagnetic potential for other charges, therefore exerting forces on itself (space charge) and other beams (Beam-Beam Interactions, BBIs). The control of the BBIs in particle colliders is fundamental to preserve beam stability and achieve the optimum collider performance. In the case of the Large Hadron Collider (LHC) at CERN, these forces are experienced as localized periodic distortions when the two beams cross each other in the four experimental areas. The forces are most important for high density beams, i.e. high intensity and small beam sizes. Each LHC beam is composed of 2808 bunches, each containing 1011 protons and with a transverse size of 16 µm at the interaction points. These extreme parameters are the key to obtain high "luminosity", i.e. the number of collisions per second needed to study \rare" physics phenomena. The BBI is therefore often the limiting factor for the luminosity of colliders. Within all BB effects, this thesis covers coherent and incoherent beam-beam effects in hadron colliders with particular emphasis on those with a large number of bunches, like the LHC. Complementary numerical tools have been developed to study the effects of BBIs on the particle beams. The use of new parallel algorithms was fundamental to allow the scale of the calculations. The objectives are a better understanding of these effects and if necessary to propose changes to the LHC beam parameters to keep detrimental effects small. We have demonstrated the numerical predictibility of bunch to bunch differences and for the first time compared the results to analytical predictions and experimental data from the Relativistic Heavy Ion Collider (RHIC). Beam-beam transfer function measurements had been reproduced and understood in terms of coherent beam-beam modes. An implementation of the measurement devices had been suggested in order to avoid misinterpreted diagnostics. An emittance growth caused by the BBIs had been observed and characterized with numerical studies and explained with a simple physical picture. A study of the parametrical dependency of this effect defines qualitatively the expected limiting factors to avoid detrimental deterioration of the beams. The variation of the LHC tune spectra for different collision symmetries and in the presence of long range as well as head-on collisions had been presented and explained. This will be of fundamental importance to correctly interprete the observations of single bunch measurements during the operation of the collider and to improve the performances.

A study of beam-beam effects in hadron colliders with a large number of bunches

In order to study dynamic effects in accelerator magnets, such as the decay of the magnetic field during the dwell at injection and the rapid so-called "snapback" during the first few seconds of the resumption of the energy ramp, a fast continuous harmonics measurement system was required. A new magnetic field measurement system, based on the use of digital signal processors (DSP) and Analog to Digital (A/D) converters, was developed and prototyped at Fermilab. This system uses Pentek 6102 16 bit A/D converters and the Pentek 4288 DSP board with the SHARC ADSP-2106 family digital signal processor. It was designed to acquire multiple channels of data with a wide dynamic range of input signals, which are typically generated by a rotating coil probe. Data acquisition is performed under a RTOS, whereas processing and visualization are performed under a host computer. Firmware code was developed for the DSP to perform fast continuous readout of the A/D FIFO memory and integration over specified intervals, synchronized to the probe's rotation in the magnetic field. C, C++ and Java code was written to control the data acquisition devices and to process a continuous stream of data. The paper summarizes the characteristics of the system and presents the results of initial tests and measurements

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A Fast Continuous Magnetic Field Measurement System Based on Digital Signal Processors

Superconducting magnets have played a key role in advancing the energy reach of proton synchrotrons and enabling them to play a major role in defining the Standard Model. The problems encountered and solved at the Tevatron are described and used as an introduction to the many challenges posed by the use of this technology. The LHC is being prepared to answer the many questions beyond the Standard Model and in itself is at the cutting edge of technology. A description of its magnets and their properties is given to illustrate the advances that have been made in the use of superconducting magnets over the past 30 years.

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The Development of Superconducting Magnets for Use in Particle Accelerators: From the Tevatron to the LHC

The decay of the sextupole component in the bending dipoles during injection and the subsequent snapback at the start of beam acceleration are issues of common concern for all superconducting colliders built or in construction. Recent studies performed on LHC and Tevatron dipole magnets revealed many similarities in the snapback characteristics. Some are expected, e.g. the effect of operational history. One particular similarity, however, is striking and is the subject of this paper. It appears that there is a simple linear relation between the amount of sextupole drift during the decay and the magnet current (or field) change during the ramp required to resolve the snapback. It is surprising that the linear correlation between snapback amplitude and snapback field holds very well for all magnets of the same family (e.g. Tevatron or LHC dipoles). In this paper we present the data collected to date and discuss a simple theory that explains the scaling found.

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A scaling law for the snapback in Superconducting accelerator magnets

During beam injection the components of the magnetic field inside the magnets decay in time. At the re-start of the ramping, this decay is cancelled resulting in a fast change of the harmonics called "snapback". This sudden variation affects mainly the sextupole and decapole components and can induce significant changes in the machine chromaticity, thus causing particle beam loss. Standard magnetic measurements with rotating coils do not have a sufficient time resolution to properly characterize the snapback phenomenon. A system based on Hall probes has been developed from an existing prototype to measure with a moderate acquisition frequency (3-10 Hz) the decay and snapback of the sextupole and, for the first time, of the decapole fields. The present system also provides local measurements of these field harmonics along a wavelength of the superconducting cable twist pitch. In this paper, we describe the assembly features of this detector and of the measurement chain. The performances are demonstrated based on preliminary measurements performed on the LHC pre-series dipoles in operating conditions.

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Field decay and snapback measurements using a fast Hall probe sensor

The decay and snap-back of sextupole in the bending dipoles are issues of common concern, albeit at different levels of criticality, for all superconducting colliders built (Tevatron, HERA, RHIC) or in construction (LHC) to date. The main difficulty is the correction of the relatively large and fast sextupole change during snap-back. Motivated by the above considerations, we have pursued an extended study of sextupole snap-back on two different magnet families, the Tevatron and the LHC bending dipoles, using the same measurement method. We show here that it is possible to generalise the results obtained by using a simple, exponential scaling law. Furthermore, we show that for magnets of the same family the parameters of the scaling law correlate linearly. This finding could be exploited during accelerator operation to produce accurate forecast of the snap-back correction based solely on beam-based measurements.

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T. Pieloni

Papers

A scaling law for the snapback in Superconducting accelerator magnets

Field decay and snapback measurements using a fast Hall probe sensor

A scaling law for predicting snap-back in superconducting accelerator magnets