Optimisation of the field description for the Large Hadron Collider during beam commissioning

Abstract

The Large Hadron Collider (LHC) at CERN is a 27km circumference particle accelerator. It is made out of 1232 superconducting dipole magnets and 392 superconducting quadrupole magnets, which guide the particles around the LHC. It is designed to have two counter-rotating beams of particles collided at nominal centre of mass energy of 14 TeV to study and explore the fundamental forces and constituents of matter. The operation of the LHC is a challenging task. All the superconducting magnets have to be controlled simultaneously in order to successfully steer the beams around the LHC. Any field variations of the superconducting magnets, which are not immediately corrected, will affect the trajectories of the beam and therefore the LHC performance. Unfortunately, a feedback control system based solely on beam-based measurements is not enough and therefore, a feed-forward control system is also required. The feed-forward control system is used to predict the behaviour of the field variations such that it reduces the load from the feedback system. The Field Description for the Large Hadron Collider (FiDeL) is the feed-forward control system designed to predict the magnetic field variations that result from the inherent properties of the superconducting magnets. This system is based on the analysis of the data obtained from the magnetic measurements of all the superconducting magnets before the magnets were installed in the LHC. This thesis is focused on determining the precision of the FiDeL model and optimising it where necessary. However, in this work, the magnetic behaviour is not studied through magnetic measurements of individual magnets, but through beam-based measurements performed during the operation of the LHC. This is done by measuring the tune and the chromaticity of the LHC, two important beam parameters which ensure beam stability. These two parameters are directly dependent on the magnetic field quality and the control of its harmonic content. Therefore, by analysing such beam-based measurements, the behaviour of the FiDeL model is compared to the behaviour of the whole LHC. Furthermore, this study also gives an outlook of the FiDeL model at the LHC nominal operation of 7 TeV. This consists of studying the main elements of FiDeL that can become critical at such operation in order to anticipate any possible issues and how these can be overcome. The work presented in this thesis has been adopted by CERN during the first LHC running period (2008-2013). It will continue to be used in 2015, following the two- year shutdown after which the LHC will be operating at the nominal design parameters.