Local Optics Corrections in the HL-LHC

In order to push the performance of current and future particle colliders, it is necessary to drive the beams at the locations of the experiments to decreasingly smaller sizes. This progress towards smaller beam sizes in the interaction points requires accelerators that must accommodate larger beam sizes in the surroundings and therefore strong magnetic errors in these regions must be suppressed. This thesis presents a study on the consequences of these magnetic errors and several mitigation techniques to correct their influence. As a specific example, in this thesis the quadrupole triplet of the future HL-LHC is studied. Simulations of its expected magnetic errors are carried out and their correctability is assessed. For this a new automatic local correction algorithm is also presented which allows to perform Monte-Carlo simulations of the errors and their corrections. In recent years it has become clear that the main limiting factor to control the beam size in the interaction points is the quality of the measurement of the β-function at this point. In this thesis some of the limitations of the current measurement method, K-modulation, are studied, some developments made to improve the accuracy of this method are discussed and two new complementary techniques that are expected to surpass its accuracy are presented. These new techniques are the computation of the minimum β near the IP using the betatron phase measured with new instrumentation and a procedure to locate the beam waist via luminosity scans. These two techniques are presented both with their theoretical framework and their experimental validation on the LHC. These are the first results of a possible complement to the K-modulation method in future colliders. The optics scheme that has been used in the LHC and is going to be used in HL-LHC pushes the β-functions to higher values also far away from the interaction regions. It has been observed in the LHC that this scheme enhances the magnetic errors of the arc magnets too, where there are no correctors to control local errors available. In the thesis, in order to counteract these errors, it is proposed to implement optics correction using orbit bumps over sextupoles, thus correcting the optics errors via feed-down for the first time in the LHC. These results suggest that drastic changes in the commissioning strategy will be necessary in the HL-LHC, as they will require intermediate luminosity measurements. To accommodate the larger beam sizes expected in HL-LHC, the new Nb3Sn superconductor technology is going to be used in the HL–LHC triplet, which allows to increase the magnets aperture while keeping the same magnetic field strength on the central axis. However, the superconductors built using this technology show an unstable behaviour during magnetic-field ramps, this effect is called flux jumps. In this thesis the effect of flux jumps on the beam emittance is studied for the first time. The studied case is of two 11 T dipoles that are going to be installed in the LHC, in the HL–LHC triplet and in the main bending dipoles of the FCC-hh. Finally, many of the software improvements developed during the studies presented in this thesis are also described. These include a thorough process of refactoring of outdated software tools, introducing modern data analysis frameworks. A graphical user interface to handle the automatic local correction algorithm which makes easier the computation of these sometimes complex corrections is briefly presented. A new harmonic analysis framework designed to replace older tools, whose development had been frozen, is also described. This new framework has allowed to further develop the harmonic analysis algorithms for optics measurements, improving their precision and performance.