Local optics corrections in the hl-lhc

Resumen

In order to increase the performance of particle colliders, it is crucial to make the beam sizes at the collision points as small as possible. This causes an increase of the beam size in the region surrounding the collision points thus enhancing the effect of magnetic errors. These errors must therefore be kept under tight control to ensure the performance and safety of the accelerator. The present thesis studies effects of the expected magnetic errors in the regions around the collision points on the beam optics that determine the beam size in the future High-Luminosity upgrade of the Large Hadron Collider (HL-LHC), a 27 km particle accelerator situated on the French-Swiss border near Geneva, Switzerland. It has become clear in recent years that in correcting the magnetic errors in this region a crucial requirement is an accurate measurement of the beam optics at the collision point. This thesis demonstrates that the technique used traditionally in recent years, called “K-modulation”, is not accurate enough to ensure the performance of the HL-LHC and therefore alternative methods of performing this measurement must be studied. To perform these studies a new automatic optics correction tool has been developed and is presented in this thesis. This new tool allows faster and more systematic calculation of corrections of the magnetic errors around the interaction regions and has been successfully tested during commissioning and experiments in the LHC. Two complementary techniques are proposed in order to improve the accuracy of the determination of the beam sizes at the collision points, namely determining the minimum beam size near the collision point using the “phase-advance” of the beam oscillations around the accelerator and locating the position of this minimum, the “beam waist”, by displacing it and maximising the collision rate characterized by the collider luminosity. In the thesis these techniques are studied theoretically, and the first results of their experimental validation performed in the LHC are presented. This push for smaller beam sizes at the collision points not only increases the beam sizes in sections around this point but also, though to lesser degree, in the arcs of the accelerator. These regions also become susceptible to smaller magnetic errors. As some regions of the accelerator do not count with adequate corrector magnets alternative solutions are needed. Here we present the first experimental results of an optics correction performed by traversing sextupoles with off-central beam in the LHC as a solution proposal. Another consequence of the growth of the beam sizes in the regions around the collision points is the eventual necessity for larger beam pipes. This is the case for HL-LHC where the magnetic lenses around the collision points are going to be replaced by new ones with the beam pipe of larger diameter. In order to keep the same magnetic strength though a new superconducting technology is going to be used to build these magnets. A downside of this novelty is that it is susceptible to a type of magnetic instability called “flux-jumps”. In the thesis the effect of the flux-jumps on the beam sizes is studied theoretically and concrete predictions using measurements of this effect on the prototypes of the new magnets of the HL-LHC are given. The study is also extrapolated to the Future hadron-hadron Circular Collider (FCC-hh), a proposed 100 km circular collider, in which superconducting magnets of this type are expected to be installed all around its circumference. Finally, the thesis presents a summary of software developments performed during the previously mentioned studies, including a user interface to facilitate the use of the automatic correction tool, a new harmonic analysis program that replaces legacy code and many refactors and rewrites that have significantly eased the development of the optics measurements and corrections programs.