Noise effects in the Large Hadron Collider (LHC) and its High-Luminosity upgrade (HL-LHC)

Abstract

In order to optimize the performance of a high-energy particle collider such as the Large Hadron Collider (LHC) and its high-luminosity upgrade (HL-LHC), a thorough understanding of all the phenomena that can act as a luminosity degradation mechanism is required. A major concern for the transverse single-particle beam dynamics is the presence of noise, a mechanism that can impact the long term stability of the circulating particles. From the plethora of noise sources that are present in the accelerator, this thesis investigates the impact of a modulation in the strengths of the dipole and quadrupole magnets of the lattice due to power converter noise. In the presence of non-linearities, depending on the spectral components of the noise and the nature of the source, such a mechanism can increase the diffusion of the particles in the distribution through the excitation of additional resonances in the vicinity of the ones driven by the lattice non-linearities. As this effect can have severe implications on the beam performance, it is important to understand and control the noise mechanisms that have been observed in the LHC and that are anticipated in the HL-LHC era. To this end, this thesis presents the investigation of the 50 Hz harmonics that have been observed in the beam spectrum since the beginning of the LHC operation. Based on several observations and dedicated experiments, the source of this perturbation is identified and its impact on the beam performance is quantified with single-particle tracking simulations. For the HL-LHC, the reduction of the beam size in the interaction points of the high luminosity experiments will lead to an increase of the maximum beta-functions in the quadrupoles of the inner triplet. The expected increase of the sensitivity to noise, combined with the new hardware that is currently implemented, justifies the need to investigate the impact of tune modulation effects on the beam performance. The modulation that may arise from the power supply ripples in the high beta-function regimes will be combined with the tune modulation that intrinsically emerges from the coupling of the transverse and longitudinal plane for off-momentum particles through chromaticity. The aim of the thesis is to determine whether tune modulation effects will pose a limitation in the luminosity production of the HL-LHC.