Single-sided collimation and the effects on beam cleaning and impedance in the LHC

Abstract

The Large Hadron Collider (LHC) employs a multistage collimation system to protect against unavoidable beam losses during operation. A regular collimator consists of two movable blocks placed symmetrically around the circulating beam in order to intercept halo particles. These blocks are known as jaws and are generally composed of a highly robust material which is designed to withstand extreme levels of temperature, pressure, and radiation. The charged particles travelling inside the accelerator generate electromagnetic fields which interact with the surrounding structure, giving rise to forces that perturb the motion of the beam, often leading to beam degradation. This perturbing effect ultimately determines the performance of the accelerator in terms of beam quality and stored current, and is characterised by the so-called beam-coupling impedance. The collimation system is the single highest contributor to the transverse impedance at top energy. This is because the collimator jaws are the closest elements to the beam, and thus interact more strongly the electromagnetic fields generated by the particles. As the beam intensity increases, so too does the coupling impedance, and when the perturbations become sufficiently strong, the beam becomes unstable. Seeing as the High-Luminosity LHC (HL-LHC) upgrade nearly doubles the bunch population, impedance must be reduced in order to ensure beam stability. Single-sided collimation aims at mitigating impedance by fully retracting one of the two jaws for any number of collimators. One would expect this to come at the cost of beam cleaning; however, simulations have shown that this is not necessarily the case, especially when selecting the right combination of jaws. This thesis explores the feasibility of single-sided collimation for the betatron cleaning insertion in HL-LHC by running simulations that quantify performance in terms of beam cleaning and impedance. Benchmark measurements have also been performed for LHC.