Beam Loss Patterns at the LHC Collimators: Measurements & Simulations

The Beam Loss Monitoring (BLM) system of the Large Hadron Collider (LHC) detects particle losses of circulating beams and initiates an emergency extraction of the beam in case that the BLM thresholds are exceeded. This protection is required as energy deposition in the accelerator equipment due to secondary shower particles can reach critical levels; causing damage to the beam-line components and quenches of superconducting magnets. Robust and movable beam line elements, so-called collimators, are the aperture limitations of the LHC. Consequently, they are exposed to the excess of lost beam particles and their showers. Proton loss patterns at LHC collimators have to be determined to interpret the signal of the BLM detectors and to set adequate BLM thresholds for the protection of collimators and other equipment in case of unacceptably increased loss rates. The first part of this work investigates the agreement of BLM detector measurements with simulations for an LHC-like collimation setup. The setup consists of one LHC collimator and three LHC BLM detectors mounted in the Super Proton Synchrotron (SPS). The geometry is modeled in the Monte Carlo particle code Fluka. The impact scenario of the beam during the measurements is determined for simulations, and the measured BLM detector signals are compared with the simulated signals. This procedure results in a determination of an overall accuracy for the prediction of the BLM signa ls, and thus also for the prediction of BLM thresholds, by simulations. It includes an assessment of BLM-signal deviation due to simplifications and misalignment of the geometry in the simulation, physics parameters of the simulation, and uncertainties for the beam impact scenario. At the same time this study is an integral check for the BLM electronics and the data acquisition system. The relative agreement of measurements and simulations ranges between 20% and 70%, depending on the detector type. The second part of this work is devoted to the prediction of BLM detector signals for the actual LHC collimation geometry and a larger set of collimator types. Again, Fluka was employed as simulation tool. The relation between the BLM signals and energy deposition in the collimators - as the crucial scaling variable for damage to the collimators - is investigated. The study focuses on the variation of the BLM signals and the BLM signal-to-energy deposition ratio due to misalignment, and different beam impact scenarios. It results in ratios of BLM signal to energy deposition in the collimator which allow to predict BLM thresholds at collimators for given damage limits of the collimators.