Beam Loss Calibration Studies for High Energy Proton Accelerators

CERN's Large Hadron Collider (LHC) is a proton collider with injection energy of 450 GeV and collision energy of 7 TeV. Superconducting magnets keep the particles circulating in two counter rotating beams, which cross each other at the Interaction Points (IP). Those complex magnets have been designed to contain both beams in one yoke within a cryostat. An unprecedented amount of energy will be stored in the circulating beams and in the magnet system. The LHC outperforms other existing accelerators in its maximum beam energy by a factor of 7 and in its beam intensity by a factor of 23. Even a loss of a small fraction of the beam particles may cause the transition from the superconducting to the normal conducting state of the coil or cause physical damage to machine components. The unique combination of these extreme beam parameters and the highly advanced superconducting technology has the consequence that the LHC needs a more efficient beam cleaning and beam loss measurement system than previous accelerators. There are several safety systems to protect the accelerator components from damage. One of them is the Beam Loss Monitoring (BLM) system that detects and quantifies the amount of lost beam particles. It generates a beam abort trigger when the losses exceed predetermined threshold values. The principal detector type is an ionisation chamber. About 4000 detectors are being installed, mostly around the quadrupole magnets. T he detectors probe the transverse tails of the hadronic showers emerging from the cryostat, which are induced by lost beam particles. The initial calibration of the BLM system should ensure that the systematic error of the system is smaller than 5. For the calibration and threshold determination several simulations are combined: Beam particles are tracked to find the most probable loss locations. At these locations hadronic showers through the machine components are simulated to determine the energy distribution in the coil of the magnet and the total energy deposition in the BLM detectors. To calculate the quench limits for short duration losses the enthalpy, the B-field and the current density in the cables is taken into account to estimate the maximal possible temperature increase. For steady state losses a possible temperature increase is mainly determined by the heat flow in the helium and in the copper of the cable. This work focuses on the effectiveness and the characterisation of the particle losses and the corresponding detector signals. Simulating the signals and comparing those to several irradiation scenarios obtained this understanding. In the process the detector response simulations were performed, which form part of the system calibration, and the uncertainty estimation of transverse hadronic shower tail simulations were conducted, which contributes to the system calibration error. The ionisation detector res ponse functions were simulated utilising the Monte Carlo program Geant4 for different particle types at various kinetic energies. Validation measurements have been performed at CERN with proton beams, gamma sources as well as in mixed radiation fields and at The Svedberg Laboratory (Sweden) the detector was calibrated with neutrons. The HERA internal proton beam dump served as a test bed for the LHC BLM system. Over a period of two years the whole BLM system was tested under real accelerator conditions, which resulted in several updates and upgrades of the hardware and software. The parasitic measurements of the tails of the hadronic showers induced by the impacting protons were compared to Geant4 simulations. The final task was to utilise the gained methods to calculate an LHC BLM detector threshold, which was compared to previously, estimated thresholds.