INVESTIGATION OF RADIATION DAMAGE EFFECTS IN HL-LHC COLLIMATOR MATERIALS

The study of the effects of radiation-matter interaction is of great importance in the nuclear energy industry. Particle physics laboratories operating high-energy and high-intensity particle accelerators developed a growing interest in this field, as the radiation level is increasing with the stored beam energy. The results presented in this work focus on the consequence of radiation damage on accelerators equipment and, in particular, on collimators. This PhD thesis has been carried out at the European Organization for Nuclear Research (CERN), located in Geneva (Switzerland), where the Large Hadron Collider (LHC), the world’s largest accelerator, is operating. The High-Luminosity upgrade of the LHC (HL-LHC) is designed to operate with high-intensity beams, whose stored energy is about 700 MJ. The interaction of particles with collimators induces microscopic defects in the materials, which affect the thermo-physical, electrical and mechanical properties. The evolution of the components during the accelerator lifetime is fundamental to understand if a correct operation can be guaranteed. The aim of this thesis is to study the radiation damage on novel composite materials, specifically developed for HL-LHC collimators. In particular, ion irradiation is used to simulate the effects of protons. The range of the analysed materials includes two grades of molybdenum carbide-graphite composites, one of those is selected as absorber of the new HL-LHC collimators. As a comparison, carbon-fibre-carbon and isotropic polycrystalline graphite are irradiated. The irradiation test is conducted also on some samples coated with metallic films, to simulate the configuration of secondary collimators. The design of the irradiation campaign is presented together with the thermo-mechanical simulations to determine the irradiation temperature. The materials are irradiated at different particle fluences, and the corresponding displacement per atom (dpa) is resented. Each material is irradiated at four or six different levels of dpa, to assess the degradation properties as the irradiation proceed. The experimental part focuses on the measurement of the electrical resistivity relying on the four-probes method. Microscopic investigations are carried out with Raman spectroscopy for graphitic materials, and with FIB-SEM for coatings. The aim of the microstructural analysis is to explain the different behavior of materials under irradiation, and to correlate the changes observed in the electrical resistivity with the microscopic evolution. A comparison between the different materials tested is carried out to understand possible improvements in view of future upgrades of the collimation system. The results presented are then discussed to evaluate the impact on collimators performance and possible mitigation actions