Ultra-High Vacuum Characterization Of Advanced Materials For Future Particle Accelerators
Creators: name: Accettura, Carlotta
Publisher: name: Zenodo
Year: 2018
Subjects: HC, HL-LHC, accelerator, vacuum, outgassing, UHV, collimator, graphite, molybdenum-graphite, microscopy, desorption

description: This Master thesis has been carried out at the CERN, the European Organization for Nuclear Research. In view of the High Luminosity upgrade of the Large Hadron Collider (LHC), a new family of materials for the collimation system must be characterized. The collimator materials need to satisfy different requirements in order to intercept particles and protect the superconductive magnets: therefore, a compromise has to be found. So far, the most promising materials are Molybdenum Carbide- Graphite (MoGr) and Copper Diamond (CuCD): these composite materials have good thermo-mechanical and electrical properties, but their compatibility with the Ultra High Vacuum (UHV) of the LHC has to be investigated. A high vacuum level in the LHC is essential to guarantee the beam stability and lifetime; therefore, the outgassing of more than 100 collimators must be taken under control. Two parameters must be verified: the total amount of gas particle outgassed by a collimator and the species of gas. Hydrocarbons, in fact, can affect more the beam and air can saturate the Non Evaporable Getters (NEG), whose pumping speed is of paramount importance to reach UHV. In order fully characterize the materials, different experimental set-ups were involved: one of the most important parts of the work was performed in vacuum test benches, where the outgassing rate value and the gas analysis of the materials have been evaluated. This analysis was integrated with thermal desorption spectroscopy (TDS): this technic gives a precise indication of all the gases present in the material and the temperature needed to remove them. The surface state of the samples, as well as the internal porosities dimension and connection, are important to determine the outgassing performance of a material. For this reason, different microscopic analysis were also performed. The secondary electron yield (SEY) is likewise important to assess the UHV compatibility of these new materials in presence of proton beams and it was measured in a dedicated test bench. From a preliminary study, MoGr was found to be out of the acceptance limits, both in term of outgassing and gas analysis; a huge presence of air was detected. One of the aims of the work has been to understand the source of this huge presence of air: in order to explain the problem, different samples have been vacuum tested and microscopically observed. Besides the characterization of the material as received from the supplier, an important part of the work has been focused on possible thermal and surface treatments able to improve the outgassing performance of the material: they were done to understand if the non-compatibility can be solved with post-production process or if a change in the production process must be foreseen. Moreover, the effects of these treatments give also advice on the steps that have to be taken into account for the final production series. Conversely, CuCD was found to have a better behaviour, but a compatible cleaning has to be investigated.