Non-Evaporable Getter coating optimisation for future particle accelerators

Non-Evaporable Getter (NEG) coating has become an integral part of particle accelerator vacuum systems since its discovery in 1998. It offers more advantages than regular vacuum pumps (including NEG cartridge pumps), like distributed pumping and reduced desorption from the chamber walls. Moreover, it provides the possibility of customisation and optimisation necessary to meet requirements for specific machines and users. However, the vacuum system designs of the existing and future particle accelerator are becoming more and more demanding, which in turn requires further tests of the NEG coating. As there is a wide variety or variable parameters that can be altered during the sample preparation process (consequently resulting in various types of coatings), systematic tests are necessary to find the optimal coating for various purposes. This work presents a few studies performed with the NEG films, each described in separate chapters. The results can be split into four parts – single metal zirconium NEG coating, tests at cryogenic temperatures, experiments with narrow samples and tests with conductive NEG. Pumping properties, electron stimulated desorption (ESD) yields, surface images have been obtained to analyse various compositions and morphologies of NEG coatings. Chapter 1 introduces the Future Circular Collider (FCC) study and presents the design challenges that are faced by the vacuum engineers when creating the vacuum chamber design. Chapter 2 gives some scientific and theoretical background on vacuum science and its most important parameters used in connection with accelerator science, vacuum generation in particle accelerators, main gas sources in accelerator vacuum chambers and beam-vacuum interactions. The evolution of getter materials up to NEG coating is briefly described. Chapter 3 presents the standard experimental procedure in STFC ASTeC Daresbury Laboratory, along with the description of the testing and deposition facilities used for preparation and testing of NEG coated samples. Chapter 4 overviews the simulation software and models used for sample characterisation and makes a link between the experiments and Monte Carlo simulations. Chapter 5 details the study of single metal zirconium NEG coating, one of the getter materials that are most promising in terms of cost effectiveness and pumping properties. Single metal targets have been of interest due to their small dimensions, which can be a limiting factor for compound targets when depositing narrow vacuum chambers. Based on some promising results of previous  experiments, pumping properties and electron (ESD) yields from Zr films have been thoroughly tested. Chapter 6 gives an overview of experiments at cryogenic temperatures. The low temperature tests of NEG coating are extremely challenging due to the temperature extremes the samples are subjected to. The chapter includes a description of the new set up used for the tests and explanation of how various design difficulties were tackled. All the experiments are described in separate sub-sections, including the motivation behind the particular experiment along with the outcome. Chapter 7 establishes testing of narrow samples as one of the priorities regarding the NEG experiments. Pumping properties of samples of various diameters and compositions are presented and compared. Chapter 8 explains the importance of studying conductive NEG samples, and the resistance of films is one of the biggest challenges currently associated with using NEG films in accelerator vacuum systems. Chapter 9 provides a joint discussion reviewing all the chapters containing experimental results and comparing the films. Chapter 10 concludes the study and indicates the areas of study that could be of interest in the future. Appendices give more details on publications and presentations in connection with the work presented in the thesis.

Please note that the full-text of two published journal papers in Appendix 3 have been removed for copyright reasons.