Thin Films on Copper for Superconducting RF Cavities within the Future Circular Collider Study

The Future Circular Collider (FCC) Study is a collaborative international initiative led by the European Organisation for Nuclear Research (CERN). Its primary objective is to assess the feasibility of a new circular collider intended to explore physics beyond the Standard Model. Toward this goal, developing superconducting radio-frequency (SRF) accelerating cavities that offer the required performance in terms of acceleration while maintaining cost-effectiveness represents a significant challenge. The baseline option being explored involves the niobium-on-copper (Nb/Cu) technology, which consists in coating the inner surface of the copper cavities with a superconducting thin film of niobium. The feasibility of using A15 compounds, particularly Nb3Sn, as coating materials is also being investigated, although this approach is still in its early developmental stages. This thesis work is focused on the initial phases of the Research and Development (R&D) of superconducting thin films produced at CERN for coating SRF copper cavities, namely when the coatings are produced and tested on their substrates in the form of small samples. To evaluate the quality of the films and study the impact of the chosen coating parameters, it is important to analyze their superconducting properties. The critical temperature (Tc) is a key parameter of interest, serving as an early indicator of the film's superconducting performance. For this, a dedicated experimental test station designed for the inductive measurement of the Tc of superconducting thin films on copper, now permanently installed at the CERN's Central Cryogenic Laboratory, is developed and described in this work. A non-contact method is chosen to avoid any direct, and possibly destructive, manipulation of the sample, which allows for further characterization tests to be performed on the same sample. On top of this, the inductive measurement via a two-coil setup represents a fast, cheap and reliable way of measuring Tc. Additionally, this work presents a feasibility study of a new method called "reverse-coating" technique, aimed at producing seamless copper substrates for Nb/Cu SRF cavities. The technique involves depositing a niobium layer on a mandrel shaped like the final cavity, followed by electro-forming the copper cavity around it. The mandrel is then removed, leaving behind a copper cavity with a functional niobium layer. This technique also eliminates the need for the usual copper surface preparation prior to superconducting layer coating. Preliminary results from flat disk samples suggest the technique's feasibility. Finally, this work investigates the use of bipolar High Power Impulse Magnetron Sputtering (HiPIMS) as a coating method for Nb3Sn films on copper. The study aims to understand potential correlations between Tc, deposition parameters, and film morphology data. The focus is on investigating if HiPIMS, as energetic deposition technique already demonstrated for niobium, can provide dense Nb3Sn films on copper which also exhibit the expected Tc and a correct stoichiometry. The study, still ongoing, offers insights for optimizing the deposition process of Nb3Sn films on copper.