The potential of energetic condensation techniques for SRF applications - The first extensive SRF performance study on Nb/Cu by ECR deposition

Particle accelerators operating in continuous wave (cw) mode or at high duty cycles usually use super- conducting radio-frequency (SRF) cavities for particle acceleration due to the lower power consumption compared to normal conducting accelerating structures.
The niobium thin film technology, where a copper structure is coated with a some μm thick niobium film, offers numerous advantages over structures made from niobium sheet in terms of thermal and mechanical stability, raw material costs, and complexity of cryomodules. However, the strong increase of surface resistance with RF magnetic field, the so-called Q-slope, currently limits the application of niobium thin films to low accelerating gradients. The established DC magnetron sputtering technology is now challenged by energetic condensation techniques which promise to improve the microstructure of a coating and subsequently improve the RF performance. Electron cyclotron resonance (ECR) deposition is the most advanced representative of the class of energetic condensation techniques.
This thesis presents the first SRF results of an ECR niobium-on-copper coating at operation-typical frequencies. The film was studied using the Quadrupole Resonator (QPR) which is the ideal tool for comprehensive investigations of the superconducting surface resistance at different temperatures, RF fields, at cavity typical frequencies and with different ambient conditions. For the latter, the QPR has been extended with a solenoid for controlling the ambient magnetic field. SEM, EBSD, FIB and EDS analyses provided further insight regarding the film microstructure and enabled linking the film prop- erties to the RF performance. The coating exhibits bulk-like material properties and a strongly mitig- ated Q-slope confirming the potential of the ECR technique in particular and of energetic condensation techniques in general. The surface resistance increase with field is however still stronger than in bulk niobium of comparable material quality. The results suggest that the grain boundaries, their density and degree of oxidation, as well as the heat transfer through the niobium-copper interface are the key aspects for the RF performance rather than the mean free path alone. Furthermore, the roles of ambient magnetic field and cooling conditions were extensively studied: It was found that despite the bulk-like properties and just like previous Nb/Cu coatings, also the studied ECR coating exhibits very low surface resistance sensitivity to ambient magnetic field. In contrast, the cooling dynamics have been identified as impacting the surface resistance severely. Finally, a discussion of the trapped flux sensitivity comes to the conclusion that the low sensitivity in Nb/Cu is not only due to a beneficial combination of short mean free path and low frequency, but is dominated by stronger pinning compared to bulk Nb.
Looking beyond niobium, this thesis additionally presents secondary electron yield (SEY) data of various SRF material candidates. The SEY is the key material parameter for determining the mul- tipacting risk. Although multipacting is not a limitation in elliptical cavities anymore, it is certainly still a limitation in non-elliptical cavities, including the Quadrupole Resonator. The results suggest that Niobium compounds are not critical. MgB2 however holds a strong risk of electron activity due to the formation of MgO when exposed to air.