Al2O3 is one of the potential insulator materials in the superconductor-insulator-superconductor (SIS) multilayer coatings of superconducting radio-frequency (SRF) cavities for pushing their performance limits. We report on the successful coating of two 1.3 GHz Tesla- shaped SRF cavities with 18 nm and 36 nm layers of Al2O3 deposited by thermal atomic layer deposition (ALD). The coating recipe was developed by thermal atomic layer deposition (ALD). The coating recipe was optimized with respect to different the applied process parameters such as exposure and purge times, substrate temperature and flow rates. After a proof-of-principle Al2O3 coating of a cavity, second the cavity maintained its maximum achievable accelerating field of more than 40 MV/m and no deterioration was observed [1]. On the contrary, an improvement of the surface resistance above 10 MV/m has been observed, which is now further under investigation.

Annealing of niobium (Nb) cavities in UHV is crucial for the performance in the later cryogenic tests and operation. Recently, a so-called “mid-T bake” treatment has exhibited very high-quality factors for Nb cavities. In this way, the first set of mid-T treated samples were produced with cavities at Zanon Research & Innovation Srl. The cavity performances have been improved with lower BCS and residual resistances, however the residual resistances were varied very different between 3-12 nΩ and didn’t achieve the low values as we expected. Thus, the characterization of these samples is discussed, and the source of residual resistance mitigation has been studied here in detail. We present our investigation on potential origins. For this, we used XPS, MOKE and Auger measurements to study the surface magnetic domains and stoichiometry of structures.

Superconducting radio-frequency cavities made out of niobium form the fundamental block of modern particle accelerators. A model proposed by Gurevich [1] suggests the use of a superconductor-insulator-superconductor (SIS) structure to achieve higher accelerating fields and a reduced surface resistance beyond the thermodynamic limits of Nb. As a first step to pursue this approach, a single-cell cavity was coated with a thin Al2O3 film via atomic layer deposition (ALD) to create an insulating layer [2] and baked for 3h at 300°C (mid-T heat treatment) [3]. In parallel, a mechanically polished two-grain-Nb sample was treated and coated analogically to the cavity. To further understand the RF performance of the coated and annealed cavity, an XRR analysis of the sample was carried out at each processing step to follow the changes in the niobium native oxides at process conditions (120°C) and throughout the chemical deposition and show that the coating technique and the resulting structure form a viable way for a further tailoring of cavity properties.