Thesis

This project will establish atomic layer deposition (ALD) as a versatile surface engineering approach for superconducting resonators, addressing two complementary objectives for particle accelerators and quantum technologies.
The first objective is controlled doping of bulk niobium superconducting radiofrequency (SRF) cavities. Conformal films up to 10 nm thick will provide solid dopant sources, followed by high-vacuum annealing to tailor dopant profiles within the RF-active region. This strategy aims to improve uniformity, explore alternative dopants and eliminate subsequent electropolishing. Particular attention will focus on suppressing hydride formation while preserving the superconducting energy gap. Target performances include quality factors of 8 × 10¹° at 2 K and accelerating gradients of 40–45 MV/m. Supported by the European EPITA project, this work addresses requirements for future accelerators, including the Future Circular Collider.
The second objective is developing protective coatings with minimal dielectric losses by mitigating defects, particularly two-level systems. Building on CEA treatments combining dielectric oxide coatings and annealing that achieved record quantum-regime quality factors, alternative compounds will be investigated.
Ultimately, one treatment could enhance performance across quantum and accelerator operating regimes. Tunneling spectroscopy, X-ray spectroscopy and TOF-SIMS will correlate local superconducting properties with surface chemistry and dopant distributions. Two ALD systems will support development on coupons and transfer to resonators, building on demonstrated coating uniformity inside cavities. RF measurements will be conducted at CEA and partner laboratories, with Fermilab evaluating promising treatments at millikelvin temperatures. These investigations will clarify performance-limiting mechanisms and guide the rational design of optimized superconducting surfaces for both application domains.