Investigating the Role of Oxygen Diffusion in SRF Cavity Performance

Superconducting radiofrequency (SRF) cavities are resonators with extremely low resistivity that have applications in particle accelerators and superconducting qubits. One performance metric of SRF cavities is its quality factor (Q₀). Q₀ is inversely proportional to resistance, and higher Q₀ corresponds to less power loss, thus lowering capital required to keep the accelerators cold. The performance of SRF cavities is largely governed by the surface composition of the first 100 nm of the cavity surface. Nitrogen doping and oxygen doping are two empirically derived surface treatments that yield high Q₀ by altering the surface composition. We compare the performances of nitrogen doped and oxygen doped cavities in terms of Q₀, heating profiles, accelerating gradients, and surface composition. A simulation of the diffusion of oxygen into the bulk of the cavity was built using COMSOL Multiphysics software. Simulated results are compared to the actual surface composition of the cavities as determined from secondary ion mass spectrometry analysis. Understanding how oxygen diffusion affects the performance of these cavities allows us to have further insight into the underlying mechanisms that enable these surface treatments to yield high Q₀ performance.