Study on the interstitial oxygen diffusion to understand the reduction of cryogenic RF loss for the superconducting radio-frequency niobium cavities

Medium-temperature baking (Mid-T baking) is an innovative method employed to enhance the unloaded quality factor Q0 of superconducting radio-frequency niobium (SRF Nb) cavities at cryogenic temperatures. This study presents an interstitial oxygen diffusion model based on the decomposition of the natural oxide to clarify the improved performance of the Nb cavities after undergoing Mid-T baking. Additionally, the correlation between the interstitial oxygen within the RF penetration depth and the surface resistance of the Nb cavities has been explored. The parameter for the oxide decomposition was determined using in-situ x-ray photoelectron spectroscopy (XPS), where the thickness of the oxide/carbide layer was calculated from the peak fitting of Nb 3d spectra and the attenuation law of the photoelectron beam. The interstitial oxygen diffusion model, validated by the semi-quantitative distribution along the depth determined by time-of-flight secondary ion mass spectrometry, quantifies the oxygen atomic concentration within the RF penetration depth in Mid-T baked Nb material. In the baking temperature range of 300 °C–400 °C, the calculated oxygen concentration from the interstitial oxygen diffusion model demonstrates a more pronounced dependence on the baking temperature than the baking time. This suggests that more precise control of the interstitial oxygen concentration can be achieved by adjusting the baking temperature. Furthermore, it has been observed that maintaining a uniform and moderate oxygen concentration throughout the depth is essential for optimal Bardeen–Cooper–Schrieffer resistance. This study paves the way for more efficient processing optimization and enhancing understanding of the mechanism behind RF loss in Nb cavities.