Surface Characterization of Impurities in Superconducting Niobium for Radio Frequency (RF) Cavities used in Particle Accelerators

Niobium (Nb) is the material of choice for Superconducting Radio Frequency (SRF) Cavities used in particle accelerators owing to its high critical temperature (Tc = 9.2 K) and critical magnetic field (≈ 200mT). However, niobium tends to harbor interstitial impurities such as H, C, O and N, which are detrimental to cavity performance. Since the magnetic field penetration depth (λ) of niobium is 40nm, it is important to characterize these impurities using surface characterization techniques. Also, it is known that certain heat treatments improve cavity efficiency via interstitial impurity removal from the surface of niobium. Thus, a systematic study on the effect of these heat treatments on the surface impurity levels is needed.;In this work, surface analysis of both heat treated and non heat treated (120°C-1400°C) large grain (single crystal) bulk niobium samples was performed using secondary ion mass spectrometry (SIMS) and Transmission Electron Microscopy (TEM). Impurity levels were compared on the surface using SIMS after various types of heat treatments expected to improve cavity performance, and the effect of these heat treatments on the surface impurities were examined.;SIMS characterization of ion implanted standards of C, N, O, D showed that quantification of C, N and O impurities in Nb is achievable and indicated that H is very mobile in Nb. It was hence determined that quantification of H in Nb is not possible using SIMS due to its high diffusivity in Nb. However, a comparative study of the high temperature heat treated (600°C-1400°C) and non heat treated (control) samples revealed that hydrogen levels decreased by upto a factor of 100. This is attributed to the dissociation of the niobium surface oxide layer, which acts as a passivating film on the surface, and subsequent desorption of hydrogen. Reformation of this oxide layer on cool down disallows any re-absorption of hydrogen, indicating that the oxide acts as a surface barrier for absorption/desorption of hydrogen and that hydrogen does not diffuse in the oxide. Subsequent ion implantation of hydrogen in an anodized niobium sample thus provided a quantification factor of hydrogen in niobium oxide, which was used to obtain an estimate of the hydrogen concentration in niobium. This estimate was found to be 40% atomic H in a non heat treated niobium sample. Such high levels of hydrogen observed in Nb before heat treatment ensures that is the main contributor to cavity degradation.;TEM analysis was performed to study the effect of heat treatment on the surface oxide thickness of niobium. Results showed a continuous oxide layer with a sharp metal-oxide interface. No significant changes in the oxide thickness were seen after heat treatment.;Time of Flight (TOF)-SIMS imaging was used to characterize the grain boundaries of large grain niobium bicrystals, since it was believed that impurity segregation at the grain boundaries of Nb might deteriorate cavity performance. Images showed segregation of carbon at the grain boundaries after 800°C heat treatment of the samples, while no segregation of hydrogen and oxygen were seen for both non heat treated and heat treated samples.;An important aspect of this study was the record-performance improvement of the 1400°C heat treated cavity, which showed a 200% increase in the cavity efficiency. SIMS analysis of the surface of this sample showed high levels of titanium, down to 1ìm depth, and it is speculated that this Ti might be responsible for high performance of the cavity by affecting the distribution of impurities within the penetration depth.