Surface analysis of Nb materials for SRF cavities

Superconducting Radio Frequency (SRF) cavities provide enhanced efficiency and reduced energy consumption in present-day particle accelerators. Niobium is the material of choice for SRF cavities due to its high critical temperature and critical magnetic field. In order to understand why certain treatments, especially a low temperature bake, improve performance, it is important to study Nb surface characteristics and identify elemental contaminants which may affect the performance of the cavity.([1]) Initial studies using SIMS and Focused Ion Beam (FIB) prepared specimens for Transmission Electron Microscopy (TEM) have helped to characterize the Nb surface and measure the surface oxide layer thickness.([2]) C, N and O are of particular interest as interstitial contaminants and earlier studies suggested very high H concentration. In the present study, ion implants of C, N, O and deuterium (D) in Nb and Si were analyzed using SIMS. D was implanted to characterize H while avoiding interference from the high H background. The D implant was easily detectable in Si, but showed a constant value and no implant shape in Nb. This result implies either that D (and by implication, hydrogen) has a high mobility in Nb, or that there is movement of D due to the primary ion beam. Nevertheless, C, N, and O could be quantified using the ion implants. Depth profiles of polycrystalline and single crystal Nb samples were also obtained. While both types of Nb samples contained low C concentration, the single crystal Nb samples showed higher N and O content. Copyright (C) 2010 John Wiley & Sons, Ltd.