Electron-activated carbon diffusion in niobium compounds for rf superconductivity

Abstract

The higher energies planned for the next generation of particle accelerators and storage rings makes the use of superconducting hi-Q RF cavities highly desirable. Past efforts to produce reliable cavities for such projects have met with limited success. Among the barriers to achieving the maximum electric field gradient are oxide layer charging, single surface multipactoring and field emission. These are suface effects. At SLAC, a multi-technique surface analysis system has been constructed to examine possible sources of these problems and to suggest processes or surface coatings which will reduce or eliminate them. As one component of this analysis, we have investigated the time evolution of species on anodized Nb/sub 2/O/sub 5/-on-Nb surfaces as a function of electron bombardment. The surface concentration of C increases at an anomalously high rate under the exciting electron beam. Examination of the surface and gas phase indicate that the C source is in the underlying material. Estimates of the penetration depth of the beam are in agreement with the fact that there is a significant rate increase in the surface C buildup when the beam penetrates the anodized layer into the Nb bulk. Bulk analytical methods indicate, however, that the C concentration in the Nbmore » is very low. Grain boundary diffusion of C to the surface and/or an enhancement of the C diffusion coefficient due to localized beam pipe heating are examined as possible explanations of this effect.« less