The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
Virginia Polytechnic Institute and State University, Blacksburg, USA
The potential of Nb3Sn for SRF cavities is widely recognized and renewed R&D efforts continue to bring new insights about material structure and its properties. We have systematically coated niobium with Nb3Sn using "vapor diffusion" under varying coating conditions to elucidate the reaction of tin with niobium at the temperatures of interest. The analysis of the coated samples is revealing new understanding about the two-stage nucleation/deposition ("vapor diffusion") process that allows us to form a hypothesis regarding Nb3Sn formation mechanism. The essential aspect of nucleation is the deposition of a high coverage, nanoscale thin tin film with particle assemblage by decomposition of tin chloride on the niobium surface at temperatures sufficient for reduction of the thick niobium oxide film, usually at about 500°C. The deposition is followed by the reaction of tin from tin vapor with the niobium surface to form Nb3Sn at about 1200°C, where the surface and grain boundaries start to play key role in the formation process initiation and progression. These findings improve understanding of the Nb3Sn growth in the typical vapor diffusion process used for accelerator cavity coatings.
Virginia Polytechnic Institute and State University, Blacksburg, USA
The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
NCSU AIF, Raleigh, North Carolina, USA
In order to fully understand nitrogen addition techniques it is vital to have a full understanding of the material, including the content, location, and speciation of nitrogen contained in the treated Nb. In this work Secondary Ion Mass Spectrometry (SIMS) is used to elucidate content and location. Dynamic SIMS nitrogen analysis is reported, for the first time, for "as-received" cavity grade niobium from three separate suppliers. In addition, a number of method and instrumental issues are discussed including depth resolution, detection limit, and quantification.
The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
Virginia Polytechnic Institute and State University, Blacksburg, USA
The promise of better performance and significant cost reduction make Nb3Sn-coated Nb SRF cavities an attractive option when compared to traditional Nb SRF cavities. Historically, the vapor diffusion technique for coating Nb cavities with Nb3Sn has proven to be the most successful, and is currently practiced in several research facilities with minor variations. Using modern characterization tools, we examined the Nb3Sn coating prepared in different systems and/or under different conditions. Identically prepared high RRR (~ 300) Nb samples were coated using existing standard protocols at different coating facilities. The microstructure and composition of Nb3Sn coatings were found to be similar when examined with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Atomic force microscopy (AFM) was performed on each sample and the topographies of the samples were then compared in terms of power spectral densities (PSDs). Secondary ion mass spectrometry (SIMS) depth profiles revealed trace amounts of Ti in some of the samples.
The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
Nb3Sn cavities are now routinely prepared by depositing few micron thick Nb3Sn coatings on Nb cavities using tin vapor diffusion process. For the¬¬ case of niobium there is a significant improvement after electropolishing (EP), but electrochemical finishing treatment on Nb3Sn coatings has not been studied. Controlled removal of the first few layers could lead to a smoother and cleaner surface that is conducive to better RF performance. Several samples, which were coated with Nb3Sn by vapor diffusion process in a JLab sample chamber, were used to explore polishing parameters, such as I-V characteristics, removal rate, topography, etc. Preliminary results from the first runs are discussed here.
JLab, Newport News, Virginia, USA
The College of William and Mary, Williamsburg, Virginia, USA
Topography arising from the final etch step in preparing niobium superconducting radiofrequency (SRF) accelerator cavities is understood to significantly impact cavity performance at high field levels. This study investigated the effect of process temperature and time on the etch rate and topography arising from the widely-used buffered chemical polishing (BCP). This study aims to understand more thoroughly the genesis of topography in BCP of polycrystalline niobium, with the ultimate aim of finding a path to surface smoothness comparable to that obtained by electropolishing (EP). It was found that the etch process is controlled by the surface reaction; and that the etch rate varies with crystallographic orientation. The familiar micron-scale roughening necessarily results. Gas evolution has an impact, but is secondary. The major outcome is that surface smoothness comparable to EP appears to be inherently unachievable for polycrystalline niobium using BCP, setting an upper limit to the gradient for which it is useful.
