CVD Process Development of Thin Film Triniobium-Tin on Copper SRF Cavities

Innovative fabrication technologies for cost-effective high-Q, high-field superconducting radio frequency (SRF) components are needed for the economic viability of future accelerator facilities. The worldwide particle accelerator community continues to investigate alternatives and performance-enhancing modifications to bulk niobium accelerator components via the application of superconducting films. DOE is interested in development of advanced process technologies to deposit superconducting materials such as triniobium-tin (Nb3Sn), which has the potential to exceed the performance capabilities of bulk niobium when formed on the interior surface of existing bulk niobium, or less costly copper, accelerator component structures, enabling substantial fabrication and operating costreductions for continuous wave and high gradient accelerators. Ultramet, in collaboration with Cornell University’s SRF Group, with support from the Applied Superconductivity Center at the National High Magnetic Field Laboratory at Florida State University (FSU), and building on previous work by Cornell, CERN, JLAB, and others, is developing techniques to create well-bonded layers of triniobium-tin on copper, niobium, and molybdenum substrates via Ultramet’s advanced chemical vapor deposition processing technologies. Chemical vapor deposition (CVD) thin film process technology using prealloyed niobium-tin precursor materials was developed and demonstrated to form triniobium-tin layers on flat molybdenum, copper, and niobium substrates as a critical first step in developing CVD processes for Nb3Sn with a tin content of 24% for accelerator component applications. Material characterization including surface resistance and RF performance properties were measured and results related to prior research. Process variables deemed critical for material optimization, future process scaling, and accelerator cavity and component fabrication efforts in Phase II were identified. Ultramet will team with Cornell and Niowave, and obtain Nb-Sn CVD precursors to be developed by Nb3Sn experts at Florida State, to fabricate and characterize multiple copper single-cell 1.3-GHz SRF test cavities of the ILCdesign utilizing the newly developed CVD Nb3Sn process. The CVD Nb3Sn on copper fabrication methodology will be scaled to produce testable flanged Nb3Sn-lined copper cavities. Extensive cavity characterization including temperature mapping, post-test dissection, and surface material analysis will be performed by Cornell. Ultramet will generate cost estimates for 1.3-GHz CVD Nb3Sn-lined copper ILC single-cell cavity fabrication as a cost indicator for complete 9-cell ILC cavities. Commercial applications and other benefits: Ultramet’s CVD-based processing to be developed in this project will represent a significant technical milestone in the cavity surface application of the superconducting material Nb3Sn. The critical temperature of Nb3Sn is 18 K versus 9.2 K for niobium, making Nb3Sn far more efficient and allowing for higher temperature operation of SRF components, avoiding the need for expensive and complex superfluid/subatmospheric helium operation to enable substantial cost reductions for SRF programs worldwide.