Fabrication and Testing of Thick-Film CVD Niobium-Lined Copper SRF Cavities for High-Gradient Applications

Period of Performance: 07/31/2017 - 07/30/2019

$1000K

Phase 2 SBIR

Recipient Firm

Ultramet
12173 Montague Street Array
Pacoima, CA 91331
Firm POC
Principal Investigator

Abstract

Innovative process development is needed to reliably deposit superconducting niobium films possessing performance capabilities near those of bulk niobium on less costly copper accelerator component structures such as cavities, complex couplers, and nosecones as an alternative to solid bulk niobium. Unlike bulk or thick-film niobium, most thin-film coatings do not afford practical post-coating surface conditioning options. Thin-film niobium coatings on copper are prone to copper diffusion through the entire ~5-μm coating thickness and poor thermal contact, limiting their use to low- to medium-gradient applications. Niobium-coated copper fabrication technologies for cost-effective high-gradient superconducting radio frequency (SRF) components are needed for the economic viability of future accelerator facilities. Ultramet, in collaboration with Cornell University’s SRF Group, is demonstrating thick-film (~150 µm) chemical vapor deposited (CVD) niobium on OFHC copper that is compatible with state-of-the-art cavity surface conditioning methods and has great potential to achieve bulk-like performance capabilities for use in high-gradient applications, but at lower cost by minimizing or eliminating many costly, time-consuming, and often performance- limiting fabrication steps as well as affording an estimated 90-95% reduction in the amount of niobium required. The feasibility of depositing thick-film niobium layers on OFHC copper using lower temperature CVD was demonstrated. The 30- to 150-µm CVD niobium coatings were shown to be diffusion bonded to the copper, and interdiffusion analysis showed that the bulk of the coating thickness remained pure niobium even after elevated temperature exposure for extended periods. Coating adherence was excellent, and the coated surface looked similar to bulk niobium. The BCS portion of surface resistance was good. Residual surface resistance was elevated, but this was not related to RRR and was likely due to surface inclusions that can be reduced or eliminated by conditioning. Process variables deemed critical for material optimization, process scaling, and accelerator cavity and component fabrication and conditioning efforts were identified. Ultramet will team with Cornell and Niowave to fabricate and characterize multiple single-cell 1.3-GHz SRF test cavities of the ILC design utilizing the newly developed lower temperature CVD niobium process. The CVD niobium on copper fabrication methodology will be scaled to produce testable flanged niobium-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 niobium-lined copper ILC single-cell cavity fabrication as a cost indicator for complete 9-cell ILC cavities. Ultramet’s thick-film niobium CVD process is uniquely well-suited for coating complex SRF accelerator component geometries because the virtually 100% dense coatings are formed on the substrate at the molecular level and purity levels in excess of 99.99% are achievable. The CVD coating process exhibits the greatest throwing power, or ability to uniformly deposit materials onto/into intricately shaped or textured substrates. This research is a necessary step toward the commercial and scientific application of advanced accelerator component-forming technologies that represents a significant technical milestone in developing reliable fabrication techniques for reproducible niobium- lined copper accelerator components offering high-gradient operation and substantial cost reduction for SRF applications worldwide. Large quantities of superconducting radio frequency components are needed for use in particle accelerators in fields as diverse as high-energy physics and airport security. Advanced chemical vapor deposition processing is being developed for economical fabrication of high-performance and low-cost thick-film niobium-lined copper accelerator components, representing a paradigm shift in SRF component manufacturing.