Development and Testing of an Advanced HOM Absorber Design for SRF Accelerators Using Dielectric-Coated Cores, Phase II

Effective methods are needed to dampen and extract power from higher order modes (HOM) in superconducting radio frequency (SRF) particle accelerators. DOE is interested in the development of innovative manufacturing and material processing technologies needed to fabricate robust broadband HOM absorber structures capable of effective operation through the entire ambient-to-cryogenic operating environment of superconducting particle accelerators. In this project, Ultramet is teaming with Cornell University’s SRF Group to develop advanced HOM absorbers for use in superconducting accelerator systems. Building upon previous research in HOM absorber development by Cornell and others in the accelerator community, Ultramet’s experience and expertise in advanced materials and process technologies are being applied to identify appropriate materials and develop fabrication methods to meet the critical HOM design criteria specified by Cornell. In Phase I, Ultramet adapted chemical vapor deposition (CVD) processes to demonstrate research objectives including the ability to fully encapsulate HOM absorber core materials with dielectric coatings by CVD and the ability to bond high thermal conductivity tungsten to the back face of dielectric-encapsulated cores by CVD to facilitate attachment to a parent component capable of surviving repeated ambient-to-cryogenic thermal cycles. The Ultramet CVD-based process capabilities demonstrated are key to identifying optimal HOM core material/dielectric coating combinations in Phase II. Cornell evaluated RF absorption characteristics and survivability under ambient-to-cryogenic thermal cycling to guide the advanced HOM absorber design optimization and build-and-test efforts planned for Phase II. Multiple core/dielectric coating material combinations will be investigated to optimize an advanced ring-style HOM absorber design with multifrequency absorption capabilities. Ultramet’s CVD dielectric coating/core encapsulation methodology will be scaled to produce testable ring-style HOM absorbers. Extensive characterization of Ultramet- fabricated dielectric-encapsulated doped ceramic core materials by Cornell will include RF measurements of complex permittivity over a wide frequency and temperature range (up to 26 GHz and ambient to cryogenic); transmission and reflection (S-parameter) RF measurements to assess broadband RF absorption; and assessments of survivability during thermal cycling to cryogenic temperatures, outgassing under ultrahigh vacuum, and DC electrical conductivity. Ultramet’s CVD-based fabrication and process technology is uniquely well-suited for gas-tight encapsulation of SRF accelerator HOM absorber 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. This research is a needed step toward the commercial and scientific application of advanced accelerator component-forming technologies that will represent a significant technical milestone in developing reliable fabrication techniques for reproducible high- performing advanced HOM absorber accelerator component designs for SRF applications worldwide.