Ductile Nb Sheet for Superconducting RF Cavities

Superconducting radio frequency (SRF) cavities, such as those fabricated at Jefferson Laboratory or the National Superconducting Cyclotron Laboratory and used in charged particle linear accelerators like CEBAF, are formed by deep drawing a high-residual-resistance-ratio (RRR), pure niobium sheet. If the starting sheet has a non-uniform or poorly textured microstructure, poor deep drawing behavior is manifested by excessive thinning, unpredictable cavity shape, inconsistent "spring-back," and the development of ¿orange peel" surface roughness, all of which can degrade SRF cavity and linear accelerator performance. In this project, severe plastic deformation (SPD) processing will be used to refine and homogenize the microstructure of bulk Nb, which will then be rolled to sheet and annealed for fabrication into SRF cavities. By carefully selecting deformation and heat treatment conditions during the processing steps, a sheet with fine and uniform grain size and preferred texture can be manufactured for subsequent fabrication into SRF cavities. Phase I will: (1) refine the microstructure of bulk high RRR Nb by SPD, (2) fabricate 2-4 mm thick sheet by rolling the SPD processed bulk, and (3) perform mechanical and microstructural characterization of the Nb sheet to determine which sequence of processing steps and fabrication conditions gives the best microstructure for increased ductility and deep drawing formability. During Phase II, a second-generation, high-RRR Nb sheet, with fine-grains and preferred texture for increased ductility, will be fabricated and formed into an experimental SRF cavity. Commercial Applications and other Benefits as described by the awardee: The technology should be applicable to the production of pure Nb sheet for superior fabrication of superconducting radio frequency (SRF) cavities. In addition to Nb, the approach should be applicable to the fabrication of other sheet materials, including Ta and steel.