Processing strategies for niobium sheet used in advanced superconducting particle accelerator cavities

Ultra-high-purity niobium sheet is used to fabricate radio frequency cavities for use in future linear accelerators. The complexity of the shapes requires the electron-beam welding of deep-drawn components, but difficulties with inconsistent formability, springback, and surface roughness have been traced to non-uniform microstructures and texture gradients that are common in commercial niobium sheet. Two approaches under study to overcome these problems are the development of fine-grained favorably textured polycrystalline sheet, and the use of a properly oriented and nearly single-crystal sheet to form pieces of the cavity. Both approaches show promise but need further work.