Fabrication of cylindrical copper waveguides with sub-millimeter internal corrugations

Over the last couple decades, powerful X-ray photon sources have been built and made available to the broad scientific community. These large (national laboratory scale) tools allow users to address some of the most important basic and applied research challenges. Even more powerful hard x-ray facilities are being designed. Many of these new facilities will rely on Wakefield accelerators to generate higher intensity / higher brightness beams. Of special importance is the Argonne Wakefield Accelerator program, which pursues the development of a variant known as electron beam-driven Structure Wakefield Accelerator (SWA). An array of accelerators based on this variant will play a central role in a free-electron laser-based x-ray user facility that is under consideration at Argonne National Laboratory. Electron beam-driven SWAs require high-precision cylindrical metallic waveguides, with internal corrugations engineered to perform as amplifying structures. Inability to procure commercial waveguides with the desired internal geometry is the major factor that has, so far, prevented the deployment of electron beam-driven SWAs. The goal of this program is to eliminate this barrier by developing a cost-effective process to fabricate high-precision cylindrical metallic waveguides with internal corrugations. The selected fabrication approach is based on the photo-chemical structuring of glass. This well-understood and very reproducible technique is used to produce sacrificial mandrels made out of fused silica. These exactingly-shaped pieces have on their external surface a pattern that corresponds to the desired internal shape (corrugation) of the SWA waveguide. A thick layer of copper is subsequently electro-deposited on the glass surface. Finally, the glass mandrel is etched away, leaving a free-standing hollowed metallic tube with precisely-shaped internal corrugations. During Phase I, the overall fabrication concept was validated. Metrology data confirmed that the desired accuracy can be achieved using the proposed approach. Several short demonstrators, based on an Argonne National Laboratory’s design, were produced. In Phase II, full size devices will be fabricated and provided to DOE end users.