Chemical Free Surface Processing for High Gradient Superconducting RF Cavities

Chemical treatment such as buffered chemical polishing (BCP) or electropolishing (EP) followed by high pressure rinsing (HPR) of niobium (Nb) superconducting RF (SRF) cavities is expensive and complex multistep process. Furthermore, the cavity RF surfaces after the treatment still have numerous bubbles and pits that result from welding. These quench-producing weld defects together with the particulate contamination, result in significant scatter of the multi-cell Nb SRF cavities performance characteristics. This scatter is the major problem in the current manufacturing of the Nb SRF cavities. FM Technologies proposes a new chemical-free processing for multi-cell Nb SRF cavities using an internal electron beam (IEB). Specifically, FMT proposes to develop a new electron gun system that will perform electron beam melting over the entire interior surface of Nb SRF cavities to produce a smooth surface, free from voids, bubbles, and other imperfections. This will allow manufacturing of the Nb SRF cavities without the above chemical treatment procedures and increase the cavities high gradient performance. In Phase I, 30 Nb samples were surface melted by an electron beam, beam parameters were determined to achieve a quality Nb surface (qualified by HIROX, AFM, SEM and EBSD). A complete Nb SRF cavity, without chemical etching, was electron beam melted at the interior surface and RF tested in the superconducting range. Several electron gun designs were evaluated. Phase II is aimed at building and testing a complete prototype computer controlled electron beam interior cavity surface melting system. Many Nb samples will be surface melted and beam parameters established to achieve a high quality surface (qualified by HIROX, AFM, SEM and EBSD). Single and multi-cell Nb SRF cavities will have the interior surface melted by this system at FMT, without chemical etching, Jefferson Lab (J-lab) will perform RF testing on these cavities. Commercial applications and other benefits: When developed the chemical free surafce processing using IEB will benefit first J-lab's CEBAF and Fermilab's Project X. Also, light sources, require 1.3 - 1.5 GHz SRF structures. Furthermore, IEB will produce much broader impact because it can be implemented into manufacturing of many other types of RF devices including both superconducting as well as normal conducting RF cavities.