Development of High-Q SRF Structures by Nitrogen Doping for Superconducting Electron Linacs

One of the most exciting recent developments in superconducting RF technology has been the discovery of a nitrogen-doping process which can reliably increase the superconducting quality factor of niobium resonators well above 1010 at high frequency (>1 GHz). This process is now well demonstrated in particular for TESLA-style 9-cell cavities at 1.3 GHz. Nitrogen doping is part of the plan for the production of the cavities for LCLS-II, significantly increasing the cryogenic efficiency of the machine, and allowing significant capital cost reductions. This Phase II proposal will expand the application of nitrogen doping to the low-frequency regime. Niowave is currently developing commercial superconducting electron linacs for a number of important applications including high-power free-electron lasers, the production of medical and industrial radioisotopes, materials processing, sterilization, and cargo scanning applications. These systems typically operate at 350 MHz, but at 4 Kelvin operating temperature, where reduction of BCS losses would be an important efficiency enhancement. Pushing commercial linacs into a regime where a small cryocooler could provide for the cryogenic load of the accelerating cavity would have a huge impact. In this project, Niowave would send a 350-MHz niobium resonator to Fermilab and Cornell University for nitrogen doping and a vertical test to demonstrate high quality factor. The collaboration would then proceed toward a full accelerator demonstration with beam, which would demonstrate the preservation of the high-quality factor through the cryomodule installation, coupler installation, and operation with high-power electron beam.