Advanced surface treatments for medium-velocity superconducting cavities for high-accelerating gradient continuous-wave operation

Abstract

Nitrogen-doping and furnace-baking are advanced high-Q0 recipes developed for 1.3 GHz TESLA-type cavities. These treatments will significantly benefit the high-Q0 linear accelerator community if they can be successfully adapted to different cavity styles and frequencies. The strong frequency- and geometry-dependence of these recipes makes transferring the technology amongst different cavity styles and frequencies far from straightforward, requiring rigorous study. Upcoming high-Q0 continuous-wave linear accelerator projects, such as the proposed Michigan State University Facility for Rare Isotope Beam Energy Upgrade, and the underway Fermilab Proton Improvement Plan-II, could benefit enormously from successful adaptation of these techniques to their βopt = 0.6∼650 MHz 5-cell elliptical superconducting rf cavities, operating at an accelerating gradient of around ∼17 MV/m. This is the first investigation of the adaptation of nitrogen doping and medium temperature furnace baking to prototype 644 MHz βopt = 0.65 cavities, with the aim of demonstrating the high-Q0 potential of these recipes in these novel cavities for future optimization as part of the FRIB400 project R&D. We find that nitrogen-doping delivers superior Q0, despite the sub-GHz operating frequency of these cavities, but is sensitive to the post-doping electropolishing removal step and experiences elevated residual resistance. Medium-temperature furnace baking delivers reasonable performance with decreased residual resistance compared to the nitrogen doped cavity, but requires further recipe refinement. The gradient requirement for the FRIB400 upgrade project is comfortably achieved by both recipes.