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

TL;DR

This study explores adapting nitrogen-doping and furnace-baking surface treatments, originally for 1.3 GHz cavities, to 644 MHz beta_opt=0.65 superconducting RF cavities, demonstrating their potential to achieve high Q0 and gradients for future accelerator projects.

Contribution

First investigation of nitrogen doping and furnace baking on 644 MHz beta_opt=0.65 cavities, assessing their effectiveness for high-Q0 and high-gradient applications.

Findings

Nitrogen doping yields superior Q0 but is sensitive to electropolishing steps.

Furnace baking provides reasonable performance with lower residual resistance.

Both treatments meet the gradient requirements for the FRIB400 upgrade.

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. Strong frequency- and geometry- dependence of these recipes makes the technology transfer amongst different cavity styles and frequencies far from straightforward, and requires 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's Proton Improvement Plan-II, could benefit enormously from adapting these techniques to their beta_opt = 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 beta_opt = 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 may require further recipe refinement. The gradient requirement for the FRIB400 upgrade project is comfortably achieved by both recipes.