  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.
