  The superconducting radio-frequency (SRF) community has shown that
introducing certain impurities into high-purity niobium can improve quality
factors and accelerating gradients. We question why some impurities improve RF
performance while others hinder it. The purpose of this study is to
characterize the impurity profile of niobium with a low residual resistance
ratio (RRR) and correlate these impurities with the RF performance of low RRR
cavities so that the mechanism of impurity-based improvements can be better
understood and improved upon. The combination of RF testing and material
analysis reveals a microscopic picture of why low RRR cavities experience low
temperature-dependent BCS resistance behavior more prominently than their high
RRR counterparts. We performed surface treatments, low temperature baking and
nitrogen-doping, on low RRR cavities to evaluate how the intentional addition
of oxygen and nitrogen to the RF layer further improves performance through
changes in the mean free path and impurity profile. The results of this study
have the potential to unlock a new understanding on SRF materials and enable
the next generation of SRF surface treatments.
