Development of wet nitrogen doping to improve the performance of half-wave resonators

FRIB is developing a new N-doping method with a simplified recipe. This recipe is called wet nitrogen doping, by adding nitric acid to the conventional EP acid. Nitrogen doping introduces impurities to the SRF surface, and reduces the BCS resistance by shortening the mean free path, which leads to a higher Qo. Conventional nitrogen doping, developed at FNAL and Jlab, requires a high-temperature treatment (900 ºC), and an additional light EP to remove the over-contaminated layer. This recipe produces a decreasing Qo at extremely low fields but successfully achieves high Qo performance up to 25 MV/m. The wet doping method does not require additional high-temperature baking and light EP afterwards, therefore it is superior in terms of processing steps. This method produced a high Qo of 8x10^10 at a low field of 0.5MV/m without the decreasing trend on FRIB beta=0.53 HWR. In this presentation, we will show the related R&D results generated from the FRIB 0.53 HWRs.