Effective medium temperature baking of 1.3 GHz single cell SRF cavities

Abstract

In order to pursue high intrinsic quality factor (Q0) as well as high accelerating gradient (Eacc) of superconducting radio-frequency niobium cavities, medium temperature baking experiments are carried out on 1.3 GHz single cell cavities at Peking University. The preliminary medium temperature treatments include 350 °C 3 h baking and 400 °C 3 h baking and the 2 K vertical test results turn out to deviate from the expectation, with Q-slope beginning at low field and acceptable but not very high values of Q0 at high field. Thus, the cavities are treated with 3 updated schemes, which refer to nitrogen doping at 330 °C, optimized medium temperature baking at 300 °C and low temperature baking at 120 °C. These experiment schemes result in considerable values of Q0 of 2.8-3.4 × 1010 at medium and high fields and maximum values of Eacc of 27.3-37.9 MV/m. Meanwhile, 5 groups of niobium samples are prepared respectively and treated identically to the cavities. The measurements of TOF-SIMS and XPS demonstrate that compared with baseline sample, the optimized 300 °C baked sample has moderately higher concentrations of carbon and nitrogen and one order lower concentration of hydrogen, similar to the situation of the 330 °C nitrogen doped sample except its lower carbon concentration. It is the lower concentration of hydrogen that yields the high values of Q0 of the cavities.