Study On The 1.3 GHz Single-cell Large Grain Niobium Superconducting RF Cavities

Nine 1.3 GHz single cell superconducting RF cavities were fabricated in the research programs of this paper and the distinct features of the manufacture were summarized and analyzed. Three large grain (LG) cavities were treated by electro polishing (EP), one of which repeatedly achieved the accelerating gradients of more than 47.0 MV/m and another with 43.0 MV/m. Two large grian cavities prepared by chemical polishing (CP) both reached the gradients above 32.0 MV/m with a maximum of 40.2 MV/m。The possibility achieving high gradient (near 50 MV/m) was demonstrated and two sets of surface treatment procedures were concluded for high gradient large grain cavities, one based on EP and another on CP. The quality factor of the LG cavities by CP is higher than the EP cavities by 50 % and the surface resistance at 2.0 K in the low field is lower, which will reduce the heat loads. The explanation and analysis was also discussed in this paper. The research of this paper shows the advantages of LG niobium over fine grain and will benefit the design of the International Linear Collider or ERL. These experiments on the LG cavities supplied the experimental data for the study of the anomalous loss mechanism of superconducting Rf cavity. The differences between the two modes of testing in this article were comparatively analyzed from the theoretical and practical vertical testing process.The cooling process of the cryogenic system of BEPCII superconducting RF cavities and magnets and its preliminary commissioning was also studied in this paper, which would promote the progress of the engineering.