Heat Treatment for a Superconducting Cavity

Imperfections on the surface of a superconducting material, niobium, have serious effects on the performance by increasing the surface resistance. Cold working of niobium to fabricate a cavity degrades the crystallinity of the niobium surface only to increase the surface resistance. Hydrogen is a well-known impurity that lowers the quality factor in the form of Q-disease. High-temperature heat treatment with a high vacuum level was carried out to remove impurity gases from the niobium surface of a cavity, and to anneal the cavity to increase the crystallinity. Optical images were taken to confirm the effect of the heat treatment on the microstructure. Grains did not grow much after the heat treatment, and the crystallinity was increased. Diffraction analyses based on X-ray diffraction (XRD) and the full widths at half maximum (HWHMs) of the XRD peaks confirmed that the annealing was successfully performed. The residual gas analysis (RGA) confirmed that a very large of amount of hydrogen, which is known as a major impurity causing the quality degradation of the superconducting cavity, was removed during the heat treatment.