Elimination of defects in a welded joint in a niobium superconducting resonator

The possible reasons for the appearance of defects during welding of a niobium superconducting coaxial halfwave resonator (HWR) are investigated, which consist in the deviation of the shape of the welded surface from the given one, in the deviation of the angle of incidence of the beam from the perpendicular to the welded surface, in fluctuations in the thickness of the welded edges, and also the presence of some instability (pulsation) accelerating voltage, beam currents and focusing of the electron beam gun. The possibilities of preventing the appearance of defects during welding are determined, which consist in regulating the operating parameters of the electron beam gun depending on possible errors in the manufacture and preparation of the welded edges. Methods for elimination of defects in welding of niobium superconducting coaxial HWR are studied. A method has been developed for repairing defects in welded joints such as burn-throughs on the type of resonator under study using electron-beam deposition. The results of measuring the resonant frequency of the HWR before the onset and after the recovery of burn-through are presented, which showed that the melting of the hole did not lead to a change in the electrical characteristics. Vacuum and cryogenic tests at liquid nitrogen temperature confirmed the tightness of the welded joints. The results obtained can be used in the development of basic technologies for the restoration and repair of expensive and difficult-to-manufacture elements of accelerator technology – superconducting high-frequency resonators made of high-purity niobium sheet.