Development of cryogenic tensile testing apparatus for lattice strain measurement using synchrotron radiation for REBCO composite conductors

REBa₂Cu₃O_7−δ (REBCO) composite tapes are candidate conductors for future accelerator magnets. Given that REBCO is a brittle oxide film, stress/strain management is important in the mechanical design of a magnet. Therefore, the strain state of a superconducting film should be predicted under various operating conditions. A quantum beam is a useful tool in evaluating the strain state of a superconducting material in a composite conductor. Although various cryogenic tensile testing machines, in combination with neutron beam and synchrotron radiation, have been developed, they are not necessarily optimized for measuring highly textured materials such as REBCO films. This study reports a novel cryogenic tensile testing apparatus that can be used at the beamline of a synchrotron radiation facility. In this system, a superconducting tape is cooled using a Gifford–McMahon (GM) cryocooler through 2.5 m long thermal links made of 5 N high-purity aluminum. This enables a compact cryogenic loading system that can be mounted on the goniometer of a beamline. Lattice strain measurements using synchrotron radiation were conducted at 40 K, 77 K, and 300 K for a commercially available (Y,Gd)BCO coated conductor at BL28B2 in SPring-8. The onset strain of film fracture determined from the lattice strain measurement at 77 K agreed well with the irreversible strain of critical current at the same temperature. The residual and fracture strain of the (Y,Gd)BCO film at 77 K was evaluated as well. (paper)