Preparation of the Cu-based Nb ₃ Sn sample via bronze route for quadrupole resonator testing

Abstract We report the successful production of a Cu-based Nb 3 Sn sample specifically designed for quadrupole resonator (QPR) testing, representing a significant step toward scalable radio-frequency superconducting coatings of Nb 3 Sn on copper substrates. The sample was fabricated using an optimized electrochemical thermal synthesis via bronze route, incorporating several key advancements: electropolishing of the Cu substrate, electroplating of the bronze precursor layer, a tailored heat treatment at 700 ∘ C to promote grain growth and suppress tin-rich secondary phases, and a newly developed chemical etching procedure for effective removal of surface bronze residues and contaminants. These improvements address longstanding challenges in the fabrication of high-quality Cu-based Nb 3 Sn thin films. Subsequent QPR measurements yielded the peak magnetic field and temperature dependent surface resistance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mtext>s</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , as well as the superconducting transition temperature and quench field. The achieved surface resistance, with a minimum of 47.5 nΩ at 4.5 K and 15 mT (413 MHz), indicates that both residual and BCS losses contribute significantly to the present performance limitations. This behavior is consistent with the observed sub-stoichiometric composition and microstructural inhomogeneity of the film. Although the procedure is not yet fully optimized the results clearly demonstrate the feasibility of this approach and its potential for further improvement through process refinement.