Superconducting Nb Thin Films on Cu for Applications in SRF Accelerators

This article describes Nb thin films deposited on Cu using coaxial energetic deposition (CED). CED is a nonequilibrium cathodic arc process that drives medium energy (50-150 eV) ions into a few atomic layers beneath the surface to promote the growth of dense, ordered films via subplantation physics. Nb films deposited on crystalline substrates by CED have shown bulk-Nb like (>; 300-500) RRR values. Here we describe Nb films of 1-10 μm deposited on coupons and complex structures made of Cu at substrate temperatures of 400 °C or less. The coated samples were sectioned and the substrate material etched away so the superconducting transition temperature (T_c), as well as residual resistivity ratio (RRR) at 10 K of the Nb films could be measured. RRR values of >;100 have been achieved on fine-grain Cu with little surface preparation. The vortex penetration magnetic field was measured by DC SQUID and these results are presented along with comparisons between bulk Nb and thin film Nb on crystalline substrates. The CED approach has applications in superconducting bellows interconnects as well as in SRF cavities.