Coaxial energetic deposition of superconducting Niobium thin films for large particle accelerators

Superconducting radio-frequency (SRF) particle accelerators rely on bulk Niobium cavities to accelerate the particle beam. Since the skin depth for the superconductor is ≪100nm, the bulk of the costly Nb material is unnecessary. An alternative is to coat less expensive copper structures with just a thin film of Niobium. The Niobium coated Copper cavities benefit from the superior thermal properties of Copper and reduced cost of Copper (∼$6/kg) over Niobium (∼$600/kg). For Niobium coated Copper cavities to replace bulk Niobium cavities, coated cavities must match the performance existing bulk cavities. Key performance specifications include critical temperature, thermal dissipation and qualify factor. Alameda Applied Sciences Corporation, in collaboration with Thomas Jefferson National Accelerator Facility, has explored the possibility of using a coaxial cathodic arc to deposit the superconducting Niobium thin film on witness plates. In this study, the properties of ambient temperature and superconducting thin films are correlated with measured rotational speed and axial speed of the arc. The data suggest that superior Niobium thin films are produced when the rotational speed of the arc is maximized. This collaboration has demonstrated thin film Niobium with critical temperature close to that of bulk Niobium cavities and a residual resistance ratio (RRR) (a measure of the thermal dissipation), of nearly 40. The goal for the program is to approach a thin film RRR of 200.