Bulk-like Nb Films might be Possible with Coaxial Energetic Deposition for Superconducting RF Cavities

T. TAJIMA; N. HABERKORN; L. CIVALE; E. VALDERRAMA; M. KRISHNAN

Nashville

Simposio; AVS 58th International Symposium and Exhibition.; 2011

University of Florida - U. S. Naval Research Laboratory

Bpen, the magnetic field at which magnetic vortices start to penetrate into Nb films prepared by coaxial energetic deposition (CED) technique was measured with a SQUID magnetometer. Unlike the films prepared by conventional sputtering technique that showed Bpen ~ 94 mT at 2.5 K, the CED films showed Bpen of 180-190 mT at 2.5 K, a value that is very close to the number for bulk Nb used for SRF cavities. This corresponds to an accelerating gradient (Eacc) of  approximately 45-48 MV/m for the SRF cavities with Bpeak/Eacc ~ 4 mT/(MV/m) such as those for the European XFEL or the ILC projects. These samples were coated on MgO, Sapphire andBorosilicate with RRR ranging between 21(Borosilicate) and 540 (MgO). The next step will be to coat on copper. If it is possible to fabricate Nb coated copper cavities that have similar performance to bulk Nb high-gradient cavities, this will lead to a significant cost saving since the cost of copper is about 2 orders of magnitude less than Nb. It will also have other benefits such as better thermal stability due to high thermal conductivity of copper and less susceptibility to ambient magnetic field than bulk Nb cavities as has already been shown by LEP Nb/Cu cavities at CERN.