• M. Krishnan, B.L. Bures, K.Y. Wilson Elliott
  AASC, San Leandro, California
• A.V. Gurevich
  NHMFL, Tallahassee, Florida
• J. Jiang, D.C. Larbalestier
  ASC, Tallahassee, Florida
• H.L. Phillips, C.E. Reece, A-M. Valente-Feliciano
  JLAB, Newport News, Virginia

We report single-step vacuum growth of a stoichiometric superconducting thin film of MgB2, using AASC’s cathodic arc deposition process. Energetic condensation using cathodic arcs produces non-equilibrium fast ions (~50-100eV) that allow growth modes on relatively low temperature substrates. We have demonstrated a film Tc of 34K with MgB2 by depositing at 275 °C in a single step, from a stoichiometric, solid MgB2 source. In a subsequent experiment, the single-step coated samples (on c-plane sapphire) were ex-situ annealed. Post-deposition anneal temperature ranged from 825–900K, for 15 minutes. The annealed film also showed a transition at 30K. An MgB2 film was also deposited over a 50mm diameter circle on a Buffered Chemically Polished Niobium substrate for future RF evaluation in a cavity. The depositions spanned a range of substrate temperatures from 550-675 K. The films began to change in appearance form silver to black as substrate temperature was increased, indicating a decreasing magnesium content in the films. Future plans are to reduce oxygen contamination and to use our filtered cathodic arc to deposit smoother films

• C.Z. Antoine, A. Aguilal, S. Berry
  CEA, Gif-sur-Yvette
• S. Bouat, J.F. Jacquot, J.C. Villegier
  Commisariat à l'Energie Atomique (CEA), Grenoble Cedex 9
• A.V. Gurevich
  NHMFL, Tallahassee, Florida
• G. Lamura
  CNR-INFM-LAMIA, Genova

Best RF bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H ~ 200 mT close to the thermodynamic critical field Hc. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > HcNb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities. Depositing good quality layers inside a whole cavity is rather difficult; we have made high quality samples with the techniques used for the preparation of superconducting electronics circuits and characterized these samples by PPMS, SQUID, and X-ray reflectivity. The dc magnetisation curves of 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating clearly exhibit different behaviours. Because SQUID measurements are influenced by edge and shape effects we propose to develop a specific local magnetic measurement of HC1 based on ac third harmonic analysis in order to reveal the screening effect of multilayers.

KEYWORDS:
Particle accelerators, niobium, niobium nitride, RF cavities, thin films, superconducting multi-layers

Poster