• J. Norem, J.W. Elam, M.J. Pellin
  ANL, Argonne
• C.Z. Antoine
  CEA, Gif-sur-Yvette
• G. Ciovati, P. Kneisel, C.E. Reece, R.A. Rimmer
  JLAB, Newport News, Virginia
• L. Cooley
  Fermilab, Batavia
• A.V. Gurevich
  NHMFL, Tallahassee, Florida
• Y. Ha, Th. Proslier, J. Zasadzinski
  IIT, Chicago, Illinois

Funding: DOE/OHEP

We have begun using Atomic Layer Deposition (ALD) to synthesize a variety of surface coatings on coupons and cavities as part of an effort to produce rf structures with significantly better performance and yield than those obtained from bulk niobium, The ALD process offers the possibility of conformally coating complex cavity shapes with precise layered structures with tightly constrained morphology and chemical properties. Our program looks both at the metallurgy and superconducting properties of these coatings, and also their performance in working structures. Initial results include: 1) evidence from point contact tunneling showing magnetic oxides can be a significant limitation to high gradient operation, 2) experimental results showing the production sharp niobium/oxide interfaces from a high temperature bake of ALD coated Al2O3 on niobium surfaces, 3) results from ALD coated structures.

• L. Cooley, D. Burk, M.H. Foley, D.T. Hicks, R. Schuessler, C. Thompson, G. Wu
  Fermilab, Batavia

Combined temperature mapping and optical inspection of SRF cavities has revealed defects near the edge of the heat affected zone (HAZ) of numerous welds. We have re-created similar defects on welded coupons that were electropolished in the lab and characterized by a variety of tools. Several features that may have bearing on the origin of these defects are discussed, such as their location relative to grain boundaries, the strain state of the niobium prior to welding and etching, the electropolishing parameters, and so forth. Since coupons are useful for distribution to academic researchers, we also describe other detailed characterizations.