• A.T. Wu, G. Ciovati, R. Manus, H.L. Phillips, C.E. Reece, R.A. Rimmer, W. Sommer, H. Tian, J.S. Williams
  JLAB, Newport News, Virginia
• F. Eozénou
  CEA, Gif-sur-Yvette
• S. Jin, L. Lin, X.Y. Lu, E. Wang
  PKU/IHIP, Beijing
• J. Mammosser
  ORNL, Oak Ridge, Tennessee

Funding: Notice: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.

A new surface treatment technique for niobium (Nb) Superconducting Radio Frequency (SRF) cavities called Buffered Electropolishing (BEP) has been developed at JLab. It was found that BEP could produce the smoothest surface finish on Nb samples ever reported in the literature. Experimental results revealed that the Nb removal rate of BEP could reach as high as 4.67 μm/min. This is significantly faster* than that of the conventional electropolishing technique employing an acid mixture of HF and H2SO4. An investigation is underway to determine the optimum values for all relevant BEP parameters so that the high quality of surface finish achieved on samples can be realized within the geometry of an elliptical RF cavity. Toward this end, single cell Nb cavities are being electropolished by BEP at both CEA-Saclay and JLAB. These cavities will be RF tested and the results will be reported through this presentation.

*Xiangyang Lu et al, to be published.

• X. Zhao, G. Ciovati, C.E. Reece, A.T. Wu
  JLAB, Newport News, Virginia

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.

SRF cavities are observed to be limited by non-linear localized effects. The variation of local material parameters between "hot" and "cold" spots is thus of intense interest. Such locations were identified in a BCP etched large-grain single-cell cavity and removed for examination by high resolution electron microscopy (SEM), electron-back scattering diffraction microscopy (EBSD), and scanning Auger electron spectroscopy (SAM). Pits with clear crystal facets were observed on both "Hotspot" and "Coldspot" specimens. The pits were found in-grain and on "Y"-shaped junction of three crystals. They are interpreted as etch pits induced by surface crystal defects (e.g., dislocations). All "Coldspots" examined had obvious low density of etching pits or very shallow tri-crystal boundary junction. EBSD revealed crystal structure surrounding the pits via crystal phase orientation mapping. This study suggests a mechanism by which BCP etching creates pits on large-grain Nb cavity surfaces and sharp-edged topography in fine-grain Nb. Field enhancements at very deep, sharp and densely populated etching pits may then cause distributed hotspots and limit cavity performance.