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2017 | 74 | 152-163
Article title

The alternative, novel technology for improvement of surface finish of SRF niobium cavities

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Pure niobium is used for making superconducting radio frequency (SRF) cavities. These cavities need to have a good surface finish to achieve maximum performance. The high smoothness, lack of subsurface hydrogen content and sulfur contamination removal of SRF niobium cavities are the three most important factors to achieving a high accelerating gradient. For finishing niobium surface, electropolishing (EP) in hydrofluoric/sulfuric acid electrolyte is commonly used as a final process. Unfortunately this process had been considered to cause hydrogen absorption that could result in hydrogen Q-disease in niobium SRF cavities. This is why a new electrolyte was compounded to substitute the one presently used. Moreover, the additional imposition of a magnetic field during the electropolishing process (magnetoelectropolishing MEP) not only increases hydrogen removal, but also results in reduction of surface roughness when compared to the roughness achieved by the electropolishing process alone (Rz much over 300 nm). This effect of sharp decrease in hydrogen content was found previously in our experiments with titanium and stainless steels after MEP processing. Our present studies, with the electrolyte consisting of a mixture of 70% methanesulfonic acid with 49% hydrofluoric acid by volume in 3:1 ratio, resulted in intensifying and speeding-up the electrochemical processes of dissolution and polishing niobium surface. It was found that the imposition of a magnetic field of ≈ 100 mT upon the EP process is able to reduce niobium surface roughness of up to 50% (Rz below 200 nm) compared to the roughness achieved by conventional electropolishing.
Physical description
  • Electrobright, Macungie PA, USA
  • Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Koszalin University of Technology, Racławicka 15-17, PL 75-620 Koszalin, Poland,
  • Institute of Tele- and Radio Technology, Vacuum Measurement Laboratory, Ratuszowa 11, PL 03-450 Warsaw, Poland
  • Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Koszalin University of Technology, Racławicka 15-17, PL 75-620 Koszalin, Poland
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