Corcoran S.G.

Paper	Title	Page
TUP17	Preliminary Results on "Polarized" Buffered Chemical Polishing of a Large Grain Niobium Cavity	154
	G. Ciovati JLab S. G. Corcoran Virginia Tech J. Halbritter Forschungszentrum Karlsruhe
	In order to further understand the relation between the high-field Q-drop and the native oxide layer on the surface of SRF niobium cavities, we tried to alter the oxidation of niobium by applying a small voltage between a large-grain niobium cavity and a niobium rod inserted in the center, during buffered chemical polishing (BCP). The cavity RF test results at 1.7 K and 2.0 K did not show any major difference in the Q-drop behavior, compared to a standard BCP treatment. In one case, dark gray regions were visible inside the cavity and were responsible for additional losses, as seen with temperature maps. In order to better understand the electrochemical process occurring during the "polarized" BCP treatment, measurements of the polarization curve have been made on a cylindrical niobium sample, with a cylindrical niobium rod in the center.
WE104	Novel Characterization of the Electropolishing of Niobium with Sulfuric and Hydrofluoric Acid Mixtures	370
	H. Tian, M. J. Kelley TJNAF and College of William and Mary S. G. Corcoran Virginia Tech C. E. Reece TJNAF
	Niobium surfaces are commonly electropolished in an effort to obtain optimally smooth surfaces for high-field SRF cavity applications. We report the first use of controlled electrochemical analysis techniques to characterize electropolishing of Nb in a sulfuric and hydrofluoric acid electrolyte. Through the use of a reference electrode we are able to clearly distinguish the anode, cathode polarization potentials as well as the electrolyte voltage drop that sum to the applied power supply voltage. We then separate the temperature and HF concentration dependence of each. We also report the first use of Electrochemical Impedance Spectroscopy (EIS) on this system. EIS results are consistent with a presence of a compact salt film at the Nb/electrolyte interface that is responsible for the limiting current. Microscopic understanding of the basic Nb EP mechanism is expected to provide an appropriate foundation with which to optimize the preparation of high-field niobium cavity surfaces. The implication of EIS for monitoring Nb surface during electropolishing shows this technology could be potentially used as a source of on-line feedback.
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