Keynote: Electropolishing of Niobium Superconducting Radio Frequency Cavities in a Low-Viscosity, Water-Based, HF-Free Electrolyte: From Coupons to Cavities
Authors: Taylor E. J. , Hall Timothy D , Snyder Stephen , Inman Maria E. 
Published: February 2020

Niobium superconducting radio frequency cavities (SRF) are required for the International Linear Collider as well as other high energy physics projects. In order for these cavities to achieve the required particle acceleration gradients, electropolishing is conducted as a final surface finishing operation. Conventional electropolishing of SRF cavities is based on the well-established viscous salt film theory
[1]
and utilizes a viscous electrolyte consisting of a mixture of sulfuric acid (95-98%) and hydrofluoric acid (49%), in a 9:1 volume ratio.[2] The HF is included to depassivate the niobium oxide film during electropolishing.[3]
Based on prior surface finishing activities using pulse reverse waveforms in low viscosity aqueous electrolytes, we speculated that a new paradigm was evolving and could be applied to electropolishing of SRF cavities. Initial research funds were provided from the Small Business Innovation Research program to demonstrate the feasibility of electropolishing niobium coupons in aqueous electrolytes using “Bipolar EP.” Although coupon polishing was successful, further funding was not provided due at least in part to skepticism by the review community familiar with the conventional viscous salt film theory. Subsequently, the American Recovery and Reinvestment Act funded a high risk/high payoff project to demonstrate the bipolar EP process on single cell SRF cavities. The ARRA funds did not allow for mechanistic studies or preliminary experiments on curved sections representative of the SRF cavity geometry.
Within these funding constraints, Faraday used guidelines and observations from 1”x1” and 3”x3” coupons to scale the bipolar EP process to single cell SRF cavities of ~1,800cm2 surface area. We recently reported these results at the 16th International Conference on RF Superconductivity in collaboration with Fermi National Accelerator Laboratory who declared that this cavity demonstrated the highest performance of any cavity previously processed at FNAL.
The presentation will focus on our development activities from coupons to cavities and the evolution of key observations required to electropolish niobium.[4] These key observations on niobium coupons were used to guide the development of bipolar EP process parameters on single cell SRF cavities even while the detailed mechanistic aspects were not completely understood. We conclude that innovation requires a careful balance between current fundamental understandings combined with evolving observation which do not necessarily fit the current paradigm.
The financial support of Faraday corporate, DOE P.O. No. 594128 and DOE Contract No. DE-SC0004588, and the technical support of Mr. Allan Rowe at Fermilab are acknowledged.

[1] P.A. Jacquet Trans. Electrochem. Soc.
69, 629 (1936).
[2] H. Tian, S. Corcoran, C. Reece, M. Kelly, J. Electrochem. Soc.,
155, D563-568 (2008).
[3] MacDougall, B. (1995), The Importance of Surface Oxide Films in Corrosion, Semiconductor and Environmental Research, Proceedings of the Symposium on High Rate Metal Dissolution Processes, Electrochemical Society Proceedings Vol. 95-19, Eds. M. Datta, B. MacDougall and J. Fenton, The Electrochemical Society, Pennington, NJ, pp 16-31.
[4] Inman, M., E.J. Taylor and T.D. Hall, Electropolishing of Passive Materials in HF-Free Low Viscosity Aqueous Electrolytes, J. Electrochem. Soc.,
160 (9), E94-E98, 2013.