Acid-Free Electropolishing of Srf Nb Cavities
Authors: Radhakrishnan Rajeswaran , Hall Timothy D. , Snyder Stephen , Inman Maria , Taylor E. J. , Furuta Fumio 
Published: February 2020

The International Linear Collider (ILC) is a 200–500 GeV center-of-mass linear electron-positron collider, based on 1.3 GHz superconducting radio-frequency accelerating technology.[1] The ILC will be used to gain a deeper understanding of the forces of energy and matter by colliding beams of electrons and positrons at nearly the speed of light. To achieve this goal ~16,000 RF superconducting cavities operating within two linear accelerators at near absolute zero are needed.[2] These SRF cavities are fabricated from pure Nb, which has an extremely low surface resistance at 2 Kelvin. To take full advantage of the Nb superconducting properties, the inner surface must be polished to a microscale roughness, and cleaned to be free of impurities that could degrade performance. The DOE seeks commercially viable, fabrication technologies for SRF cavities, specifically new or improved electropolishing technologies that do not use hydrofluoric acid. Current methods use high viscosity electrolytes containing hydrofluoric acid, which is not conducive to low-cost, high volume manufacturing and is potentially harmful to workers.
Faraday is developing an electropolishing process for niobium SRF cavities, based on a new and evolving paradigm of non-viscous salt solution processing, enabled by a pulse reverse electric field as high-throughput vertical Acid-Free Final Electropolishing. The use of a low viscosity, near-neutral aqueous salt-based electrolyte enables a vertical cavity orientation, with no required rotation, which is conducive to high throughput manufacturing and industrialization. Furthermore, the use of this electrolyte combined with the pulse reverse or bipolar electric field changes the electropolishing mechanism by which material is removed and the surface profile reduced. Based on our limited understanding to date; [3] we have speculated that the process works via oxide film formation controlled during a designed anodic pulse, followed by an off time to remove heat and waste byproducts, followed by a cathodic pulse that removes the oxide film from the surface. This cycle is repeated many times per second, effectively removing niobium. The waveform design is such that the niobium is preferentially removed from the peaks on the surface, thus smoothing the surface.
In addition, we will also discuss recent results from our development of high-throughput vertical acid-free final electropolishing process for SRF cavities and button cell cavity studies. The button cell cavity was designed and built by Cornell for their own cavity finishing process optimization using the conventional sulfuric acid HF approach. This tool enables rapid analysis of the effect of the electrofinishing applied parameter on the finish produced as a function of anode to cathode gap and active area ratio position of the button, orientation, gravity, electrofinishing conditions, and flow rate.
Acknowledgements: Funding for this work is gratefully acknowledged from DOE SBIR Grant Number DE-SC0011235.
[1] http://home.web.cern.ch/about/updates/2013/06/international-linear-collider-ready-construction
[2] http://www.linearcollider.org/ILC/What-is-the-ILC/The-project
[3] M. Inman et al “Electropolishing of Passive Materials in HF-Free Low Viscosity Aqueous Electrolytes, J. Electrochem. Soc.,
160 (9) E94-E98 (2013).