Development of an HF Free Electropolishing Process for Nb Srf Cavities
Authors: Hall Timothy D , Snyder Stephen , Inman Maria , Taylor E. J. , Furuta Fumio 
Published: February 2020

The International Linear Collider (ILC) requires ~16,000 RF superconducting radio frequency niobium cavities that must be a) polished to a microscale roughness, and b) cleaned to be free of impurities that would degrade performance of the ILC. The DOE is seeking new commercially viable, fabrication technologies for SRF cavities, specifically new or improved bulk processing technologies. Current methods, such as buffered chemical polishing (BCP), use high viscosity electrolytes containing hydrofluoric acid, which is not conducive to low-cost, high volume manufacturing and is potentially harmful to workers. Therefore, Faraday is developing an electropolishing process for niobium SRF cavities, based on a new paradigm of non-viscous dilute acid processing, enabled by a pulse reverse or bipolar electric field. The use of this electrolyte combined with the sophisticated electric field changes the electropolishing mechanism by which material is removed and the surface profile reduced. The design of the waveform is such that the niobium is preferentially removed from the peaks on the surface structure, thus smoothing the surface to an Ra of less than 1 nm has been obtained in coupon studies. Furthermore, when used as a Final electropolishing step, the cavity testing performance, according to our colleagues at Fermilab, was at least equivalent to conventional electropolishing and in fact exhibited outstanding performance.
The overall objective of the program is to develop, optimize and validate a low-cost, high throughput vertical cavity electropolishing process for eco-friendly Bulk Processing of single- and nine-cell SRF cavities at the alpha/beta scale. To achieve this objective Faraday, along with subcontractor Cornell University are focusing on: 1) Designing and building alpha-scale Bipolar EP Cell based on vertical cavity orientation without rotation for single and multi-cell cavity processing, 2) Optimizing the Bipolar EP waveform parameters and cathode geometry to improve electropolishing performance and test in single-cell cavities, and 3) Appling the optimized Bipolar EP process to multi-cell cavities and determine performance.
During this talk we will specifically discuss recent results from our button cell cavity study studies. The button cell cavity, as shown in Figure 2, 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. Our recent studies that demonstrate an effect on the position of the button, orientation, gravity, electrofinishing conditions, and flow rate will specifically be discussed. Figure 2, shows the difference in removal rate and material removal ratio from the beam tube to the iris, as a function of half-cell orientation. Greater uniformity is demonstrated using the P/PR Electropolishing process as compared to the conventional H2SO4/HF process.

Acknowledgements: The financial support of DOE Contract No. DE-SC0011342is acknowledged.

Figure 1