Acid-Free Electropolishing of SRF Cavities

The DOE seeks commercially viable, fabrication technologies for SRF cavities, specifically eco-friendly final polishing technologies. Current methods, such as conventional and buffered electropolishing, use high viscosity electrolytes containing hydrofluoric acid, which is not conducive to low-cost, high volume manufacturing and is potentially harmful to workers. robust, high throughput vertical Final Electropolishing process for SRF cavities, based on an Acid-Free Bipolar Electropolishing process, to replace conventional electropolishing for single and nine-cell cavities at the alpha/beta scale. The development of Bipolar EP for final surface finishing step will enable a manufacturing process that controls costs and reduces the environmental and health hazards associated with using acid solutions. Phase I coupon studies identified a range of salt-based aqueous electrolytes that were able to electropolish niobium to an Ra as low as 0.05 m, at removal rates as high as 0.7 m/min. The performance via cavity testing was unable to be tested due to scheduling conflicts. However, we mimicked the geometric conditions inside a cavity using coupon work, and successfully transitioned the Bipolar EP process to those conditions while maintaining removal rates of 0.3 to 0.4 m/min at a surface finish below 0.2 m. A preliminary economic analysis showed the commercial viability as compared to conventional practice, with up to 4x lower cost for the International Linear Collider. Phase II will develop a process and apparatus for cost-effective, scalable, high yield Nb cavity processing based on Bipolar EP. We will design and build an alpha-scale Bipolar EP cell, based on vertical cavity orientation without rotation, applicable to single- and multi-cell cavity processing, optimize Acid-Free Bipolar EP parameters and cathode geometry and surface area to improve final electropolishing performance, test in single-cell and multi-cell cavities, and refine the economic and manufacturing analysis. Commercial Applications/Other Benefits: The proposed product is a cost effective and robust process for bulk processing of niobium SRF cavities. The first market for cavities is for the International Linear Collider (~16,000), with applicability to many other cavity configurations and applications. Other markets for Bipolar EP include niobium-alloys that are hypoallergenic and commonly alloyed with titanium and zirconium to make implantable medical devices, and require HF acid for polishing. Elimination of HF acid for electropolishing of medical implants represents a commercial opportunity with a US market of ~$45B by 2014