Eco-Friendly Bipolar Electrochemical Bulk Processing of SRF Cavities
- Faraday Technology, Inc., Englewood, OH (United States)
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. 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, we have developed an electropolishing (EP) process for Nb SRF cavities, based on a new paradigm of non-viscous environmentally benign electrolyte based process, enabled by a bipolar electric field. The use of this electrolyte combined with the sophisticated electric field changes the EP mechanism by which material is removed and the surface profile reduced. Based on our understanding to date; we speculate 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 material. The design of the waveform is such that the niobium is preferentially removed from the peaks on the surface structure, thus smoothing the surface, and as has been demonstrated in prior work, an Ra of less than 1 nm has been obtained in coupon studies. Furthermore, when used as a final EP step, the cavity testing performance, according to our colleagues at Fermilab, Cornell, Oak Ridge, and Jefferson Lab agree that the performance was at least equivalent to conventional EP. The overall objective of this program was to develop, optimize and validate a low-cost, industrially compatible cavity EP process for eco-friendly processing of single- and nine-cell SRF cavities at the alpha/beta scale. To achieve this objective Faraday, along with subcontractors Cornell University and Advanced Energy Systems (AES), Inc. and CRADA member TJNAF focused on: 1) Designed and built alpha-scale Bipolar EP Cell based on vertical cavity orientation for single and multi-cell cavity processing, 2) Optimized the Bipolar EP parameters to improve EP performance and test in single-cell cavities, 3) Applied the optimized Bipolar EP process to multi-cell cavities and measured their performance, and 4) Refined the economic and manufacturing analysis completed during Phase I. Specific accomplishments during this Phase II program are highlighted as: Tooling was designed and built to process button cells, stacked single cell, and nine cell cavities. Process parameters like flow rate, button location, electrolyte, and processing conditions were optimization within a button cell cavity; Current density and oxidation characteristic simulations were carried out with Comsol® Multi-Physics modeling - software to understand the effect of gap on the primary current distribution. Dye flow study experiments were completed on single cell cavities to understand the effect of flow rate and baffling on electrolyte concentration uniformity. Stack single, nine, and nitrided single cell cavities were processed and tested for their performance at Cornell and TJNAF. Cost analysis of existing BCP processes was compared to the Bipolar EP process with the help of AES. Enabled beta scale transitioning of Bipolar EP cavity processing at TJNAF. The societal benefits of this eco-friendly alternative to the current EP technology is that it facilitates a cost-effective, scalable, industrially viable process to meet ILC cavity demands. Furthermore, the medical community uses HF to EP devices and implants fabricated from strongly passive materials, including alloys containing niobium and nickel/titanium alloys. The Bipolar EP process will eliminate the environmental hazards posed by the use of HF acid employed by chemical polishing and conventional EP.
- Research Organization:
- Faraday Technology, Inc., Englewood, OH (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- SC0011342
- OSTI ID:
- 1435156
- Type / Phase:
- SBIR (Phase II)
- Report Number(s):
- DOE-FTI-11342; Faraday-1023 Final
- Country of Publication:
- United States
- Language:
- English
Similar Records
Commissioning of JLab Vertical Cavity Processing System for SRF Nb Single Cell and Multicell Cavity With HF-Free Pulse-Reverse Electopolishing
Surface polishing of niobium for superconducting radio frequency (SRF) cavity applications