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Vertical EP and Green EP Progress at Cornell

This presentation discusses the progress made in the development of a HF-free and acid-free bipolar electropolishing (BEP) process for superconductive radio frequency cavities. The focus is on the successful treatment of both 9-cell and single-cell cavities using these environmentally-friendly methods, with improved Q values and high field gradients achieved.

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Vertical EP and Green EP Progress at Cornell

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  1. Vertical EP and Green EPProgress at Cornell J. T. Maniscalco CLASSE, Cornell University On behalf of: F. Furuta, M. Ge, M. Liepe (CLASSE, Cornell University) T. Hall, M. Inman, E. J. Taylor, R. Radhakrishnan, S. Snyder (Faraday Technology, Inc.) James Maniscalco

  2. Overview James Maniscalco Motivation: • As EP becomes standard across SRF, HF and other corrosive acids will be used in high volumes • Example: 9-cell VEP uses ~3 L of HF and ~22 L of H2SO4 • Environmental impact (waste disposal, cleaning) • Health & safety impact (spills, vapors) • Desire to limit use of dangerous acids, especially HF. • Faraday Technology, Inc. bipolar EP process (patented) In this talk: • HF-free EP worked well on single-cells… let’s try a 9-cell! • Even more “green” and safe: acid-free BEP.

  3. Faraday Tech. BEP an anodic pulse (+V to cavity) to grow an oxide layer on the niobium surface, voltage time off to dissipate the heat, remove reaction products, and replenishes reacting species, and a cathodic pulse (-V to cavity) to remove the oxide layer on the niobium surface, thus eliminating the need for HF. E. J. Taylor, M. E. Inman, T. D. Hall “Electrochemical system and method for electropolishing superconductive radio frequency cavities”, U.S. Pat No. 9 006 147, April 14, 2015; Japanese Pat No. 6 023 323, October 14, 2016; European Pat No. 2 849 908, February 15, 2017. James Maniscalco

  4. New Results James Maniscalco • 9-cell HF-free BEP • Motivation from earlier strong single-cell results (40 MV/m, Q > 1010) • 20 MV/m • Q > 1010 • 1-cell acid-free salt-based BEP • 40 MV/m • Q > 6×109 after 120 °C baking

  5. Early single-cell results 2K results James Maniscalco • HF-free BEP: single cells reach 40 MV/m • + 120 °C bake: improve Q, high field Q slope (HFQS)

  6. 9-cell BEP James Maniscalco • KEK 9-cell MHI-02 used for 3 HF-free BEP treatments. All runs proceeded as follows: • BEP with HF-free H2SO4 solution • Ultrasonic cleaning (USC) • Optical inspection • USC + high pressure rinse (HPR)

  7. 9-cell BEP: optical inspection Heavy discoloration at equator weld – oxide layer or O-rich Nb Mild discoloration at equator weld – oxide layer or O-rich Nb Looks like high-quality traditional EP BEP w/ 11% H2SO4 BEP w/ 4% H2SO4 BEP w/ 9% H2SO4 James Maniscalco Boroscope images taken at equator

  8. 9-cell BEP: RF test results HF-free BEP process can produce 9-cells with Q > 1e10 @ 2 K! Process parameters (pulse time, pulse V, acid %) have room for optimization. James Maniscalco Best performance from 9%wt. soln. Lower RBCS at 11%wt. but higher R0 4% & 9% soln.: no FE, quench at 19 MV/m 11% soln.: FE @ 12 MV/m, power limited

  9. 1-cell salt-based BEP Equator looks like high-quality traditional EP Mid-cup has discoloration (likely oxide) * 2-amino-2-(hydroxymethyl)propane-1,3-diol;hydrochloride; C4H11NO3 · HCl ** 2-amino-2-(hydroxymethyl)propane-1,3-diol; C4H11NO3 James Maniscalco • Cornell cavity LTE1-15 treated at Faraday: • 200 g/L NaNO3 + 19.5 g/L Tris acid* + 1 g/L Tris base** • 30 μm removal @ 1.6 μm/hr

  10. 1-cell salt-based BEP: RF results James Maniscalco • 2 RF tests: • Test 1: cavity as received after NaNO3 BEP (+USC, +HPR) • Test 2: 120 °C UHV bake 48 hr (+USC, +HPR) • Test 1: 40 MV/m, high Rs(results shown at 1.6 K),limited by quench • Test 2: higher Q0(results shown at 2 K),limited by Q-slope at 11 MV/m • Mean free path ≈ 18 nm; impurities left by BEP?

  11. 1-cell salt-based BEP: RF results 2K results 1.6 K 2K results Acid-free BEP process can produce 1-cells with high gradient (40 MV/m), 120 °C baking can improve Q Process parameters (pulse time, pulse V, salt conc. / species) need optimization. James Maniscalco Comparison with previous BEP 1-cell results:

  12. Traditional VEP @ 500 MHz HPR after chemistry steps 3) 2/6 doping 1) BCP (100 μm) 2) VEP (50 μm) 4) VEP (5 μm) including waveguide! James Maniscalco BNL B-cell N-doped (2/6 recipe), inc. 50 μm & 5 μm VEP steps

  13. Traditional VEP @ 500 MHz 4.2 K results James Maniscalco RF performance: Peak field of 12 MV/m Power limited (~200 W) No quench detected. Record 500 MHz Q0!

  14. Summary James Maniscalco • Progress with HF-free and acid-free bipolar EP process in collaboration with Faraday Technology, Inc. • 9-cell HF-free H2SO4 BEP: Q > 1e10 @ 2 K, gradient up to 20 MV/m • 1-cell acid-free salt-based BEP: achieved high gradient, Q improved with 120 °C baking • Future work: improving parameters of BEP to achieve higher gradient and Q0 • Pulse widths • Pulse voltage • Acid concentration • Salt species and concentration • Traditional VEP successful at 500 MHz • Record-high Q0 = 1.6e9, quench > 12 MV/m.

  15. James Maniscalco Thanks for your attention! Acknowledgements: DOE SBIR (Faraday BEP) BNL (B-cell VEP) NSF CBB (travel to TRIUMF) See also: “Latest Results of Salt Based Bipolar Electro-polishing…”, M. Ge, et al. LINAC 2018 “HF Free Bipolar Electro-polishing…”, F. Furuta, et al. IPAC 2018 “RF Test Result of a BNL N-Doped 500 MHz B-Cell…”, F. Furuta, et al. IPAC 2018 “RF Performance of Multi-cell Scale Niobium SRF Cavities…”, F. Furuta, et al. SRF 2017

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