1. 1 Superconducting Nb thin film coatings to decrease impedance of unshielded accelerator bellows* C. James, M. Krishnan, B. Bures, Alameda Applied Science Corporation (AASC), San Leandro, California 94577 L. Phillips, X. Zhao, J. Spradlin, and C. Reece, Thomas Jefferson National Accelerator Facility (Jefferson Lab), Newport News, Virginia 23606 * Work supported by the Department of Energy

2. 2 Outline AASC’s motivation for pursuing SRF thin films Brief history of thin film development at AASC Nb on crystalline coupons Nb on metallic coupons Moving towards realistic structures (Nb coated bellows) Future Plans

3. 3 Motivation More than 1000 particle accelerators worldwide; most use normal cavities Facility for Rare Isotope Beams (FRIB), ILC and other large facilities: NSAC report states that as a result of technical advances, a world-class rare isotope facility can be built at ≈ half the cost of the originally planned Rare Isotope Accelerator (RIA), employing a superconducting linac Superconducting RF (SRF) cavities offer a ~10x improvement in energy efficiency over normal cavities, even accounting for cryogenic costs at 2K Operating at higher temperatures (~10K) would further improve accelerator energy efficiency as the cryogenic cooling becomes less demanding and moves away from liquid He and towards off the shelf cryo-coolers such as those used in cryo-pumps Replacing bulk Nb with Nb coated Cu cavities would also reduce costs The ultimate payoff would be from cast Al SRF cavities coated with higher temperature superconductors (NbN, Mo3Re, Nb3Sn, MgB2, oxypnictides) AASC’s thin film superconductor development is aimed at these goals

4. 4 Challenges for thin film SRF: path to success Amorphous How do we grow low-defect Nb films on such substrates? Study adhesion, thickness, smoothness, RRR, stability Understand these issues at the coupon level Validate these films at low field levels on realistic geometries Proceed to RF cavity level and measure Q at high fields Install 9-cell Nb coated Cu modules in SRF accelerator and validate the thin film solution –Spur acceptance of thin film Nb by accelerator community Continue R&D towards higher T c films and Al cavities Polycrystalline Cu and/or Al cavity substrates might be of two different forms

5. 5 Outline AASC’s motivation for pursuing SRF thin films Brief history of thin film development at AASC Nb on crystalline coupons Nb on metallic coupons Moving towards realistic structures (Nb coated bellows) Future Plans

6. 6 Coaxial Energetic Deposition (CED) Coaxial Energetic Deposition (CED TM ) CED coater uses “welding torch” technology Arc source is scalable to high throughputs for large scale cavity coatings Present version deposits ~1 monolayer/pulse ~1 ms UHV and clean walls important

7. 7 CAD: Pulsed Biased depostion; Dual Targets Dual Target source for Nb 3 Sn, Mo 3 Re, MgB 2 etc. Cathodic Arc Deposition (CAD) Pulsed Bias capability * A. BENDAVID, P. J. MARTIN, R. P. NETTERFIELD, G. J. SLOGGETT, T. J. KINDER, C., ANDRIKIDIS, JOURNAL OF MATERIALS SCIENCE LETTERS 12 (1993) 322-323 For more information, see the talk by M. Krishnan at 4:00pm

8. 8 Outline AASC’s motivation for pursuing SRF thin films Brief history of thin film development at AASC Nb on crystalline coupons Nb on metallic coupons Moving towards realistic structures (Nb coated bellows) Future Plans

9. 9 Excellent results on coupons motivates moving to accelerator structures 2 RRR-585 (±1% error) measured on 5µm film on MgO RRR-330 measured on a- sapphire 2 M. Krishnan et al, “Very high residual-resistivity ratios of heteroepitaxial superconducting niobium films on MgO substrates,” Superconductor Science and Technology, vol. 24, p. 115002, November 2011 M. Krishnan, et al., “Energetic condensation growth of Nb films,” Phys Rev ST Accel Beams, v15 032001 (2012)

10. 10 RRR of Nb thin films on a-sapphire substrates  XRD Pole Figures show that higher RRR correlates with higher crystalline order RRR=10, 150/150 RRR=31, 300/300 RRR=155, 700/500 PolycrystalPolycrystal with twins Monocrystal  "Twin symmetry texture of energetically condensed niobium thin films on sapphire substrate (a-plane Al2O3)”, X. Zhao, L. Philips, C. E. Reece, Kang Seo, M. Krishnan, E.Valderrama, Journal of Applied Physics, Vol 115, Issue 2, 2011, [in-press],

11. 11 RRR of Nb thin films on MgO substrates  Pole Figures show change in crystal orientation from 110 to 200 at higher temperature M. Krishnan et al, “Very high residual-resistivity ratios of heteroepitaxial superconducting niobium films on MgO substrates,” Superconductor Science and Technology, vol. 24, p. 115002, November 2011 RRR=7, 150/150 RRR=181, 500/500 RRR=316, 700/700 200110 & 200110 PolycrystalMonocrystal with two orientations Monocrystal with 100 orientation

12. 12 RRR–316: Cross sectional EBSD: dense, monocrystal film MgO Nb MgO Conductor – Mounting material Nb 0.15μm scan step size

13. 13 RRR–7: Cross sectional EBSD (OIM): textured crystal BSE image – Cross sectional view MgO Conductor – Mounting material Nb Nb thin film layer MgO IPF map 0.1μm Scan step size 35nm Scan step size Lower CI values between Nb matrix and MgO substrate indicate that there could be an amorphous or non-structured layer between them MgO layer

14. 14 Outline AASC’s motivation for pursuing SRF thin films Brief history of thin film development at AASC Nb on crystalline coupons Nb on metallic coupons Moving towards realistic structures (Nb coated bellows) Future Plans

15. 15 Numerous Nb/Cu samples have been created and we have learned a great deal about the process All of the samples that have been properly prepared have passed a tape-pull and simple scratch which demonstrates good adhesion of the film to the substrate Sample showing film delamination due to inadequate sample preparation Sample after tape-pull test showing no delamination of the film

16. 16 EBSD analysis shows large grain (> 50 um) Nb deposited on Cu at only 300 o C Pole figures shows a highly textured surface EBSD results of Nb on mechanically polished Cu

17. 17 RRR=110 achieved on mechanically polished, fine grain Cu at 400 o C RRR measurements must be performed on an insulating substrate Nb/Cu samples must have the Cu substrate removed before analysis (a 3-4 week process in (NH 4 ) 2 S 2 O 8 ) RRR measurements taken like this are considered to be a lower bound only due to the damage incurred by the film

18. 18 Nb on metal coupons has shown similar high- quality results Fine grain Cu coupons polished at AASC EBSD image of Nb on mechanically polished Cu showing large (~75 um) grains Nb/Cu results reinforce that the Nb film quality improves with both deposition temperature and thickness

19. 19 Preliminary tests, Nb/Stainless Steel Stainless steel type 304L and 321 coupons are being used to validate sample prep methods and coating adhesion before coating the ‘high-value’ bellows So far, all films have shown excellent adhesion (tape-pull, scratch test, etc) on un-polished (diamond cut) surface Still needed: thermal cycling tests and mechanical flexure tests to simulate bellows operating parameters

20. 20 Outline AASC’s motivation for pursuing SRF thin films Brief history of thin film development at AASC Nb on crystalline coupons Nb on metallic coupons Moving towards realistic structures (Nb coated bellows) Future Plans

21. 21 One idea for lowering bellows impedance  Bellows serve as interconnects and alignment points in particle accelerators  Bellows have the requirements of being mechanically robust and present a low beam impedance 1  Many designs implement a Nb shield to reduce beam impedance, however, these shields have the drawback of particulate generation  Alameda Applied Sciences Corporation proposes to deposit a superconducting layer of Nb on the inside surface of these bellows  This superconducting layer should lead to a decrease in the surface resistance at cryogenic temperatures and a decrease in the power consumption of the bellows while remaining particulate free 1 Wu, et al. “Low impedance bellows for high-current beam operations,” presented at Int. Particle Accel. Conf., New Orleans, LA, 2012.

22. 22 Initial bellows coating Coatings capabilities include: –Coupons (up to 65mm OD) –Tubes/Bellows/Cavities (ID > 45mm, OD < 250mm) Substrate heating –Radiative (up to 900 o C) –Contact (up to 400 o C) SS Bellows CED Anode Radiative Heaters A stock bellows (OD 95mm, ID 74mm) being coated for film uniformity analysis. (Radiative heater, 350 o C)

23. 23 Initial bellows coating Type 304SS bellows coated with 3 μm Nb film Tube and convolution sectioned for EDX analysis EDX showed uniform Nb coating on tube section and trough of convolution SEM shows smooth film covered in macroparticles

24. 24 Low field test cell Series of before and after tests planned to validate an improvement in Q with the Nb film Low-field bellows tests to be completed at JLAB Test assembly presently under construction

25. 25 Conclusions and future plans Initially explored Nb films on crystalline and metal substrates –Tests with SS substrates and ‘low-value’ tests bellows have been positive We have moved forward to begin testing real world components –Test stand and ‘high-value’ bellows are on track to begin testing soon Questions yet to be answered: What is the mechanical stability of a film on an adjustable substrate at cryogenic temperatures? Knowledge gained from bellows coatings will be applicable to simpler geometries (ie, accelerator cavities) –Cu is typically thought to be easier to coat than SS –Cavities will experience much less flexing during installation and operation

26. 26 Questions?