SRF Materials R&D

1. SRF Materials R&D Alex Gurevich1 & Pierre Bauer2 1Applied Superconductivity Center, UW/NHMFL 2Fermi National Accelerator Laboratory AARD Meeting Fermilab, Feb. 15, 2006

2. Background Best KEK - Cornell and J-LabNb cavities are close to the depairing limit (H  Hc = 200 mT) How far further can rf performance of Nb cavity be increased? Theoretical SRF limits are poorly understood … Understand SRF mechanisms to replicate record cavities on the industrial scale KEK&Cornell • Address underlying SRF physics and materials science • Understand the RF critical fields and develop new materials and surface • treatments to increase Q: integrate SRF physics and materials science • Feedback between the fundamental R&D and cavity design and testing • Develop new ideas and attract students and people from different fields • Bring different groups and tools together in a national R&D SRF program

3. Superconducting Materials Very weak dissipation Strong vortex dissipation Hc Hc2 H 0 Hc1 Very weak dissipation at H < Hc1(Q = 1010-1011) Q drop due to vortex dissipation at H > Hc1 Nb has the highest lower critical field Hc1 Thermodynamic critical field Hc (surface barrier for vortices disappears) - M Higher-Hc SC Nb

4.  0.1-1 m l COOLANT Mechanisms of Surface Resistance • Effect of impurities and rf field on surface resistance E(x,t) • Break-in RF field for • vortices. Vortex oscillations • produce hotspots at • grain boundaries Heat Flux • Pento-oxides (5-10 nm) • RF field penetration • depth  = 40 nm defines Rs • Heat transport through cavity wall  3mm and Kapitza thermal resistance

5. Multiscale SRF Mechanisms • Nanoscale: NonlinearBCS surface resistance and the effect of impurity scattering in the 40 nm surface layer of rf field penetration • Microscale:RF dissipation due to vortex penetration. Critical RF fields and effect of grain boundaries and surface defects • Macroscale: Thermal rf breakdown. Effect of thermal conductivity and the Kapitza thermal resistance. Mechanical and acoustic properties. • Technological scales: Effect of cavity processing on SRF performance Multiple experimental and theoretical approaches are needed

6. MAIN ISSUES • Fundamental limits: high-field surface resistance and the RF critical field. • Surface materials science of Nb: effects of grain boundaries, impurity distribution profiles at the surface and surface defects on Q • Close knowledge gap: how processing affects microstructure and superconducting properties • New materials and surface treatments to increase Q and the breakdown field beyond the intrinsic limits of Nb • APPROACH • Combine multiple experimental techniques with theory to reveal SRF mechanisms on relevant length scales • OUTCOME • Use the acquired knowledge to improve cavity performance; streamline and reduce cost of processing

7. Emerging SRF Materials & Surface Techniques • Thermal Maps - Cornell, JLab • Magneto-Optical Imaging (MOI) – ASC • Eddy Current Scanning - Fnal • Near Field RF Microscopy - ASC • 3D Atomic Probe - NU • X-ray Photoelectron Spectroscopy (XPS) - JLab • Mechanical Properties - JLab, MSU • Thermal Properties - MSU • Plasma Thin Film Coating - JLab • High-field surface resistance measurements – SLAC, SNS, JLab • Theory - ASC Diverse experimental and theoretical tools require coordinated efforts to understand SRF mechanisms and the ways to increase Q and the breakdown field of SC cavities

8. Thermal Maps (Cornell and J-Lab) Thermometer array to detect hotspots, which ignite cavity breakdown G. Ciovati - JLab - ODU H. Padamsee - Cornell

9. MOI of Vortex Penetration (ASC&FNAL) A P Hz(x,y) A. Polyanskii & P. Lee – ASC/UW • MOI of Nb bi-crystals cut from a Nb cavity. • MOI reveals vortex penetration along grain boundaries (x,y)=VHz(x,y)d Faraday rotation of the light polarization angle

10. FNAL – Eddy Current Scanning Eddy Current Scanning: mostly detects surface imperfections (pits, scratches, height / thickness variations) Examples of calibration disc measurement and optical measurement of a pit C. Boffo / Fnal

11. Near Field RF Microscope: effect of surface defects on SRF hotspots Scanning tip applies low-power GHz field in a few micron region Reveals lateral variations of surface resistance If combined with SEM, XPS, NFRFM reveals defects responsible for hotspots Shows evolution of the hotspot distribution for different baking treatments ASC plans to build a low-T NFRFM to investigate surface of Nb cavities

12. 3DAP: impurity profiles at the surface (NU) • Atom Probe Tomography of • electro polished Nb (RRR300) tip: • Oxide Layer thickness: 25 nm • Interstitial O content: 15-8% • in first 15nm at the surface Courtesy of K. Yoon, D. Seidman (NU)

13. X-ray Photoelectron Spectroscopy • XPS reveals the surface chemistry non-destructively to study effect of cavity processing • Oxide thickness is less with EP than BCP and single than poly crystal • Low-T bake decreases oxide thickness and creates more sub-oxides. Vacuum preserves the baked state, but sustained air exposure restores it Hydrocarbons & impurities Nb hydroxides Nb2O5, dielectric NbOx (0.2 < x < 2), metallic NbOx precipitates (0.02 < x < 0.2) Nb 3d spectrum H. Tien & C. Reece/ JLab - CWM - BU - NSLS-BNL)

14. Mechanical Properties Poly-crystal – JLab Poly-crystal – MSU Poly-weld – MSU Single crystal – JLab • Mechanical properties testing at JLab (single crystal, cryogenic) & MSU (weld analysis, texture) • Inspired the single crystal approach! G. Myneni / JLab H. Jiang-T. Bieler / MSU

15. Thermal Properties at MSU Thermal conductivity and Kapitza conductance Surface roughness strongly affects the Kapitza conductance A. Aiziz & T. Grimm/ MSU

16. JLAB – Thin Films Plasma coating as a new approach to produce high quality Nb film: To improve Nb on Cu cavities To explore NbN or Nb3Sn thin film coating to Increase RF critical field (Gurevich, 2006) L. Phillips, G. Wu, A.-M. Valente - JLab

17. High Field Surface Resistance Measurements Probing the fundamental RF field limits of superconducting materials • “Sample-in-Host” • (TE011) cavity systems: • JLab I (7.5 GHz) • JLab II (3.5 GHz) • CU (10 GHz) • SLAC (11.4 GHz) • LANL (20 GHz) No results yet at ultimate fields! Goal: reach 200 mT with nW sensitivity R. Campisi SNS, C. Nantista SLAC L. Phillips / JLab

18. SRF THEORY (UW) • Microscopic theory of a high-field nonlinear surface resistance • Thermal feedback model with the nonlinear Rs(H): improved agreement with the multiple source cavity data from JLab, CU, FNAL, DESY and Saclay • Model of hot spots around defects to explain the medium and high field Q-drop • RF vortex dissipation at grain boundaries; penetration field and medium Q slope. How much vortex dissipation can be tolerated? • Theory of multilayer coating to increase the RF breakdown field A. Gurevich (UW)

19. PROPOSAL FOR FUTURE R&D Multi-institutional collaboration (preliminary list) National labs: FNAL (P. Bauer),JLab (L. Phillips, P. Kneisel, G. Ciovati, C. Reece), SNS (R. Campisi), SLAC (C. Nantista),ANL (K. Shepard, J. Norem), LANL (T. Tajima) Universities: Cornell (H. Padamsee), Penn State (X.X. Xi), Michigan State (T. Grimm, T. Bieler), Northwestern (D. Seidman), ASC/NHMFL (A. Gurevich, P. Lee, D.C. Larbalestier)

20. PROPOSAL FOR FUTURE R&D - I • Fundamental SRF physics: • -Theory of critical RF field • Theory of nonlinear Rs(T,Ha, mfp) and nonequilibrium effects • Direct critical field measurements (SLAC and JLab) • Mechanisms of residual surface resistance • Hyper sound generation by rf field • Physics and materials science of the 40-50 nm surface layer • -Local field penetration (MOI, LTSM, NFRFSM) • - Correlate SRFM with surface defects and thermal mapping; • - Local chemical analysis (3DAP). Effect of surface pento-oxide structures and impurities on surface resistance • - SEM and surface topography;

21. PROPOSAL FOR FUTURE R&D - II • 3. Thermal stability and cavity quench • Theory of nonuniform thermal breakdown caused by hotspots; minimum quench energy and lateral quench propagation velocity • Correlate thermal maps with cavity processing • Optimizing thermal properties (thermal conductivity and Kapitza resistance) 4. New ways of improving cavity performance -Local grain boundary alloying; - Thin film multilayer coating of conventional Nb cavities with Nb3Sn, NbN or MgB2

22. LANL/CU/STI – First MgB2 Trails Encouraging first results with MgB2! Courtesy of T. Tajima – LANL Good MgB2 films from X.X.Xi - PSU

23. Beyond the Nb technology Nb3Sn 50 nm Nb3Sn monolayer G. Muller and P. Kneisel Hc1Nb3Sn • Thin high-Hc layers (d < ) separated • by insulating layers increase Hc1 well • above the bulk Hc1. • Nb3Sn thin film coating may triple the breakdown field of Nb and increase • Q  exp(/kBT),by 3-10 times because • Nb3Sn  1.8Nb A. Gurevich, Appl. Phys. Lett.88, 012511(2006)

24. Summary • Further progress in SRF can be possible by addressing underlying physics and materials science • It is time to bring many excellent (but disconnected) groups and tools together in a national R&D materials SRF program • SRF can grow by developing new ideas and attracting students and people from different fields • Address SRF challenges: understanding the RF critical fields, and developing new materials and surface treatment/coating techniques • Establish a feedback between the fundamental SRF science and cavity design and testing (similar to the successful DOE LTSM program). Bring together different US groups through collaboration and yearly SRF materials workshops.