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

Background Best KEK - Cornell and J-Lab Nb cavities are close to the depairing limit (H  H c = 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 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 KEK&Cornell

H c2 H c1 0 H Strong vortex dissipation HcHc Very weak dissipation - M Superconducting Materials Material T c (K) H c (0) [T] H c1 (0) [T] H c2 (0) [T] (0) [nm] Pb na 48 Nb Nb 3 Sn NbN MgB YBCO Very weak dissipation at H < H c1 (Q = ) Q drop due to vortex dissipation at H > H c1 Nb has the highest lower critical field H c1 Thermodynamic critical field H c (surface barrier for vortices disappears) Nb Higher-H c SC

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

-Nanoscale: Nonlinear BCS 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 Multiscale SRF Mechanisms Multiple experimental and theoretical approaches are needed

MAIN ISSUES 1.Fundamental limits: high-field surface resistance and the RF critical field. 2.Surface materials science of Nb: effects of grain boundaries, impurity distribution profiles at the surface and surface defects on Q 3.Close knowledge gap: how processing affects microstructure and superconducting properties 4.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

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

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

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

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

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

3DAP: impurity profiles at the surface (NU) Atom Probe Tomography of electro polished Nb (RRR300) tip: 1)Oxide Layer thickness: 25 nm 2)Interstitial O content: 15-8% in first 15nm at the surface Courtesy of K. Yoon, D. Seidman (NU)

X-ray Photoelectron Spectroscopy XPS reveals the surface chemistry non-destructively to study effect of cavity processing H. Tien & C. Reece/ JLab - CWM - BU - NSLS-BNL) 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 Nb 2 O 5, dielectric NbO x (0.2 < x < 2), metallic NbO x precipitates (0.02 < x < 0.2) Nb 3d spectrum

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

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

JLAB – Thin Films L. Phillips, G. Wu, A.-M. Valente - JLab Plasma coating as a new approach to produce high quality Nb film: To improve Nb on Cu cavities To explore NbN or Nb 3 Sn thin film coating to Increase RF critical field (Gurevich, 2006)

High Field Surface Resistance Measurements “Sample-in-Host” (TE 011 ) cavity systems: JLab I (7.5 GHz) JLab II (3.5 GHz) CU (10 GHz) SLAC (11.4 GHz) LANL (20 GHz) Probing the fundamental RF field limits of superconducting materials R. Campisi SNS, C. Nantista SLAC L. Phillips / JLab No results yet at ultimate fields! Goal: reach 200 mT with n  sensitivity

Microscopic theory of a high-field nonlinear surface resistance Thermal feedback model with the nonlinear R s (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 SRF THEORY (UW) A. Gurevich (UW)

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)

PROPOSAL FOR FUTURE R&D - I 1.Fundamental SRF physics: -Theory of critical RF field -Theory of nonlinear R s (T,H a, mfp) and nonequilibrium effects -Direct critical field measurements (SLAC and JLab) -Mechanisms of residual surface resistance -Hyper sound generation by rf field 2.Physics and materials science of the 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;

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 Nb 3 Sn, NbN or MgB 2

LANL/CU/STI – First MgB 2 Trails Courtesy of T. Tajima – LANL Good MgB 2 films from X.X.Xi - PSU Encouraging first results with MgB 2 !

Beyond the Nb technology A. Gurevich, Appl. Phys. Lett. 88, (2006) Thin high-H c layers (d < ) separated by insulating layers increase H c1 well above the bulk H c1. Nb 3 Sn thin film coating may triple the breakdown field of Nb and increase Q  exp(  /k B T), by 3-10 times because  Nb3Sn  1.8  Nb 50 nm Nb 3 Sn monolayer Nb 3 Sn G. Muller and P. Kneisel H c1 Nb3Sn

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.