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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)

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SRF Materials R&D

Alex Gurevich1 & Pierre Bauer2

1Applied Superconductivity Center, UW/NHMFL

2Fermi National Accelerator Laboratory

AARD Meeting

Fermilab, Feb. 15, 2006


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


  • 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

Superconducting Materials

Very weak


Strong vortex







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


Mechanisms of surface resistance

0.1-1 m



Mechanisms of Surface Resistance

  • Effect of impurities and rf field on surface resistance


  • 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

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


  • 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


  • Combine multiple experimental techniques with theory to reveal SRF mechanisms on relevant length scales


  • 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)

Thermometer array to detect

hotspots, which ignite cavity breakdown

G. Ciovati - JLab - ODU

H. Padamsee - Cornell

MOI of Vortex Penetration (ASC&FNAL)




A. Polyanskii & P. Lee – ASC/UW

  • MOI of Nb bi-crystals cut from a Nb cavity.

  • MOI reveals vortex penetration along grain boundaries


Faraday rotation of the

light polarization angle

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

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) hotspots

  • 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)

X-ray Photoelectron Spectroscopy hotspots

  • 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)

Mechanical Properties hotspots

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

Thermal Properties at MSU hotspots

Thermal conductivity and Kapitza conductance

Surface roughness strongly affects

the Kapitza conductance

A. Aiziz & T. Grimm/ MSU

JLAB – Thin Films hotspots

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

High Field Surface Resistance Measurements hotspots

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

SRF THEORY (UW) hotspots

  • 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)


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)


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)


  • 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;


  • 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

LANL/CU/STI – First MgB hotspots 2 Trails

Encouraging first results with MgB2!

Courtesy of T. Tajima – LANL

Good MgB2 films from X.X.Xi - PSU

Beyond the Nb technology hotspots


50 nm Nb3Sn


G. Muller and P. Kneisel


  • 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)

Summary hotspots

  • 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.