1 Investigation of Microscopic Materials Limitations of Superconducting RF Cavities Steven M. Anlage Department of Physics Center for Nanophysics and Advanced Materials University of Maryland 29 October, 2008 SRF Materials Workshop

2 Questions to Address What new experiments shed light on limits played by topology, surface phases and structures, chemical makeup, or physics? How can we separate intrinsic behavior from that which is dependent on history or processing pathway? Are there new studies … telling us about the intrinsic qualities of the niobium surface? Do we still need to focus on heat transfer effects? My Answers: New Microscopic Techniques to link RF Properties to Local Structure 1) Near-Field Microwave Microscope 2) Laser Scanning Microscope Use these to establish connections between surface structure and RF performance on a microscopic scale

3 Near-Field Microwave Microscope 1 Create Strong ( B RF ~ 200 mT ) Highly Localized ( < 1  m ) RF ( GHz ) Magnetic Fields on Nb Surfaces at Low Temperatures ( < 2 K ) 2 Measure Local Response: Nonlinearity (Harmonics, Intermodulation) Sensitive to RF breakdown, flux generation, … 3 Correlate Local RF Properties to: Topography Defects (welds, grain boundaries, …) Surface Treatment Processing Microwave Microscope Probe Intense B RF Drive Nb Sample Localized Excitation on Surface Fundamental Response Harmonic / Intermod Response

4 Third harmonic power (dBm) Near-Field Microwave Microscope Example Result Localized Harmonic Generation from a single bi-crystal grain boundary in a high-T c (YBa 2 Cu 3 O 7-  ) thin film Fundamental Tone In Harmonic Tones Out > Superconductors have an intrinsic nonlinearity, clearly visible as J → J c The superfluid density is suppressed and very sensitive to perturbations  s becomes time-dependent, giving rise to harmonics and intermods Phys. Rev. B 72, (2005) f in = 6.5 GHz T = 60 K

5 Near-Field Microwave Microscope ► Next Generation for SRF Applications ◄ Magnetic Write Heads create i) Strong RF magnetic fields: ~ 1 T in existing write heads ii) Highly localized RF magnetic fields (|| to surface!): 100’s of nm Schematic of a longitudinal write head RF Drive Superconductors show nonlinearity due to both intrinsic and extrinsic effects Noise Level Temperature (K) Example Result Seagate Longitudinal Recording Head 3 rd -Harmonic Response of a Homogeneous high-T c film 3 rd -Harmonic Power P 3f (dBm) f in = 6.4 GHz Goals: Achieve ~200 mT surface RF fields on Nb at microwave frequencies at 2 K Image RF breakdown fields and correlate with surface properties… TcTc

6 Laser Scanning Microscope 1 Create a Microwave ( ~ GHz ) Resonance at Low Temperatures ( < 2 K ) 2 Perturb the Surface with a Modulated Laser Spot to cause Local Heating 3 Image: J RF (x, y) Local RF Current Density Local sources of Nonlinear Response RF vortex Entry and Flow Thermal Healing Length 4 Measure the change in f 0 and Q as the laser spot is scanned over the surface “Short-Sample” RF / Materials Science of Nb Surfaces Co-planar Waveguide Resonator f 0, Q Microwave Input Ground Plane Material of interest

7 1 x 8 mm scan Laser Scanning Microscope “Short-Sample” RF / Materials Science of Nb Surfaces Reflectivity Image RF Image 100  m x 200  m I RF B RF RF contrast developed from grain bdes, cracks, scratches, etch features, corners, etc. Low Temperature Physics 32, 592 (2006)

8 Laser Scanning Microscope Preliminary Results on Bulk Nb Surfaces

9 Determination of the Thermal Healing Length T = 79 K P = - 10 dBm f = GHz f mod = 99.9 kHz YBCO/LaAlO 3 CPW Resonator 1 x 8 mm scan W strip = 500  m Thermal Conductivity Specific heat Mass Density Modulation Freq. Fit gives J. Supercond. 19, 625 (2006)

10 Conclusions A proposal to DOE/HEP on SRF Materials Issues is in preparation I am looking for materials collaborators who need to solve specific materials / low-T RF property problems and think that a microscopic approach is fruitful Some topics of interest: RF properties of grain boundaries and step edges RF properties of etch pits Understanding the microscopic physics of intrinsic RF breakdown Can coatings (S / I / S /…, or novel superconductor) prolong RF breakdown? I believe the Near-Field and Scanning Laser Microwave Microscopes can help to solve vexing SRF materials problems Existing Collaborators: Dragos Mircea [Seagate → Hitachi] Alexander Zhuravel [Kharkov, Ukraine] Alexey Ustinov [Karlsruhe, Germany]

11 Agilent E8362 Microwave Synthesizer LakeShore340 Temperature Controller Low-Pass Filters High-Pass Filters Microwave Amplifiers Gain ~ 60dB Directional Coupler (-6dB) sample Writer Cryogenic chamber Agilent E4407B Spectrum Analyzer f f, 2f, 3f,..

12

13

14 New Approach: inductive writer from a HDD B ~ 1 T !!!

15 P input loop coaxial probe sample surface

16 The inductive reader/writer : general concept Attractive features : ~ T magnetic field ~ T magnetic field sub-micron pole tips sub-micron pole tips

17 An inductive magnetic writer/reader : Example Pins for reader/writer magnetic disk / superconducting sample

18 An inductive magnetic writer/reader : Detail writer

19 An inductive magnetic writer/reader : Detail microcoils high-  magnetic core wiring of the microcoils to the pins

20 An inductive magnetic writer/reader : Schematics R. Hsiao IBM J. Res. Develop., vol. 43, no.1/2, Jan/March 1999 SAMPLE

21 Laser-induced signal generation model The power distribution induced by a focused modulated laser beam can be described as: temporalspatial x-y z t focused laser beam (l LAS = 670 nm, P L = 1 mW) substrate HTS film d heat source x z The thermally induced changes of S 21 (f) in the probe are understood as LSM photo-response (PR) that can be expressed as: inductive PR + resistive PR + insertion loss PR where A.P. Zhuravel, S. M. Anlage and A.V. Ustinov ~ 21 Scanning Laser Microscopy of Superconducting Microwave Devices

22 Results: Power dependence of PR R (x,y) LAO YBCO 10 mm 0 dBm +6 dBm+4 dBm +2 dBm LAO Images of resistive LSM PR penetrating into HTS film (area B) at the different input HF power indicated in the images. White dotted boxes show the YBCO/LAO patterned edge. Brighter regions correspond to larger amplitude of PR R (x,y). 3D plot of resistive LSM PR at +6 dBm LAO YBCO PR R (x,y) A.P. Zhuravel, S. M. Anlage and A.V. Ustinov Scanning Laser Microscopy of Superconducting Microwave Devices

23 Microwave Microscope Probe Intense B RF Drive Nb Sample Localized Excitation on Surface Fundamental Response Harmonic / Intermod Response

24 - YBCO film - LAO substrate 1 mm 1x1 mm X Y X Y J RF 0 max Frequency Power [dBm] f1f1 f2f f 1 -f 2 2f 2 –f x1 mm IMD PR J rf x y 1x1 mm 0 max (a) (b) (c) RF IN RF OUT X Y J RF X Y J IMD

25 TM 010 Resonant Mode Current Distribution Fiducial Materials Nb Strip with Two different Treatment

26

27 Nb Pb Sapphire defect

28

29 reflectivity RF = -3dBm Nb stripGap 1 mm F1

30 Line Scan Pb Nb 7.2 K 9.2 K 5.6 K 10.6 K Temperature 1 mm Line scan LSM PR 0 + peak - peak

31 Nb foil (160  m thickness) RF input Pb foil (50  m) Pb foil (50  m) Area scan Line scan Nb foil (160  m thickness) RF input Pb foil (50  m) Pb foil (50  m) Area scan Line scan Pb foil (50  m) Pb foil (50  m) Area scan Line scan