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Xiaoxing Xi - Magnesium Diboride Thin Films for Superconducting RF Cavities
 

Xiaoxing Xi - Magnesium Diboride Thin Films for Superconducting RF Cavities

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Magnesium Diboride Thin Films for superconducting RF cavities (Xiaoxing Xi - 40')
Speaker: Xiaoxing Xi - Temple University | Duration: 40 min.
Abstract
MgB2 has a Tc of 40 K, a low residual resistivity, and a high Hc . RF cavities coated with MgB2 films have the potential for a higher Q and gradient than Nb cavities with an operation temperature of 4.2 K or higher. At Temple University, we have started a project to study issues related to the application of MgB2 to RF cavities, and to coat single-cell RF cavities with MgB2 film for characterization by the collaborators in accelerator-compatible environment. The key technical thrust of this project is the deposition of high quality clean MgB2 films and coatings using a hybrid physical-chemical vapor deposition technique. I will review the progress to date in this project.

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  • Content Slide Notes “ UNCLASSIFIED ” marking of slides is not a security requirement and may be deleted from the Slide Master (View › Master › Slide Master). In general, slides should be marked “ UNCLASSIFIED ” if there is potential for confusion or misinterpretation of something that could be deemed classified. For guidance on marking slides containing classified and unclassified controlled information, see the Protecting Information Web site at http://int.lanl.gov/security/protectinfo/ .
  • From a thermodynamic point of view, the deposition techniques that have certain degrees of success in growing MgB2 films can be categorized into high-temperature, intermediate temperature and low temperature processes according to the highest processing temperature used. In a typical high temperature annealing in Mg vapor technique, B films is first deposited at room temperature, then sealed in Ta tube with bulk Mg and annealed at 850 degrees C an up. Because Mg is heated in an enclosure, the Mg vapor pressure can be very high, which allows the high temperature processing. The phase space of the operating conditions is indicated here. Because of the high processing temperature, epitaxial growth is achieved, and the films have excellent superconducting properties.
  • Content Slide Notes “ UNCLASSIFIED ” marking of slides is not a security requirement and may be deleted from the Slide Master (View › Master › Slide Master). In general, slides should be marked “ UNCLASSIFIED ” if there is potential for confusion or misinterpretation of something that could be deemed classified. For guidance on marking slides containing classified and unclassified controlled information, see the Protecting Information Web site at http://int.lanl.gov/security/protectinfo/ .

Xiaoxing Xi - Magnesium Diboride Thin Films for Superconducting RF Cavities Xiaoxing Xi - Magnesium Diboride Thin Films for Superconducting RF Cavities Presentation Transcript

  • Magnesium Diboride Thin Films for Superconducting RF Cavities Xiaoxing Xi Department of Physics Temple University, Philadelphia, PA October 5, 2010 Workshop on Thin Film RF Padua, Italy Supported by DOE/HEP Contributors Chenggang Zhuang, Alex Krick, Teng Tan, Ke Chen, Ed Kaczanowicz Collaborators Enzo Palmieri (INFN), Som Tyagi (Drexel), Steven Alage (Maryland)
  • Outline
    • Why is MgB 2 promising
      • R s
      • Gradient
      • Successful fabrication of MgB 2 films
    • Progress report
      • DOE/HEP supported project: “Magnesium Diboride Thin Films for Superconducting RF Cavities”
      • Film deposition and coating of cavity
      • Microwave characterization
    • Summary
  • MgB 2 : An Outstandting Superconductor
    • SCIENCE
    • BCS, T c = 40 K
    • Two bands with weak inter-band scattering: σ band and π band
    • Two gaps (∆ σ ~ 7meV, ∆ π ~ 2meV) : two order parameters
    HIGH FIELD
    • High performance in field ( H c2 over 60 T)
    • Cryogen free, mobile MRI, high field magnets, etc.
    • ELECTRONICS
    • 25 K operation , much less cryogenic requirement than LTS Josephson junctions
    • Superconducting digital circuits
    • High T c , low resistivity
    • High Q , high E acc
    • Possibly replacing Nb
    • ILC, light sources
    SRF CAVITY  //
  • Potential Low BCS R s in MgB 2 for RF Cavity R s (BCS) versus ( ρ 0 , T c ) Vaglio, Particle Accelerators 61, 391 (1998)
    • 230 μΩ at 15 K and 18 GHz
    • Scale to 0.5 GHz: 177 n Ω at 15 K.
    • ~20 time reduction in R s for 15 K to 4 K has been shown for MgB 2
    R s from π Gap R s from σ Gap Nb T = 4.2 K, f = 0.5 GHz Nb 3 Sn MgB 2 18 GHz
    • MgB 2 films from STI by reactive co-evaporation
    • A stripline resonator at 2 GHz for films on dielectric substrates
    • A dielectric resonator at 10.7 GHz for films on metallic substrates.
  • Jin et al , SC Sci. Tech. 18, L1 (2005) Surface Resistance vs. Mean Free Length A minimum in the surface resistance when the mean free path is comparable to the coherence length . Nb , 1.5 GHz Nb /Cu Padamsee, SC Sci. Tech. 14, R28 (2001) MgB 2
  • Jin et al , SC Sci. Tech. 18, L1 (2005) Penetration Depth  =  /  ≈ 6 H c = H c2 /√2  ≈ 820 mT H sh ≈ 0.75 H c ≈ 620 mT H c2 (0) =  0 /2 π  ab (0) 2  ab (0) ≈ 7 nm Upper Critical Field Potential High Maximum E acc in MgB 2 for RF Cavity Xi, SC Sci. Tech. 22, 043001 (2009)
    • Issues to be studied:
    • Effects of existence of two gaps.
    •  is larger for π band than for σ band ( Koshelev and Golubov, PRL 90, 177002 (2003) ) .
    •  is larger for π band than for σ band ( Eisterer et al, PRB 72, 134525 (2005) ) .
    (Nb: H c ≈ 200 mT)
  • Single Crystal, Magnetization Powder, Specific Heat Experimental Measurement of H c of MgB 2 Zehetmayer et al , PRB 66, 052505 (2002) Bouquet et al , PRL 87, 047001 (2001)
  • Dahm & Scalapino, APL 85, 4436 (2004) Effects of Two Gaps on Microwave Nonlinearity Nonlinear Coefficient of MgB 2 YBCO, MgB 2 , & 40-K BCS SC MgB 2 of Different Intraband Scattering
    • It has been predicted theoretically that
      • nonlinearity in MgB 2 is larger than 40 K BCS superconductor due to existence of two bands.
      • compares favorably with HTS at low temperature
    • Manipulation of interband and intraband scattering could improve nonlinearity.
  • Microwave Nonlinearity of HPCVD MgB 2 Films Cifariello et al , APL 88, 142510 (2006)
    • Result in agreement with Dahm – Scalapino prediction.
    • Modification of interband and intraband scattering key to low nonlinearity.
    theoretical d wave theoretical one-band s wave theoretical two-band s wave Г π / Г σ =2 YBCO Nb MgB 2
  • Larbalestier et al ., Nature 414, 368 (2001) Properties of Various Superconductors 40 0.1 Nb 9.2 0.4 40 2 NbN 16.2 15 200 70 7 22 3.6
  • Keys to Growth of MgB 2 Films
    • Keep a high Mg pressure for phase stability
      • For example, at 600 °C Mg vapor pressure of 0.9 mTorr or Mg flux of 500 Å/s is needed
    • No need for composition control , as long as the Mg:B ratio is above 1:2.
    • Keep oxygen away : Mg reacts strongly with oxygen - forms MgO, reduces Mg vapor pressure.
    • Avoid carbon : Carbon doping reduces T c and increases resistivity
    Liu et al ., APL 78, 3678 (2001)
  • Hybrid Physical-Chemical Vapor Deposition get rid of oxygen prevent oxidation make high Mg pressure possible generate high Mg pressure: required by thermodynamics pure source of B B supply (B 2 H 6 flow rate) controls growth rate Pure source of Mg high enough T for epitaxy Schematic View Substrate H 2 (~100 Torr) B 2 H 6 (~ 5 - 250 sccm) Mg Susceptor 550–760 °C
  • Clean, Epitaxial HPCVD MgB 2 Films Xi et al , Physica C 456, 22 (2007) Wu et al. , APL 85, 16 (2004) SiC MgB 2 interface
  • Jin et al , SC Sci. Tech. 18, L1 (2005) Microwave Properties vs Cleanness of MgB 2 Films Surface Resistance @ 18 GHz Penetration Depth π - Band Gap
    • Dielectric single-crystal sapphire puck resonator at 18 GHz.
    • Cleaner films (low resistivity) leads to lower surface resistance, shorter penetration depth, and smaller π band gap (less interband scattering).
  • Technical Approach
    • Advantages:
    • Localized source of high-pressure Mg vapor
    • Negligible MgO in the pocket
    • B evaporation in vacuum
    • No need to control Mg/B flux ratio
    • Ability to separate substrate T from Mg T
    • Uniform growth on multiple large-area substrates
    • Non-contact: Double-sided deposition possible
    • Can try a variety of substrate materials at once
    Rotating blackbody heater Reactive evaporation using the pocket heater Directly addresses MgB 2 growth difficulties
  • MgB 2 Film by Reactive Co-Evaporation Moeckly and Ruby, SUST 19, L21 (2006)
  • Large Area HPCVD Films Using STI Pocket Heater
    • Differences from reactive co-evaporation:
    • B 2 H 6 used as boron source instead of e-beam evaporation
    • Hydrogen used as the carrier gas instead of HV
    • Deposition temperature in broader range
    • Advantages:
    • Large area and double sided films
    • Potential for scale up for wires
    Wang et al . Supercond. Sci. Tech. 21, 085019 (2008) B 2 H 6
  • High-Temperature Ex-Situ Annealing Kang et al , Science 292, 1521 (2001) Eom et al , Nature 411, 558 (2001) Ferdeghini et al , SST 15, 952 (2001) Berenov et al , APL 79, 4001 (2001) Vaglio et al , SST 15, 1236 (2001) Moon et al , APL 79, 2429 (2001) Fu et al , Physica C377, 407 (2001) B Mg Low Temperature ~ 850 °C in Mg Vapor Epitaxial Films
  • MgB 2 Film by Reaction of CVD B Film Clean B precursor layer leads to clean MgB 2 film.
  • Outline
    • Why is MgB 2 promising
      • R s
      • Gradient
      • Successful fabrication of MgB 2 films
    • Progress report
      • DOE/HEP supported project: “Magnesium Diboride Thin Films for Superconducting RF Cavities”
      • Film deposition and coating of cavity
      • Microwave characterization
    • Summary
  • “ Magnesium Diboride Thin Films for Superconducting RF Cavities”
    • Supported by DOE HEP, “Fundamental Research in Superconducting RF Cavity Design” Program
    • Project period: 6-15-10 to 6-14-13
    • Project objective: Develop HPCVD technology for MgB 2 SRF cavities
    • Research activities:
      • Study of MgB 2 film properties related to the RF cavity application
      • Coating single-cell RF cavities with MgB 2 film
    • Film research:
      • Deposit large-area (2" - 4" diameter) films using both in situ HPCVD and two-step annealing process
      • Deposit on sapphire and metal substrates, such as Nb, Mo, and Cu
      • Effects of smoothness, thermal expansion mismatch, hydrogen, film stability, two-gap nature of MgB2, and multilayer structure on RF properties
    • Coating cavity:
      • Both in situ HPCVD and two-step annealing process
      • First step: 6 GHz cavity from INFN
      • Next step: larger cavities
  • Scaling up HPCVD for Large Area Films
    • Inductive heater
    • Successful
    • 5mm x 5mm
    • Resistive heater
    • Successful
    • 15mm x 15mm
    • Resistive heater
    • Designed for 2" dia.
    B 2 H 6 B 2 H 6 B 2 H 6 Penn State Mg Susceptor 550–760 °C Mg Resistive Heater Peking University Mg Resistive Heater Temple University
  • HPCVD Reactors at Temple
    • Two HPCVD reactors with resistive heaters
    • Smaller reactor for 15mm x 15mm films
    • Larger reactor for 2” diameter films
  • First Small MgB 2 Films Made at Temple
  • overall size 3’’x 5’’ New Design of Pocket Heater
    • Inspired by Michio Naito of Tokyo University of Agriculture and Technology
    • Much more compact than the STI pocket heater – the dimension of the heater for 2” films is less than 5”
    • Under construction
    B 2 H 6 inlet Mg oven 2’’ wafer Top heater Bottom heater
  • In Situ Coating of 6 GHz Nb Cavity
    • 6 GHz Nb cavity provided by Enzo Palmieri, INFN. Characterization capability exists at INFN.
    • Small size: Can be coated using existing equipment in the lab; Low cost for optimization; First step towards coating of larger cavities.
    • Setup design near finish
    4" B 2 H 6 Mg
  • Cavity Coating Setup at Temple Resistive tube furnace will be installed in the existing HPCVD system Integrated HPCVD system: Two HPCVD systems, one sputtering system, and one distribution chamber connected together. Cavity Up Cavity Down
  • Coating SRF Cavity with a Two-Step Process Coating cavity with B layer at ~400-500°C using CVD Reacting with Mg to form MgB 2 at ~ 850-900 °C in Mg vapor Mg vapor H 2 , B 2 H 6
  • Low-Field Microwave Magnetoabsorption
    • Energy loss caused by the vortex movement leads to increase in microwave reflected power P from a resonated cavity
    • H mw ⊥ film surface
    • H ext // film surface
    Bai, Patanjali, Bhagat, and Tyagi, J. Supercond. 8, 299 (1995). Prof. Som Tyagi, Drexel University 9.3 GHz source Power meter Circulator Helmholtz coil Microwave Cavity Q~2500 H // ab Cryo-cooler Head Waveguide Quartz Rod Quartz Tube
  • Low-Field Microwave Magnetoabsorption 1000 G 1500 G 0 G 9.3 GHz Thickness: 100 nm Value much higher than experimental and predicted value (~ 20 mT) To be understood. dH c1 / dT =25 mT/K 20 30 40 Temperature (K) Absorption T (K) H c1 (G) 28 1500 30 1000 32 0
  • Microwave Surface Impedance Measurement Advantages: Measures transition and homogeneity of sample Fast: built for a continuous flow cryostat Can be used for quantitative R s measurements if required Disadvantages: sample is pressed against a plate (this could be relaxed) The dielectric puck is attached to the sample with grease 24.7 GHz resonance A resonance of the dielectric puck is excited. It’s properties are modified by the superconducting film underneath Prof. Steven Anlage, U. of Maryland
  • Summary
    • Clean MgB 2 thin films have excellent properties:
      • low resistivity (<0.1  cm ) and high T c promise low BCS surface resistance
      • long coherence length and short penetration depth promise high H c : possibly as high as ~ 820 mT
      • Two gap effects need to be investigated.
    • Current status of MgB 2 coating for SRF:
      • July 2009: Move from Penn State University to Temple University
      • June 2010: Start of DOE/HEP SRF project
      • HPCVD at Temple for small films (15mm x 15mm) operational
      • Scaling up for in situ deposition of 2&quot; films underway
      • Coating of 6 GHz cavity by in situ processes being designed
    • Low-Field Microwave Magnetoabsorption at Drexel University shows possible high H c1
    • Dielectric puck resonator from University of Maryland installed.