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Tantawi - Measurements of RF properties of Novel Superconducting Materials

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http://www.surfacetreatments.it/thinfilms …

http://www.surfacetreatments.it/thinfilms

Measurements of RF properties of Novel Superconducting Materials (Sami Tantawi - 20')
Speaker: Sami Tantawi - SLAC National Accelerator Laboratory | Duration: 20 min.
Abstract
We have developed an X-band SRF testing system using a high-Q copper cavity with an interchangeable flat bottom for the testing of different materials. By measuring the Q of the cavity, the system is capable to characterize the quenching magnetic field of the superconducting samples at different power level and temperature, as well as the surface resistivity. This paper will present the most recent development of the system and testing results.

Published in Technology
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  • 1. Measurements of RF properties of Novel Superconducting Materials
    Jiquan Guo, Sami Tantawi, Charles Yoneda, David Martin(SLAC)
    Tsuyoshi Tajima(LANL)
    Oct. 4, 2010
  • 2. Sami Tantawi, Thinfilms for SRF
    Outline
    Motivation
    System description
    Overview
    Cavity design
    Experiment results
    Bulk Niobium
    Thin film
    Summary
  • 3. Motivation
    Test bed for SRF materials
    Magnetic quenching field characterization
    Possibly higher than Nb’s 170-180mT
    Different thin film or bulk sample
    Quick testing cycles with small samples
    Able to explore higher Tc materials (MgB2)
    Surface resistance characterization
    Non-superconducting materials
    RRR of Copper in different forms
    Other materials
    Sami Tantawi, Thinfilms for SRF
  • 4. System overview
    Characterize surface impedance by measuring the Qs of a cavity
    Capable of low power(NWA) and high power(Klystron) measurements
    X-band compact design
    Interchangeable flat cavity bottom, fits 2-3” diameter samples
    Cavity design maximizes H-field and minimizes E-field on the sample surface
    Can achieve ~360mT Hpeak with 50MW Klystron running 1.6µs flat pulses and Qe~320,000, Q0~320,000
    Sami Tantawi, Thinfilms for SRF
  • 5. Cavity Design
    High-Q hemispheric cavity under a TE013 like mode
    Zero E-field on sample
    Maximize H-field on the sample, Hpeak on bottom is 2.5 times of peak on dome
    Maximize loss on the sample, 36% of cavity total
    No radial current on bottom
    Copper cavity body
    No temperature transition or quenching
    Higher surface impedance
    Coupling sensitive to iris radius
    Possible future Nb cavity body
    More precise Rs characterization
    High-Q cavity under TE013 like mode
    H
    E
    Sample
    R=0.95”
    Q0,4K=~224,000
    Q0,290K=~50,000
    (measured from
    bulk Cu samples)
    Fres, design=~11.399GHz
    Fres, 290K=~11.424GHz
    Fres, 4K=~11.46GHz
    Q0,4K=~342,000
    (Estimated for zero
    resistivity samples, using measured Cu sample results)
    Tc~3.6µs(using Q value for copper at 4K)
    Qe~310,000
    Sami Tantawi, Thinfilms for SRF
  • 6. Cavity Assembly
    Sami Tantawi, Thinfilms for SRF
  • 7. System overview
    Sami Tantawi, Thinfilms for SRF
    Measurement ports:
    Forward Power: 2 or 5
    Reflected power: 4 or 3
    Waveform measured by either a Peak Power Meter or a scope with mixers
    Low power NWA measurement:
    6, 7, or 3
    Klystron
    1
    Cryostat
    Cryostat
    55dB
    Cavity
    2
    3
    Waveguide to Klystron/NWA
    10dB
    4
    5
    6
    7
    45dB
    45dB
    Mode converter
    Bend
    System Diagram
    Load
  • 8. Cavity heater power supply
    Power trace
    PPM
    T read/control
    Temperature Monitor
    Computer
    Amp and phase
    Scope
    I/Q control
    REF
    RF
    Frequency Control
    LO
    FWD
    RF
    I
    Cavity
    LO
    AFG
    TWT
    Klystron
    Q
    Load
    Load
  • 9. Frequency tuning
    phase
    Amplitude
  • 10. Measurement Results: Bulk Nb, high power test
    Sami Tantawi, Thinfilms for SRF
    Power traces of the high power test
  • 11. Nb Measurements vs. Pulse Length and Repetition Rate
    Sami Tantawi, Thinfilms for SRF
  • 12. Gradual Quenching Theory
    Sami Tantawi, Thinfilms for SRF
  • 13. Measurement Results: Bulk Cu
    This reference Cu sample is used to estimate the surface impedance of the cavity body. It uses similar material as the body, and the same annealing process.
    Sami Tantawi, Thinfilms for SRF
  • 14. Measurement Results: Bulk Nb, low power test
    FNAL bulk large grain Nb sample
    Sample surface impedance is estimated from the measured Q0 of the cavity with Nb sample and the measured copper surface impedance.
    Without magnetic shielding, the residual resistivity is high. After adding a magnetic shielding and 800˚C vacuum bake, surface impedance reduced by a factor of 3.
    Sami Tantawi, Thinfilms for SRF
  • 15. Measurement Results: Bulk Nb, high power test
    Sami Tantawi, Thinfilms for SRF
    FNAL bulk large grain Nb sample
    The residual resistivity is causing pulse heating and degrades the quenching field.
    Before magnetic shielding and baking, the sample start to quench at ~65mT with temperature rises ~5K.
    After shielding and baking, quenching starts at about 120mT when temperature rises ~3K.
  • 16. Measurement results: 300nm MgB2 on Sapphire
    Sami Tantawi, Thinfilms for SRF
    300nm MgB2 thin film on Sapphire substrate, provided by LANL and deposited at STI.
  • 17. Measurement results: MgB2/Al2O3/Nb
    Sami Tantawi, Thinfilms for SRF
    200nm MgB2/300nm Al2O3/Nb sample provided by LANL, Al2O3 coated at ANL, MgB2 coated at STI.
  • 18. Summary
    Demonstrated a system which can precisely measure the quenching field of up to 300-400mT
    Magnetic shielding is crucial for Nb residual resistivity. At X-band, pulse heating from residual resistivity can easily degrade the quenching field.
    Precision of Rs measurement is currently at the level of 0.1mΩ. It can be improved with a separate Nb cavity.
    Sami Tantawi, Thinfilms for SRF