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.

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