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

1. 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)

6. 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

8. 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)

11. 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

13. 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

14. 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

17. MgB 2 Film by Reactive Co-Evaporation Moeckly and Ruby, SUST 19, L21 (2006)

19. 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

20. MgB 2 Film by Reaction of CVD B Film Clean B precursor layer leads to clean MgB 2 film.

25. First Small MgB 2 Films Made at Temple

28. 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

29. 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

31. 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

32. 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