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Thin superconducting niobium-coatings for RF accelerator cavities

Thin superconducting niobium-coatings for RF accelerator cavities. J. LANGNER, M.J. SADOWSKI, R. MIROWSKI, P. STRZYŻEWSKI AND J. WITKOWSKI The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 Otwock-Swierk, Poland L. CATANI, A.CIANCHI, J. LORKIEWICZ, R. RUSSO AND S. TAZZARI

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Thin superconducting niobium-coatings for RF accelerator cavities

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  1. Thin superconducting niobium-coatings for RF accelerator cavities J. LANGNER, M.J. SADOWSKI, R. MIROWSKI, P. STRZYŻEWSKI AND J. WITKOWSKI The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 Otwock-Swierk, Poland L. CATANI, A.CIANCHI, J. LORKIEWICZ, R. RUSSO AND S. TAZZARI University Tor Vergata and INFN, Via della Ricerca Scientifica 1, Roma 2, Italy D. PROCH DESY, 22 603 Hamburg, Germany International Congress on Optics and Optoelectronics 28 August – 2 September 2005, Warsaw, Poland

  2. OUTLINE - Introduction - UHV arc devices - Formation of niobium films - Properties of the deposited films - Filtering of micro-droplets - Coating of Cu cavities - Summary

  3. OURAIM Deposition of thin, high quality superconducting films of pure niobium on the inner surface of RF cavities for particle accelerators

  4. SUPERCONDUCTING RF CAVITIES -mainly based on Nb bulk technology - early 90s- copper RF cavities coating withthin niobium film ( LEP 2 ) - so far UHV cylindrical magnetron sputtering technology - in 2000 new approach UHV arc deposition, Italian INFN grant – project „ARCO” - since 2004 FP6 program „CARE’, contract number RII3-CT-2003-506395

  5. CARE- JRA1-SRF WP4 - Thin film cavityproduction Two work packages:WP4.1 - Linear cathode arc coating J.LangnerWP4.2 - Planar cathode arc coating S.Tazzari coordinated by M.J. Sadowski

  6. MERITS of Nb/Cu CAVITIES Nb/Cu cavities offer several advantages such as: - better mechanical stability, - insensitivity to external magnetic fields, - better thermal stability, - easier conditioning on the machine, - easier connection to the cryostat - lower cost

  7. WHY CATHODIC ARC ? -no working gas -ionized niobium -high ion energy -excellent adhesion -high purity -possible to apply bias and magneticfields

  8. WHY UHV CONDITIONS ? Purity of a deposition process plays the crucial role during formation of thin superconducting niobium films. Reaching UHVstandard (below 10-9 Torr) can cause the practical elimination of impurities, like H2O, nitrogen, oxygen, hydrocarbides, CO2from the vacuum chamber.

  9. HV and UHV conditions 10-8 Torr 10-10 Torr

  10. FIRST UHV SET-UP WITH PLANAR ARC SOURCE (ROME)

  11. UHV SET-UP WITH PLANAR ARC SOURCE

  12. UHV SET-UP WITH LINEAR ARC (IPJ)

  13. TRIGGERING SYSTEM FOR UHV ARC The triggering of arc discharges creates often many problems. At the UHV conditions these problems are multiplied. The triggering system for UHV arc - must be infallible - cannot produce any impurities

  14. Two Nd YAG lasers: - energy -60mJ and 100mJ - pulse duration -5 ns - repetition rate 20 Hz - 110 V booster applied LASER TRIGGERING After testing many known triggering methods from the point of view of operational reliability and cleanliness, we have finally decided to use a laser beam focused upon the cathode through a vacuum-tight glass window.

  15. Vacuum arc-based devices investigated in Rome and Swierk

  16. UHV LABORATORY IN ROME

  17. UHV LABORATORY AT SWIERK

  18. FORMATION OF NIOBIUM SUPERCONDUCTING FILMS substrats: sapphire and Cu -base pressure 10-10 hPa -arc current 60 - 140 A -bias 20 – 100 V -temperature 50 -2000 C -deposition rate (planar arc) 10 nm/s

  19. VACUUM CONDITIONS Behavior of the partial pressure (in ion-current units) of residual gases during the arc discharge. The downward slope is due to the pumping effect of a freshly deposited film

  20. PROPERTIES OF THE DEPOSITED FILMS RRR The Nb Residual Resistivity Ratio (RRR definedas the Resistivity at room temperature divided by the Resistivity at 10K) is very sensitive to impurities. Typical RRR values for Nb films deposited by sputtering at room temperature range from 2 to 10. RRR of arc deposited Nb films range from 20 to 50, while heating the substrate to ≈150oC resulted in a record value of ≈80. We believe such a high value of RRR is obtained thanks to the absence of contaminating auxiliary gases and to the atomic-scale heating due to the high kinetic and potential energy of Nb ions impinging on the film surface which results in a local temperature higher than the average temperature reached by the substrate and recorded by thermometry.

  21. RESIDUALRESISTIVITY RATIORRR = 80 The transition to superconducting state for the thin niobium film deposited on sapphire

  22. PROPERTIES OF THE DEPOSITED FILMS TC The critical temperature, Tc, of the deposited material is very sensitive to impurities. Tc, transition width ΔTc and surface current density (Jc) values of our best film samples have shown values identical, within the measurement error, to those of bulk Nb, i.e. Tc = (9.26+- 0.03) K, ΔTc ≈ 0.01K and Jc = 3x107 A/cm2. The narrow transition width (<0.01K) is a strong indicator of uniform and clean film while the absolute Tc value indicates that stresses in arc-deposited films are lower than in magnetron sputtered ones.

  23. CRITICAL TEMPERATURE (2002) Measured transition temperature curves of various arc deposited Nb film samples in 2002 and 2004

  24. RF MEASURMENTS Q values as function of temperature

  25. High Field RF measurements(6 GHz) High field RF measurements, at 6 GHz, were performed in Cornell on the four filtered, large Nb coated Cu substrates. The quality factor (Q) of the best sample was comparable withinthe errors to the present limit value of the host cavity of ≈3*108

  26. SURFACE MORFOLOGY ( AFM ) Magnetron sputtering Arc deposition AFM pictures of niobium films deposited on supphire substrate. Nb grains in arc deposited film are visible and their avarege dimension is 200nm

  27. SAMPLE MORFOLOGY SEM images of niobium samples deposited on sapphire substrate without and with the magnetic filter

  28. MICRO-DROPLETS Micro-droplets distribution for 4 samples deposited at the same conditions for different times. The main difference is the amount of particles with a radius larger than the film thickness.

  29. MAGNETIC FILTER FOR PLANARCATHODE (2003)

  30. MAGNETIC FILTER FOR PLANAR CATHODE (2005) I Aksenov-type magnetic filter

  31. MAGNETIC FILTER FOR PLANAR CATHODE (2005) II T-type magnetic filter

  32. General view of the cylindrical magnetic filter before installation. The cylindrical magnetic filter fixed upon the linear (cylindrical) cathode. MAGNETIC FILTER FOR CYLINDRICAL CATHODE (2005)

  33. COATING OF CU CAVITIES(SWIERK) First coating of this single cell has just been performed without any micro-droplet filtering. Single-cavity has been equipped with appropriate UHV flanges, and after that it has been installed within the prepared facility. The coated single-cell has been cut along its symmetry axis in order to perform an analysis of inner surfaces.

  34. COATING OF CU CAVITIES(ROME) Simulated distribution of magnetic field lines in „cusp” configuration used for plasma transport for deposition Set-up in „cusp” configuration

  35. SUMMARY -very high RRR 20 - 50 (T<50C) , 80 (T≈200C) -less stressed and more homogeneous than standard Nb films produced by sputtering -larger grains (200 nm) -critical temperature,Tc = 9.25 K ±0.03 K -very narrow transitions widths ( ~ 0.01 K ) -RF properties ( Tc , Q ) are comparable with niobium bulk

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