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Thin Films for Superconducting Cavities

Thin Films for Superconducting Cavities

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Thin Films for Superconducting Cavities

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  1. Thin Films for Superconducting Cavities HZB

  2. Outline • Introduction to Superconducting Cavities • The Quadrupole Resonator • Commissioning • Outlook

  3. Basics of RF Cavities • Acceleration of charged particles using a radio frequency field • There are normal conducting and superconducting (sc) cavities • Qnc ≈ 105vs. Qsc ≈ 1010 • The needed power for operation is about a factor of 200 less than for superconducting cavities

  4. Materials for sc Cavities • Niobium (from sheet) • Critical temperature Tc = 9.2K • Accelerating gradients reaching the theoretical limit (≈45MV/m) due to improved treatment techniques • Expensive • Niobium on copper • 1-2μm niobium on copper cavity • Less need of niobium • Accelerating gradients up to 10MV/m

  5. Where can we improve? Understanding the dominant loss mechanisms in niobium Reducing losses Improving the performance of niobium films Reducing material costs Finding new materials ?

  6. How can we improve? • Power consumption in a superconducting cavity is proportional to its surface resistance RS • RS shows a complex behavior on external parameters, such as temperature, frequency, magnetic and electric field Systematic studies on cavities are no option

  7. The Quadrupole Resonator • The Quadrupole Resonator enables RF characterization of small, flat samples over a wide parameter range • Samples of 75mm diameter are welded to a niobium cylinder with flange so that they can be mounted to the host cavity 361 mm

  8. Field Configuration & Features • Resonant frequencies: 400MHz, 800MHz, 1.2 GHz • Almost identical magnetic field configuration • Ratio between peak magnetic and electric field proportional to frequency B׀׀ max 50 mm 1 E. Mahner et al. Rev. Sci. Instrum., Vol. 74, No. 7, July 2003 2 T. Junginger et. al Rev. Sci. Instrum., Vol. 83, No. 6, June 2012 0

  9. The Calorimetric Technique • Measuring the temperature on the sample surface • Precise Calorimetric measurements over wide temperature range DC Heater Temperature Sensors Heat Flow Sample Surface Quadrupole Resonator Thermometry Chamber

  10. The Calorimetric Technique Temperature Power Temperature of Interest DC Heater PRF Bath Temperature PDC,1 PDC,2 Heat Flow time DC on RF on ≈60 s ≈40 s Temperature Sensors Measured directly • Measurement of transmitted power Pt • Pt=c∫H2ds, c from computer code

  11. Imperfect Meissner Effect Meissner effect Trapped magnetic flux

  12. Flux Trapping in the Quadrupole Resonator Sample DC Coil

  13. Flux Trapping in the Quadrupole Resonator DC Coil

  14. First test with trapped flux • Bulk niobium sample • Reactor grade, RRR • Standard BCP, no bake out

  15. RS(B) at 400MHz, 2-4K • Convex curve for • Concave curve for • Different loss mechanisms dominant

  16. Trapped Flux at 400MHz and 4K

  17. Outlook • MgB2 from Superconductor Technologies, Inc. • Tc = 39K, Bc up to 1T • expected: Eacc > 75MV/m, RBCS(4K,500MHz) = 2.5nΩ • HiPIMS: Nb/Cu from CERN and Berkeley • Dense film, low cost, but competitive to bulk Nb? • ECR: Nb/Cu from Jlab • High RRR, low cost, but competitive to bulk Nb?