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Determination of Niobium films surface resistance by a calorimetric method

Determination of Niobium films surface resistance by a calorimetric method. M. Fouaidy, IPN Orsay, France P. Bosland, M. Ribeaudeau, S. Chel, J.P. Charrier, CEA Saclay, France.

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Determination of Niobium films surface resistance by a calorimetric method

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  1. Determination of Niobium films surface resistance by a calorimetric method M. Fouaidy, IPN Orsay, France P. Bosland, M. Ribeaudeau, S. Chel, J.P. Charrier, CEA Saclay, France An alternative device, for Nb/Cu samples RF properties characterization purpose was developed. The main feature of this technique, which is based on thermometry, is an improved accuracy and sensitivity as compared to the usual RF method. Thethermometric method, conjointly with a thermal model, is used for the measurement of the absolute RS distribution on superconducting thin film samples. Precise calibration of test-samples RF losses is performed by means of a removable DC heater and temperature sensors pressed on the back of the disk and placed in a vacuum chamber. This new facility allows in-situ determination of all the thermal parameters involved in the model (substrate thermal conductivity and heat transfer coefficient at the solid-Lhe interface). The thermometric technique was first successfully validated and RF properties of several Nb/Cu sample was studied with this new device. Interesting data was obtained and analyzed. In particular, the effect of the copper substrate surface conditions on the Nb/Cu sample RF properties was investigated and the corresponding results discussed.

  2. TOPICS  Motivation for developing such an instrument  Purpose  Main advantages of the calorimetric method  Method principle and thermal modelling  Thermometric system  Measured versus simulated temperature profiles  Sensitivity and accuracy of the calorimetric method  Validation of the calorimetric method  Test results with sputtered niobium films

  3. Motivations for developing such an instrument Why did we need to develop a new instrument for measuring the RF surface resistance (Rs) of sputtered superconducting films with such SRF cavity ?  Improve accuracy and sensitivity of Rs measurement,  Lack of accuracy and sensitivity at 4.2 K for measurements performed by the end plate replacement method !  Measure exclusively the test-sample RF losses by excluding any extra RF losses : Some of extra RF losses are inherent to the ‘classical’ method (rest of the cavity, indium gasket, RF coupling loops) Potentially, anomalous RF losses induced by Field Emitted electron impacting area other than the sample

  4. Purpose • Improve accuracy, reliability and sensitivity of Rs measurement  Thorough and precise RF characterization of sputtered Nb and NbTiN films onto Copper substrate Study the effect of sputtering process parameters andsubstrate surface preparation on the films RF properties Improve SRF performance and master the technology • Investigate Rs(T) in the temperature range: 1.6 K - 4.5 K • Study Rs spatial distribution on the sample . • Progress in the understanding of SRF properties and get more insight into superconducting film physics and develop new superconducting materialinteresting for accelerators

  5. Main advantages of this method Absolute, direct and local method as compared to the usual RF technique  No reference disk needed  Save time,  No assumption concerning the rest of the Niobium cavity RF surface  Vacuum insulation  and hence a precise temperature measurement (thermometers in contact with a non-wetted solid wall)  In-situmeasurement of substrate thermal parameters

  6. Heater Vacuum Thermometric part qSTAT LHe Rseal=58 qHF Nb Cavity RF part R=0 Rcav=55 Method principle and thermal modelling Sample

  7.  No RF dissipation at the indium seal  2nd order polynomial law parametrization of Rs=f(Hs)  Total dissipated RF power PRF depending on the surface magnetic field Hs p11:J1‘s first zero For radius r>40mm PSTAT=PRFDTSTAT= DT RF Determination of RS coefficients(a, b, c) by least square method

  8. Thermometric system (1) Dismountable assembly of 24 Surface thermometersin a vacuum chamber  Four subsets at 90° apart at 6 radial positions from 12.4mm up to 47.4mm (Step Dr=7mm)  Calibration heater (F12 OFHC rod) located at the centre of the test-sample  Calibrated thermometer (1.5K-60K) placed near the heater/sample boundary control of the heater temperature and determination of the heat leaks Two reference thermometers (Calibrated Germanium and Carbon resistor)  Accurate measurements of Tbath during thermometers calibration (R vs T curve) and during measurements sequences (DT vs qSTAT and DT vs qHF)

  9. Thermometric system (2)  Bulk Niobium cavity : TE011 mode f=4GHz TE012 mode f=5.6GHz Calibration heater  24 thermometers Vacuum chamber Heater thermometer (Heat leaks)

  10. Thermometric system (3) Thermometers Sample Heater

  11. Measured and simulated temperature profiles

  12.  Minimum detectable heating : ~0.1mK at T=1.7K and T=4.2K Sensitivity and accuracy of the method T=1.7K T=4.2K Accuracy: calorimetric versus RF method at f=4 GHz T=1.7K T=4.2K The accuracy of calorimetric method is ~5 times better than RF method

  13. Validation of the calorimetric method by comparison with RF measurement (1) Tests of a Bulk niobium sample(Solid dots: usual RF method, Solid line:calorimetric method) RS(nW) f=5.6 GHz RS(nW) f=5.6 GHz T=1.7K T=4.2K f=4 GHz f=4 GHz Bs(Oe) Bs(Oe)  Good agreement between the two methods For bulk niobium the field is limited by RF heating (Disk cooled by liquid helium at the lateral rim only)

  14. Validation of the calorimetric method by comparison with RF measurement (2) Tests of a Bulk niobium sample(dots: usual RF method, Solid line:calorimetric method) f=4 GHz f=4 GHz T=1.7K T=4.2K Rim cooling Disk cooling For bulk niobium, the cooling conditions have a strong effect on the maximum RF field achieved and on surface resistance at high field (Joule heating)

  15. Test of a niobium film sputtered onto a copper substrate at T=1.7K f=5.6 GHz T=1.7K  Good agreement between the two methods: for six tests performed at 1.7 K the difference is 15%-20% For sputtered niobium films the field is not limited by RF heating Efficient conduction cooling by copper substrate (Disk cooled by liquid helium at the lateral rim only)

  16. Test of a niobium film sputtered onto a copper substrate at T=4.2K RS(nW) Bs(mT) Field Limitations  For Bs<5mT: thermal instabilities due to switching from natural convection to nucleate boiling (Not hard limit!)  For Bs>15mT: power limitation due to dissipations in the cylindrical part of the cavity (bulk Nb)

  17. Effect of substrate roughness on surface resistance at 1.7 K (1) Copper substrate Niobium film

  18. Rs vs B measurement of Nb Films on Cu substrate (T=1.7K, f=4GHz) RS(nW) RS(nW) Bs(mT) Bs(mT)  Nb film residual surface resistance increases with the substrate roughness and defects density Use clean and smooth substrate with intermediate layer for of lattice matching and improve superconducting properties

  19. Rs vs B measurement of Nb Films on Cu substrate (T=1.7K, f=5.6 GHz) RS(nW) RS(nW) Bs(mT) Bs(mT)  Nb film residual surface resistance increases with the substrate roughness and defects density  Sameeffect observed at 4 GHz and 5.6 GHz

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