M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 improved accuracy and sensitivity as compared to the usual RF method RS distribution validated and RF properties of several Nb/Cu sample was studied effect of the copper substrate surface Nb/Cu sample RF properties 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. The thermometric 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. 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

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 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

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 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 Motivations for developing such an instrument

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 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 and substrate surface preparation on the films RF properties  Improve SRF performance and master the technology Investigate Rs(T) in the temperature range: 1.6 K 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 material interesting for accelerators Purpose

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 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-situ measurement of substrate thermal parameters Main advantages of this method

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 R=0 Rcav=55 Rseal=58 q HF Nb Cavity RF part LHe Thermometric part q STAT Vacuum Heater Method principle and thermal modelling Sample

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 No RF dissipation at the indium seal 2 nd order polynomial law parametrization of Rs=f(Hs) Total dissipated RF power P RF depending on the surface magnetic field Hs p 11 :J 1 ‘s first zero For radius r>40mm P STAT =P RF   T STAT =  T RF  Determination of R S coefficients (a, b, c) by least square method

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Dismountable assembly of 24 Surface thermometers in a vacuum chamber  Four subsets at 90° apart at 6 radial positions from 12.4mm up to 47.4mm (Step  r=7mm)  Calibration heater (  12 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 T bath during thermometers calibration (R vs T curve) and during measurements sequences (  T vs q STAT and  T vs q HF ) Thermometric system (1)

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006  Bulk Niobium cavity : TE011 mode f=4GHz TE012 mode f=5.6GHz  Calibration heater  24 thermometers  Vacuum chamber  Heater thermometer (Heat leaks) Thermometric system (2)

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Sample Thermometers Heater Thermometric system (3)

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Measured and simulated temperature profiles Q STAT (mW) Type  T 12 (mK)  T 11 (mK)  T 10 (mK)  T 9 (mK)  T 8 (mK) 220Computed Measured Computed Measured Hs (A/mm) Type  T12 (mK)  T11 (mK)  T10 (mK)  T 9 (mK)  T 8 (mK) 14Computed Measured Computed Measured

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Minimum detectable heating : ~0.1mK at T=1.7K and T=4.2K Accuracy: calorimetric versus RF method at f=4 GHz Sensitivity and accuracy of the method T=1.7KT=4.2K T=1.7KT=4.2K  The accuracy of calorimetric method is ~5 times better than RF method

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Validation of the calorimetric method by comparison with RF measurement (1) f=5.6 GHz f=4 GHz f=5.6 GHz T=1.7K T=4.2K Tests of a Bulk niobium sample (Solid dots: usual RF method, Solid line : calorimetric method) 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) Bs(Oe) R S (n  )

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 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) T=1.7K T=4.2K f=4 GHz 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)

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Test of a niobium film sputtered onto a copper substrate at T=1.7K T=1.7K f=5.6 GHz 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)

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 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) Test of a niobium film sputtered onto a copper substrate at T=4.2K R S (n  ) Bs(mT)

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Effect of substrate roughness on surface resistance at 1.7 K (1) Copper substrate Niobium film

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Rs vs B measurement of Nb Films on Cu substrate (T=1.7K, f=4GHz)  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 Bs(mT) R S (n  )

M. FOUAIDY Thin films applied to superconducting RF cavitiesLegnaro Oct.10, 2006 Rs vs B measurement of Nb Films on Cu substrate (T=1.7K, f=5.6 GHz) R S (n  ) Bs(mT) R S (n  )  Nb film residual surface resistance increases with the substrate roughness and defects density  Same effect observed at 4 GHz and 5.6 GHz