Superconducting RF cavities made of bulk Nb has reached a breakdown field of about 200 mT which is close to the superheating field for Nb. As it was theoretically shown a multilayer coating can be used to enhance the breakdown field of SRF cavities. The simple example is a superconductor-insulator-superconductor (S-I-S), for example bulk niobium (S) coated with a thin film of insulator (I) followed by a thin layer of a superconductor (S) which could be a dirty niobium. To verify such an enhancement in a presence of a DC magnetic field at 4.2 K a simple experimental facility was designed, built and tested in ASTeC. The details of experimental setup and results of the measurements will be shown at the conference

Dumbell, Keith D.

Gurevich, Alex

Gurran, Lewis

Malyshev, Oleg B.

Pattalwar, Ninad

Pattalwar, Shrikant

Valizadeh, Reza

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsPhysics Faculty Publications Physics2015A Facility For Magnetic Field PenetrationMeasurements on Multilayer S-I-S StructuresOleg B. MalyshevLewis GurranShrikant PattalwarNinad PattalwarKeith D. DumbellSee next page for additional authorsFollow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubsPart of the Engineering Physics CommonsThis Conference Paper is brought to you for free and open access by the Physics at ODU Digital Commons. It has been accepted for inclusion inPhysics Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationMalyshev, Oleg B.; Gurran, Lewis; Pattalwar, Shrikant; Pattalwar, Ninad; Dumbell, Keith D.; Valizadeh, Reza; and Gurevich, Alex, "AFacility For Magnetic Field Penetration Measurements on Multilayer S-I-S Structures" (2015). Physics Faculty Publications. 264.https://digitalcommons.odu.edu/physics_fac_pubs/264Original Publication CitationMalyshev, O., Dumbell, K., Gurevich, A., Gurran, L., Pattalwar, N., Pattalwar, S., & Valizadeh, R. (2015, December). A Facility forMagnetic Field Penetration Measurements on Multilayer S-I-S Structures. In Proceedings of the 17th International Conference on RFSuperconductivity (SRF2015), Whistler, BC, Canada, Sept. 13-18, 2015 (pp. 716-719).AuthorsOleg B. Malyshev, Lewis Gurran, Shrikant Pattalwar, Ninad Pattalwar, Keith D. Dumbell, Reza Valizadeh, andAlex GurevichThis conference paper is available at ODU Digital Commons: https://digitalcommons.odu.edu/physics_fac_pubs/264  A FACILITY FOR MAGNETIC FIELD PENETRATION MEASUREMENTS  ON MULTILAYER S-I-S STRUCTURES Oleg B. Malyshev1,3, Lewis Gurran1,2,3, Shrikant Pattalwar1,3, Ninad Pattalwar1,  Keith D. Dumbell1,3, Reza Valizadeh1,3 and Alex Gurevich4 1 ASTeC, STFC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK 2 Lancaster University, Lancaster, UK 3 Cockcroft Institute, Warrington, Cheshire, UK 4 Old Dominion University, Norfolk, VA 23529, USA  Abstract The best-performing superconducting RF cavities made of bulk Nb have reached breakdown fields which sometimes exceed 200 mT and are close to the superheating field for Nb. As it was theoretically shown [1], a multilayer coating can be used to enhance the breakdown field of SRF cavities. In the simplest case, such multilayer may be a superconductor-insulator-superconductor (S-I-S) coating, for example, bulk niobium (S) coated with a thin film of insulator (I) followed by a thin layer of another superconductor (S) which could also be a dirty niobium [2]. To verify such an enhancement under a DC magnetic field at 4.2 K an experimental facility was designed, built and tested in ASTeC. The details of experimental setup and first results of the measurements are presented here. METHOD A number of well-known techniques such as RRR measurements, DC magnetization in parallel and perpendicular magnetic field and AC susceptibility studies have been used to study the superconducting properties of thin films. These methods are useful for the characterization of the films; however, they are not very suitable to select the best coating material in the superconductor-insulator-superconductor (S-I-S) coating, which would provide the higher breakdown field in the SRF cavities. There are several reasons for that:    (1) The most interesting case corresponds to the magnetic field parallel to the surface; however, as the film samples are usually small, then their alignment parallel to the magnetic field is difficult. (2) These small samples would only represent a small fraction of total surface area of a larger sample, not representing the whole area.   (3) The samples are exposed to the magnetic field from both sides of the film, so in the case of S-I-S coating the magnetic field can easily penetrate between the layers from the open edges. To address the problems listed above, the following method was suggested by A. Gurevich and implemented by ASTeC at STFC Daresbury Laboratory to study and compare various superconducting materials under DC magnetic field. A short superconducting magnetic coil is placed in the middle of a relatively long sample tube made of or coated with superconducting material with a large aspect ratio. Two magnetic field strength sensors (e.g. Hall-effect sensor/probe) are placed in on the central plane of the magnet: one inside the tube and the other outside as shown in Figure 1.    Figure 1:  A schematic layout of magnetic measurements with tubular superconducting samples. In this geometry the maximum magnetic field in the middle plane of the coil is uniform over the cross section of the bore. In the middle of the magnet, the magnetic field is the strongest and tangential to the sample tube walls. The magnetic field decreases with a distance from the middle plane of the coil and is no longer tangential. So, when Hall probes are placed in the middle of the TUPB059 Proceedings of SRF2015, Whistler, BC, CanadaISBN 978-3-95450-178-6716Copyright©2015CC-BY-3.0andbytherespectiveauthorsFundamentals SRF R&D - Other MaterialsD04-Multi-layer coatingsSample tube ~ Field Probe 1 Superconducting magnet coil Field Probe 2   magnet, Probe 1 measures the maximum magnetic field of the coil and Probe 2 measures the field that penetrates through the superconductor sample tube walls.     Figure 2: A detailed layout of the magnetic penetration facility. A detailed layout of the magnetic penetration facility is shown in Figure 2. In this facility the tubular sample, magnetic field probes and superconducting magnets are placed in LHe bath, therefore all measurements are performed at 4.2 K.   Figure 3: An image of the magnetic penetration facility.  Figure 3 shows the facility built at ASTeC cryogenic laboratory in the STFC Daresbury Laboratory site. Magnetic field of the superconducting magnet can be varied between 0 and +/1.7 T. The magnet and magnet field probes can move up and down +/12 cm from the middle of the tube. Two Hall probes were mounted to be always in the central plain of the magnet and move together with it. One probe is inside the tube and the other is outside. Magnetic field sensitivity of the Hall probes is 10-4 T. SAMPLES Four samples have been prepared for the measurements. Sample 1: The tube was manufactured from niobium with RRR=384 by Niowave Inc. [3]. The tube was 230 mm long and had the outer diameter of 12 mm and the inner diameter of 7.5 mm.  No chemical treatment was done after fabrication. Three other samples were Nb films deposited on a polished copper tube which was 240 mm long and had the outer diameter of 12 mm and the inner diameter of 10 mm. Sample 2 was a ~1 m thick Nb film. Sample 3 was a ~100 nm thick Nb film. Sample 4 was a S-I-S structure with ~1 m thick Nb film, ~10 nm thick AlN insulation layer and ~100 nm thick Nb film. The detailed characterization of the films is ongoing. EXPERIMENTAL  Experiments with Sample 1 For the measurement two experimental procedures were developed.  Experimental procedure 1:  Magnet held stationary.  Magnet current is stepped up slowly and turned off between steps.  The lower critical field 𝐻𝐶1 was measured at first indication of residual magnetisation detected  outside the Nb tube with Probe 1 when magnet current is turned off between the steps measured with magnet off,  i.e. with zero external field.  The full penetration field in the Bean model defined as 𝐻P = d Jc, where Jc is the critical current density due to pinning of vortices and d is the wall thickness, was measured with two techniques:   o when the magnetic field first detected  inside the Nb tube with Probe 2 o when residual magnetisation is first measured with magnet off,  i.e. with zero external field.  The upper critical field 𝐻𝐶2 was defined as a field of complete penetration through the Nb tube, so that   Probes 1 and 2 detect the same field.  Based on the Experimental procedure 1, the following critical magnetic fields of Nb were obtained:   𝐻𝐶1 ≈  1.15 × 105 Am−1 or 𝐵𝐶1 ≈ 0.145 T 𝐻𝑃  ≈  2.11 × 105 Am−1 or 𝐵𝑃 ≈ 0.264 T 𝐻𝐶2 ≈  3.74 × 105 Am−1 or 𝐵𝐶2 ≈ 0.470 T  Proceedings of SRF2015, Whistler, BC, Canada TUPB059Fundamentals SRF R&D - Other MaterialsD04-Multi-layer coatingsISBN 978-3-95450-178-6717 Copyright©2015CC-BY-3.0andbytherespectiveauthorsWiringPort2 SampleTubeSlider -----Vapour Cooled Magnet Current Leads LHe Cryostat 4.2 K GHe Exhaust - ----- WiringPortl Cryostat Top Plate ·- ~- RadiationBaffleSet Magnet Support Sample Support Field Probes 1 and 2 LHe Level Proble Magnet Sample Tube Sample Tube Holder Sample Tube Positioner    Figure 4: Field penetration measurements with Sample 1 (bulk Nb tube). Experimental procedure 2:  Magnet current stepped up to 1.7 T.   Then magnet was switched off: field remains trapped in tube.  Probes moved up and down to scan through trapped field profile. The results are show in Figure 5. The initial position of the centre of magnet and probes is zero. One can see that the residual field is almost constant at +/- 2 cm from the centre of magnet, but as the distance along the magnet further increases beyond, +/-6 cm from the centre, the field reduces rapidly to below the sensitivity level. That means that the magnetic field was below Hc1 for distances greater than 6 cm away from the centre of magnet. This leads to a conclusion that in such a geometry of magnet and a sample tube a sample of ~12 cm long is sufficient. For practical application this means that our standard 24-cm long sample of can contain two different ~12-cm long coating, i.e. two samples can be measured in one experiment.    Figure 5: Trapped field measurements after exposure to the magnetic field of 1.7 T. Experiments with Sample 2 Sample 2 was intended to be studied with Experimental procedure 1:  Magnet held stationary  Magnet current was stepped up slowly up to ~1 T, and turned off between steps. The results are presented in Figure 6. Because the loss of data in this experiment, the first point in the measurements with a magnetic field of H = 0.136 T was obviously above both 𝐻𝐶1 and HP because Probe 2 detected magnetic field inside the tube. Then in whole range of measurements the magnetic field between two probes was different by a factor 2.0. When the external magnetic field was set to zero, both Hall probes measure zero field.    Figure 6: Field penetration measurements with Sample 2 (~1 m thick Nb film on copper tube). CONCLUSIONS AND FUTURE PLANS The new magnetic field penetration facility was built at STFC Daresbury Laboratory. It was tested with a bulk Nb tube and demonstrated the critical field results comparable with one from the literature. The 1-m thick Nb film deposited on Cu tube exhibited the reduction of magnetic field by a factor of 2, demonstrating that the new facility is capable for studying various samples such as bulk tubes, thin films deposited on Cu tubes and the S-I-S structures.  These results are very preliminary; the experiments with more detailed automatized data acquisition will be performed in near future with four prepared samples and other samples.  ACKNOWLEDGMENTS  The ASTeC project team would like to acknowledge the support given by the ASTeC management, namely P.A. McIntosh, J.D. Herbert and A.E. Wheelhouse. We also acknowledge timely support of ETC, namely Steve Bibby-Trevor and Phil Davies. L. Gurran would like to TUPB059 Proceedings of SRF2015, Whistler, BC, CanadaISBN 978-3-95450-178-6718Copyright©2015CC-BY-3.0andbytherespectiveauthorsFundamentals SRF R&D - Other MaterialsD04-Multi-layer coatings0.6 0.5 0.0 0.1 1 3 2.5 E 2 .. C ~ 1.5 ~ N fl: o.s 0 • • -8 -6 _j/·: .. ... + + FP l {outside) • • • • • • • • -- • FP2 (inside) 0.2 0 .3 0.4 0.5 Magnet field strength (T) • • • • • • • • + + + • • • -4 -2 0 6 Probe 2 position (cm) _j ~: 0.8 E o.1 .. i 0.6 .. ~ 0.5 .tl 0 0: 0.4 ~ 0.3 0.2 0.1 0 • L oo 8 I 0 7 +- + + + 0 +- + +- + + +- t 0 0 +- + 0 + . 0 +- 0 . 0 0 . 0 o- · . 00 . . FPl (outside) 00 .. 00 .. . • FP2 (inside) 0 ...... + .. 0.1 0.2 0.3 0.4 o.s 0.6 0.7 0.8 0.9 ~Magnetic field strength (T)  thank ASTeC for providing a grant for his PhD studentship. The work at Old Dominion University was supported by the US Department of Energy under contract No. DE-SC0010081.  REFERENCES [1] A. Gurevich, APL 88, 012511 (2006). [2] A. Gurevich, AIP Advances, 5, 017112 (2015). [3] NIOWAVE Inc., 1012 N Walnut St, Lansing, MI 48906. Proceedings of SRF2015, Whistler, BC, Canada TUPB059Fundamentals SRF R&D - Other MaterialsD04-Multi-layer coatingsISBN 978-3-95450-178-6719 Copyright©2015CC-BY-3.0andbytherespectiveauthors

A Facility For Magnetic Field Penetration Measurements on Multilayer S-I-S Structures

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A Facility For Magnetic Field Penetration Measurements on Multilayer S-I-S Structures

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