The performance of superconducting RF cavities made of bulk Nb is limited by a breakdown field of Bp ≈200 mT, close to the superheating field for Nb. A potentially promising solution to enhance the breakdown field of the SRF cavities beyond the intrinsic limits of Nb is a multilayer coating suggested in [1]. In the simplest case, such a 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 be e.g. dirty niobium [2]. Here we report the first results of our measurements of field penetration in Nb thin films and Nb-AlN-Nb multilayer samples at 4.2 K using the magnetic field penetration facility designed, built and tested in ASTeC

Dumbell, K. D.

Gurevich, A.

Gurran, L.

Malyshev, O. B.

Pattalwar, N.

Pattalwar, S.

Valizadeh, R.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsPhysics Faculty Publications Physics2016First Results of Magnetic Field PenetrationMeasurements of Multilayer SIS StructuresO. B. MalyshevL. GurranR. ValizadehS. PattalwarN. PattalwarSee next page for additional authorsFollow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubsPart of the Condensed Matter Physics Commons, and the Quantum 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, O. B.; Gurran, L.; Valizadeh, R.; Pattalwar, S.; Pattalwar, N.; Dumbell, K. D.; and Gurevich, A., "First Results of MagneticField Penetration Measurements of Multilayer SIS Structures" (2016). Physics Faculty Publications. 290.https://digitalcommons.odu.edu/physics_fac_pubs/290Original Publication CitationMalyshev, O., Gurevich, A., Pattalwar, S., Valizadeh, R., Gurran, L., Pattalwar, N., & Dumbell, K. (2016). First results of magnetic fieldpenetration measurements on multilayer SIS structures. In Proceedings of the 7th International Particle Accelerator Conference, Busan,Korea, May 8-13, 2016 (pp. 2245-2247). http://accelconf.web.cern.ch/AccelConf/ipac2016/papers/wepmb057.pdfAuthorsO. B. Malyshev, L. Gurran, R. Valizadeh, S. Pattalwar, N. Pattalwar, K. D. Dumbell, and A. GurevichThis conference paper is available at ODU Digital Commons: https://digitalcommons.odu.edu/physics_fac_pubs/290FIRST RESULTS OF MAGNETIC FIELD PENETRATION MEASURE-MENTS ON MULTILAYER S-I-S STRUCTURES O.B. Malyshev†,2, L. Gurran1,2, R. Valizadeh2, S. Pattalwar2, N. Pattalwar, K.D. Dumbell2, ASTeC, STFC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK       1 University of Manchester, Manchester, UK       2 Cockcroft Institute, Warrington, Cheshire, UK A. Gurevich, Old Dominion University, Norfolk, VA 23529, USA  Abstract The performance of superconducting RF cavities made of bulk Nb is limited by a breakdown field of Bp  200 mT, close to the superheating field for Nb. A potentially promising solution to enhance the breakdown field of the SRF cavities beyond the intrinsic limits of Nb is a multi-layer coating suggested in [1]. In the simplest case, such a multilayer may be a superconductor-insulator-superconductor (S-I-S) coating, for example, bulk niobi-um (S) coated with a thin film of insulator (I) followed by a thin layer of another superconductor (S) which could be e.g. dirty niobium [2]. Here we report the first results of our measurements of field penetration in Nb thin films and Nb-AlN-Nb multilayer samples at 4.2 K using the magnetic field penetration facility designed, built and tested in ASTeC. INTRODUCTION Recently a new facility for magnetic field penetration measurement was presented in Ref. [3]. The schematic layout of the experiment is shown in Fig. 1. A short su-perconducting magnetic coil, 30 mm long and 30 mm inner diameter, is placed over the middle of a relatively long sample tube with a large length to diameter ratio, made of or coated with superconducting material. Two magnetic field sensors (Hall Field Probes 1 and 2 in Fig. 1) are placed on the central plane of the magnet: one on the outside of the tube and one inside the tube. After cool-ing down a sample to 4.2 K in zero magnetic field, the magnetic field penetration measurements can be per-formed.    Figure 1:  A schematic layout of magnetic measurements with tubular superconducting samples. SAMPLES Four samples were prepared for the measurements -  Sample 1: The tube was manufactured from cavity-grade niobium with RRR=384 by Niowave Inc. [4]. The tube was 230 mm long and had an outer diameter of 12 mm and an inner diameter of 7.5 mm.  No chemical treatment was done after fabrication.     Three other samples were Nb films deposited on a pol-ished copper tube which was 230 mm long and had an outer diameter of 12 mm and an inner diameter of 10 mm.  Sample 2 was an ~ 840 nm thick Nb film deposited with pulsed DC at 10-3 mbar of Kr. It has dense columnar structure. Sample 3 was a ~56 nm thick Nb film, deposited with DC at 10-2 mbar of Kr. It has columnar structure. Sample 4 was a S-I-S structure with: an ~840 nm thick Nb film deposited with the same parameters as Sample 2; followed by a ~3 nm thick AlN insulation layer deposited from an aluminium target in krypton and nitrogen gas in a 50/50 ratio, at a deposition pressure of 10-3 mbar; and finally, ~56 nm thick Nb film deposited with the same parameters as Sample 3. The high resolution FIB X-sectional view SEM images of samples 2-4 are shown in Fig. 2. The images were taken at 45, hence the thickness has been scaled up by √2.   Figure 2: High resolution FIB X-section SEM images of deposited film: (a) Sample 2, (b) Sample 3, (c) Sample 4. Proceedings of IPAC2016, Busan, Korea WEPMB05707 Accelerator TechnologyT07 Superconducting RFISBN 978-3-95450-147-22245 Copyright©2016CC-BY-3.0andbytherespectiveauthorsSample tube Field Probe 1 Superconducting magnet coil Field Probe 2 Preliminary results of trial runs on these samples were first described in [3]. Here we report new results of more detailed field penetration measurements for all samples.   EXPERIMENTAL Experimental Procedure Initial conditions: A sample was cooled down from room temperature to 4.2 K with no magnetic field (i.e. zero magnet current). The magnet was held stationary in the middle of the sample tube.  Measurements: Magnet current was ramped up in vari-able steps to increase the magnetic field up to 1 T in steps of between 5×10-4 and 2×10-2 T. Magnetic field meas-urements B1 and B2 were performed with Field Probes 1 and 2 after the current had been stabilised after each step, which was checked with a routine written in LabVIEWTM.  After initial magnetisation up to B1  1 T, measure-ments of a full hysteretic magnetisation loop were per-formed between B1  1 T and B1  -1 T, i.e. the magnetic field was ramped down to -1 T followed by ramping up to B1  1.7 T. The magnet current was then turned off to measure the residual magnetisation of the sample. Experimental Runs The results of our measurements are shown in Fig.3. Measurements of the initial magnetization of the samples in a magnetic field B1 = 0 T showed that for all samples, except the thin film Sample 3, the magnetic field B2 measured inside the sample tube was zero (within the sensitivity of the Hall probes 10-4 T) up to a certain applied magnetic field B1 = BFP.  This is the so-called full penetration field defined in the Bean model as BFP = d Jc, where Jc is the critical current density due to pinning of vortices and d is the wall thickness [5].  Figure 3: Results of measurements with Experimental procedure 1 for Samples 1-4 shown for (a) -0.5 < B1 < 0.5 T and (b) 0 < B1 < 0.06 T.  The measured values of BFP shown in Table 1 correspond to the mean of the last measurement of B1 with B2 = 0 and the following measurement of B1 with B2 0. For Sample 3, in the whole range of measurements, both Hall probes detected approximately the same field: B1B2. This suggests a very small full penetration field for this sample, BFP3 < 310-4 T.  Discussion The results for full penetration magnetic field demon-strated that BFP increases with the thickness of the niobi-um: from the below-sensitivity-level BFP3 < 110-4 T for the 56-nm thick film (Sample 3) to BFP2 = 410-3 T for the 840-nm thick film (Sample 2). This is much lower than the results for the 2.25-mm thick bulk Nb (Sample 1) which has a BFP1 = 0.28 T.  One might expect that the full penetration magnetic field of a thin film with a thickness of 840 nm would be close to the sum of those of Samples 2 and 3: BFP2 + BFP3 = 4.110-3 T. However, the S-I-S multilayer film (Sample 4), consisting of two Nb layers deposited at the same conditions as for Samples 2 and 3  and separated by a  3-nm thick AlN insulating layer, shows a much higher value of BFP4 = 3.2 10-2 T. Enhancement of the full penetration magnetic field with S-I-S structures has therefore been demonstrated.   Table 1: Full Penetration Magnetic Field and Residual Magnetic Field   Sample Hall Probe Sample 3: Thin film Sample 2: Thick film Sample 4:  S-I-S film Sample 1: Bulk Nb Full penetration magnetic field BFP (T) B1 110-4 110-4 0.0040 0.0010 0.032 0.002 0.2793 0.0009 Residual magnetic field BR (T) after exposure to 1.7 T B1R 810-4 0.0011 -0.0005 -0.0002 B2R 510-4 0.0136 0.0588 0.2769 BR -310-4 1.2410-2 5.9310-2 0.2771  For SRF cavity applications it is important to know how well the superconducting films can screen the mag-netic field. The magnetic field screening efficiency can be quantified either by the magnetic screening field B = B1  B2                  (1) or by a dimensionless parameter, the screening efficiency S, defined as  S = 1  B2 / B1                 (2) The B results for Samples 1, 2 and 4 are shown in Fig. 4. The results for Sample 3, where B < 10-3 T over the whole range of measurement, are not shown. One can see that B is equal to the applied magnetic field, B1, up to B1 = BFP, then it increases more slowly than B1 until reaching a maximum value, Bmax, at B1(Bmax). B then reduces to zero as B1 continues to increase.  WEPMB057 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-22246Copyright©2016CC-BY-3.0andbytherespectiveauthors07 Accelerator TechnologyT07 Superconducting RF(a) 0.S - Thinfllm ---TNdt f~m 0.4 •• • M~i ev-,, flhl - - lul: Nb lubl! ,::-0.3 ;:: 0.2 .. ... ~ 0.1 ... ~ -0.1 iS -0.2 ::; -0.3 -0.4 -0.S -0.S -0.4 -0.3 -0.2 -0.1 0 0. 1 0.2 0.3 0.4 o.s AppUed Field Bl (T) (b) 0.005 --Thin film 0.004 E N 0.003 .. ... ~ 0.002 ... i 0.001 i 0 ::  ::~~I:~:, film / ~ Bulk Nb 1ube /j / /l _ ... ·" _______ ... ~ - - Ii>- - - ; __ ,.. --0.001 0 0.01 0.02 0.03 0.04 0.0S 0.06 Applied Fi eld Bl (T) The results for the screening efficiency, S, for all sam-ples are shown in Fig. 5. S = 1 corresponds to full screen-ing up to B1 = BFP, then it reduces with magnetic field B1. For example, if the magnetic field at the surface of a Nb cavity was 0.1 T then if the cavity was coated with a film like Sample 2 the bulk Nb would be exposed to ~0.09 T. If the same cavity was instead coated with a S-I-S film like Sample 4 the bulk Nb would be exposed to ~0.06 T. The magnetic field screening efficiency S for each sample at B1 = 0.1, 0.2, 0.3 and 0.4 T, as well as Smax and B1(Smax) are shown in Table 2.   Figure 4: Magnetic screening field B as a function of applied magnetic field.   Figure 5: Magnetic field screening efficiency, S, as a function of applied magnetic field. The residual magnetisation BR of the samples measured after exposure to 1.7 T is shown in Table 1. These values are higher than BFP for the film samples (Samples 2-4).  Superconducting thin films which would be potentially interesting for SRF applications should provide screening in the range of 0.2 < S < 1, corresponding to B = 0.05 T at B1 = 0.25 T. Notice that the direct comparison of B for thin films on Cu and the Nb tube is not appropriate as the penetra-tion field B = BFP = 0Jcd for the Nb tube is proportion-al to its much larger wall thickness d = 2.5 mm. A more relevant for SRF applications would be to compare B for the Nb tube before and after deposition of multilayers, which we plan to do soon. Yet the measurements reported here demonstrate a capacity of our magnetic field penetra-tion method and facility for future research and for the thin film optimisation. Table 2: Magnetic Field Screening Efficiency S and Mag-netic Screening Field B at 0.1, 0.2, 0.3 and 0.4 T, and Bmax  Thick film S-I-S film Bulk Nb B1 (T) B  (T) S B (T) S B (T) S 0.1  0.010 0.1 0.038 0.4 0.1 1.0 0.2  0.0063 0.03 0.020 0.1 0.2 1.0  0.3 0.0038 0.015 0.014 0.045 0.05 0.22  0.4 1.8×10-3 8×10-3 0.010 0.025 0.007 0.02     Bmax 0.011 0.054 0.28 B1(Bmax) 0.064 0.065 0.28 CONCLUSIONS The magnetic field penetration facility was designed, built and tested at ASTeC. It can accommodate tubular samples made of e.g. bulk superconductors, non-superconductors coated with superconducting films or S-I-S structures or bulk Nb coated with S-I-S structures. First tests demonstrated that the full penetration magnetic field BFP increases with the thickness of the niobium films and that the results for a bulk Nb sample are as expected: BFP = 0.28 T. They also demonstrated that the full pene-tration magnetic field for a S-I-S multilayer film, BFP = 0.032 T, is higher than the sum of BFP for the two single layer Nb films that comprise it. The S-I-S enhancement has therefore been demonstrated. Although the obtained results for the studied films are not as good as required for SRF applications, they demonstrate the capability of a magnetic field penetration measurement method and facility for future research.   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 the timely support of ETC staff, namely Steve Bibby-Trevor and Phil Davies. L. Gurran would like to 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 con-tract No. DE-SC0010081.  REFERENCES [1]  A. Gurevich, APL 88, 012511 (2006). [2]  A. Gurevich, AIP Advances, 5, 017112 (2015). [3]  O.B. Malyshev, L. Gurran, S. Pattalwar, et al. Proc. of SRF2015, p. 716 (2015). [4] NIOWAVE Inc., 1012 N Walnut St, Lansing, MI 4890. [5]  C.P. Bean. Magnetization of High-Field Supercon-ductors, Rev. Mod. Phys. 36, 31 (1964).  Proceedings of IPAC2016, Busan, Korea WEPMB05707 Accelerator TechnologyT07 Superconducting RFISBN 978-3-95450-147-22247 Copyright©2016CC-BY-3.0andbytherespectiveauthors0.1 E N a, ..:. a, 0.01 0.001 L 0.001 V) ~ C ., ~ ., ~ 0.10 'i: ., ~ V) 0.01 • Bulk • Multilayer ~ .. , ~ \ • Thick .. ._ ... - ~ ··.,. .~ . . . . · .. . · .0.01 0.1 Applied Field Bl (T) . . .. :. .. _j - Bulk Nb tube 0.00 0.05 0.10 0 .15 0.20 0.25 0.30 0.35 0.40 0.45 0 .50 Applied Field Bl (T) 

First Results of Magnetic Field Penetration Measurements of Multilayer SIS Structures

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First Results of Magnetic Field Penetration Measurements of Multilayer SIS Structures

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