Astapovych, D.

Glamann, N.

Grau, A.

Jauregui, D. Saez de

Krasch, B.

English

KITopen

MAGNETIC FIELD INVESTIGATION IN A COMPACTSUPERCONDUCTING UNDULATOR WITH HTS TAPED. Astapovych∗, N. Glamann, A. W. Grau, B. Krasch, D. Saez de Jauregui,Karlsruhe Institute of Technology (KIT), Karlsruhe, GermanyAbstractThe superconducting undulator (SCU) based on thesecond-generation high-temperature superconducting (HTS)tapes is a promising application for building tabletop free-electron laser (FELs). The short period < 10 mm undulatorswith a narrow magnetic gap < 4 mm are especially relevant.The advantage of the HTS tape is that it shows both highcritical current density and high critical magnetic field. Eachtape has 50 µm thickness and 12 mm width and is furtherscribed by a laser to achieve a meander structure, hence,providing the desired magnetic field pattern.Thus, a new approach to a superconducting undulatorhas been presented in the past and is further developed atKIT: each coil is wound with a single 15 m structured HTStape. As a result, 30 layers of scribed sections lay aboveeach other, and therefore, provide the required magnetic field.The results of the magnetic field measurements together withthe results of the numerical investigation will be presentedand discussed.INTRODUCTIONAn idea of the undulator using the meander-structuredHTS tapes was proposed by Prestemon [1,2]. Figure 1 showsa photo of a second-generation HTS tape with a meanderstructure, which has been made in-house at KIT using thepulsed YAG laser system at the Institute of Technical Physics(ITeP). The main parameters of the laser-scribed tapes arepresented in Table 1 and used later in the computations.Figure 1: Segment of laser-scribed HTS tape.Table 1: Main parameters of laser-scribed HTS tapesParameter ValueHTS tape thickness (`m) 55Period length (mm) 8.05Period number 12.5HTS tape groove section (`m mm) 25 4A concept of a jointless undulator using the structuredHTS tapes has been proposed by T. Holubek [3]. The designis the following: a 15 m single-piece of structured HTS tape∗ daria.astapovych@kit.eduis folded in half, resulting in the opposing current directionbetween two layers. Further, two non-magnetic stainlesssteel yokes are used to wind coils with such tapes, hence, 30layers of scribed sections lay above each other and providethe needed magnetic field. Figure 2 shows the photo of theundulator prototype based on this winding concept.Figure 2: Undulator prototype: structured HTS tapewounded around non-magnetic stainless steel core.Setup for Magnetic Field MeasurementsThe magnetic field measurements station CASPER(ChAracterization Set-uP for Field Error Reduction), whichwas built and installed at our institute and is operated atKARA [4], is shown in Fig. 3. In principle, CASPER is aFigure 3: Magnetic field measurements station CASPER I.Figure 4: Magnetic field measurement system.liquid helium bath cryostat that allows to perform magneticfield measurements at 4 K. The maximum current of 1500 Acan be achieved through a pair of vapor cooled current leads.In addition, the power supply is provided with a quenchdetection system. More information on this system can befound in Ref. [4].In order to place the undulator prototypes inside of thecryostat, a complete new measurement system was designedand fabricated, and is shown in Fig. 4. The magnetic fieldis measured with an array of ten Hall probes in-house cali-brated at KIT at 4.2 K and attached to a sledge. The sledgeis later moved by a computer controlled stepper motor.MAGNETIC FIELDMeasurementsDuring the experiment, the undulator with the woundstructured HTS tapes is vertically submerged into theCASPER I. As previously mentioned, the measurementsare conducted at 4 K in liquid helium. The magnetic fieldgenerated by the HTS tapes is measured by the array of Hallprobes.Figure 5 shows the mean values of the peaks of the mea-sured magnetic field generated at different current values.As one can notice, the Hall probes at the ends of the arrayare less affected by the magnetic field. Therefore, in furthercomputations, the Hall probes number 5 – 7 have been usedto compare the results as ones, which are (located) in thecenter of the array.It is important to mention that these measurements havebeen conducted with a slightly larger gap between theHTS tapes stacks: 5.6 mm instead of 4 mm. The reason isto protect the Hall probes’ array from any possible dam-Figure 5: Mean values of peaks of measured magnetic fieldgenerated at different current.age due to the possible shrinking of surrounding parts at 4 K.SimulationsThe magnetic field was simulated using the softwareOpera 3d (TOSCA solver) [5] and using a Biot-Savart solverwritten in Python language. Figure 6 shows the model usedin the Opera simulations. The magneto-static TOSCA SolverFigure 6: Model of the structured HTS tapes used in Opera3d (TOSCA).allows to use the so-called Biot-Savart conductors, which donot require meshing, hence, saving time on computations.The model considered the racetrack current path through allthe 60 layers of the structured HTS tapes.Another tool to compute the magnetic field is a Biot-Savart solver written in Python [6]. This code also consid-ers a simplified model of line currents, which assumes thatline currents are sinusoidal. Figure 7 shows the magneticfield along the axis calculated for different current flowingthrough the tapes. For example, at nominal operating current𝐼 = 500 A, a peak magnetic field is estimated to be 170 mT.Figure 8 shows the comparison of the measured and com-puted magnetic field. As one can notice, the results are in agood agreement. While the Biot-Savart Solver in Python hasa straightforward computation of the magnetic field throughFigure 7: Magnetic field for different applied current, com-puted with the Biot-Savart Solver written in Python.the applied current, the Opera computation considers thecurrent density and very dependant on a small change in themodel dimensions. The magnetic field at 400 A obtained inthe measurements is about 160 mT in comparison to Biot-Savart Solver’s ≈140 mT and Opera’s ≈170 mT. Here, theresults for the middle Hall probe are presented.Figure 8: Comparison of the measured and computed mag-netic field.SUMMARY AND OUTLOOKFor the first time, the measurements have been conductedwith all the 30 layers of the HTS tapes in each of two stacksin a long run. The first results confirm the principle of sucha superconducting undulator using HTS tapes. The criticalcurrent at this stage is slightly higher than 400 A, whichmight be improved. One has to take into account that theHTS tapes are very sensitive to stretching, folding etc.. Also,they are not perfectly homogeneous in terms of carryingcritical current [7].The small differences between the measurements and com-putations can be explained by both the measurement uncer-tainties and the numerical errors. In order to converge theresults from Opera simulations, the model based on meshingmight be used together with the properties of each layer inthe HTS tape.Even though further measurements with the current proto-type are planned, the next approach is foreseen - the designwith individually structured HTS tape pieces, which arestacked and soldered alternating at the tape ends. For moredetails see Ref. [6].ACKNOWLEDGEMENTSThe authors would like to thank R. Nast (ITeP, KIT) forhis help with producing the laser-scribed structure on theHTS tapes.This work is supported by the German federal ministry ofeducation and research (BMBF) under grant No.05K19VK1.REFERENCES[1] S. Prestemon, D. R. Dietderich, A. Madur, S. Marks, and D.Schlueter, “High Performance Short-Period Undulators UsingHigh Temperature Superconductor Tapes”, in Proc. PAC’09,Vancouver, Canada, May 2009, paper WE5RFP075, pp. 2438–2440.[2] S. Prestemon et al., “Development and analysis of HTS-undulator components for FEL applications”, IEEE transac-tions on applied superconductivity, vol. 21, no 3, pp. 1880-1883, 2013. doi:10.1109/TASC.2010.2098014[3] T. Holubek et al., “A novel concept of high temperature su-perconducting undulator”, Superconductor Science and Tech-nology, vol. 30, no 11, p. 115002, 2017. doi:10.1088/1361-6668/aa87f1[4] E. Mashkina et al., “CASPER-A magnetic measurement fa-cility for superconducting undulators”, Journal of Physics:Conference Series, vol. 97, p. 012020, 2008. doi:10.1088/1742-6596/97/1/012020[5] Dassault Systèmes, 78946 Vélizy-Villacoublay Cedex,France. Available: https://www.3ds.com/fr/produits-et-services/simulia/produits/opera/[6] A. Will et al., “Design and Fabrication Concepts of a CompactUndulator with Laser-Structured 2g-HTS Tapes”, presented atthe 12th Int. Particle Accelerator Conf. (IPAC’21), Campinas,Brazil, May 2021, paper THPAB048, this conference.[7] SuperPower, Inc., http://www.superpower-inc.com/.

Magnetic Field Investigation in a Compact Superconducting Undulator with HTS Tape

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Magnetic Field Investigation in a Compact Superconducting Undulator with HTS Tape

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