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Superconducting YBa2Cu3Oy (YBCO) foams were prepared using commercial open-cell, polyurethane foams as starting material to form ceramic Y2BaCuO5 foams which are then converted into superconducting YBCO by using the infiltration growth process. For modelling the superconducting and mechanical properties of the foam samples, a Kelvin-type cell may be employed as a first approach as reported in the literature for pure polyurethane foams. The results of a first modelling attempt in this direction are presented concerning an estimation of the possible trapped fields (TFs) and are compared to experimental results at 77 K. This simple modelling revealed already useful information concerning the best suited foam structure to realize large TF values, but it also became obvious that for various other parameters like magnetostriction, mechanical strength, percolative current flow and the details of the TF distribution, a refined model of a superconducting foam sample incorporating the real sample structure must be considered. Thus, a proper description of the specific microstructure of the superconducting YBCO foams is required. To obtain a set of reliable data, YBCO foam samples were investigated using optical microscopy, scanning electron microscopy and electron backscatter diffraction (EBSD). A variety of parameters including the size and shape of the cells and windows, the length and shape of the foam struts or ligaments and the respective intersection angles were determined to better describe the real foam structure. The investigation of the foam microstructures revealed not only the differences to the original polymer foams used as base material, but also provided further insights to the infiltration growth process via the large amount of internal surface in a foam sample.

International audience; The Pulsed Field Magnetization (PFM) is a compact and fast method to magnetize superconducting bulks compared to the field cooling method. However, the heat generation induced by the strong applied variable magnetic field during the PFM makes high trapped magnetic field harder to achieve. In order to make the REBaCuO bulks easier to magnetize by PFM, superconducting bulks including a superconducting weld are studied taking into account the thermal and electromagnetic properties of the weld different from those of the bulk body. This artificial grain boundary obtained by superconducting welding method might increase the trapped magnetic flux without increasing the applied magnetic field. In this paper, we are modelling the superconducting weld behavior during PFM using a 3D finite element model with the software COMSOL Multiphysics. The simulations are based on an H-formulation from Maxwell's equations and the heat diffusion equation. We analyse the impact of the critical current Jc of the weld on the trapped magnetic field.

The use of high-temperature superconducting (HTS) cables in power systems increases the transmission capacity, whereas reducing the volume of the installation. In addition, in high current applications, HTS cables considerably reduce power losses, right-of-ways and total system mass. This paper presents different multi-physics studies to be performed to accurately design the direct current (DC) HTS cables for the future railway network planned by the French company SNCF. The process used to design DC cables for high operating current (between 5 kA and 20 kA at 1750 V) made of commercial (RE)BaCuO tapes is presented. In this design process, the critical current density dependence J c(B, θ, T) of the superconducting tapes, the thermal properties of the materials used as well as different cable cooling configurations are considered. Finally, we discuss the selection of the appropriate cooling configuration to ensure adequate cooling of the cable.

International audience; The use of High Temperature Superconducting (HTS) cables in power systems increases transmission capacity while reducing the volume of the installation. In addition, when transmission currents exceed a few kA, HTS DC cables significantly reduce power losses, rights-of-way and total system mass. This summary describes the various studies to be carried out in order to correctly dimension DC HTS cables for the new railway network envisaged by the French company SNCF, which has to take into account the ultra-urban needs. The process used to design DC cables for different operating current values between 5 kA and 20 kA at 1 750 V using commercial (RE)BaCuO tapes is presented. In this design stage, the dependence of the critical current density Jc(B, θ, T) of the superconducting tapes, the thermal properties of the materials used, and the different cooling modes as a function of the cable length were taken into account. Finally, we are discussing a cryogenic solution to protect the cable in case of short-circuit or overload.

Bulk high temperature superconductors (HTS) can be magnetized and act as permanent magnet much stronger than conventional ones as NdFeB. The design of the inductor is a key point to perform the desired magnetization of the HTS bulk. In this paper, we focus on modeling a pulsed field magnetization (PFM) process of an HTS bulk using a coil powered with a magnetizer. The built model is a 2-D axisymmetric problem, based on the H formulation and coupled with electrical equations though the magnetic flux seen by the magnetizing coil. The calculation of this magnetic flux in the H formulation is not trivial and was validated using magnetic vector potential formulation on a coil in the air. Assuming different operating conditions, the bulk HTS is then modeled using four different properties corresponding to air, perfect diamagnetic, copper, and HTS. It was shown that simulating a PFM process could lead to different value of peak current and applied magnetic field to the bulk HTS, depending on the critical current density of the bulk, for example. These variations are in the range of the air and diamagnetic cases. Therefore, the proposed method should be used in order to predict a realistic trapped magnetic field in the HTS bulk by taking into account its reaction seen by the coil during the PFM process.

We compare the temperature and field dependence of the critical current densities of high- $T_c$ superconductor materials intended for various bulk applications such as trapped-field magnets. This comprises bulk samples of YBa $_2$ Cu $_3$ O $_x$ (YBCO), MgB $_2$ , and iron-based materials, including also various versions of the YBCO compound such as melt-textured ones, infiltration-growth processed ones, and YBCO foams. Critical current densities and flux pinning forces were obtained from magnetization loops measured using Quantum Design SQUID and physical property measurement system (PPMS) systems with applied magnetic fields of up to $\pm$ 9 T. The obtained data are compared to each other with respect of the optimal cooling temperature possible using modern cryocoolers. Furthermore, we plot the temperature dependencies of the critical current densities versus the normalized temperature $t=T/T_c$ . This enables a direct judgement of the performance of the material in the trapped-field applications.

High-Temperature Superconductors (HTS) considerably accelerate the development of superconducting machines for electrical engineering applications such as fully electrical aircraft. This present contribution is an overview of different superconducting materials that can be used as magnetic screens for the inductor of high specific power electrical machines. The impact of the material properties, such as the critical temperature (Tc) and the critical current density (Jc), on the machine performances is evaluated. In addition, the relevance to flux modulation machines of different HTS bulk synthesis methods are addressed.

International audience; Electrical machines are important parts of different power systems. The application of high temperature superconductors (HTS) in electrical machines is very promising due to high transport currents. This paper reviews various topologies of superconducting motors and generators using HTS published in the literature in recent time. It begins with a brief presentation of the HTS material used in electrical machines. The description of AC losses and cryogenic systems is done afterwards. Then we offer a striking description of the various realizations of HTS electrical machines such as half HTS synchronous machines, fully HTS synchronous machine, machines with HTS bulks and stacks. Some of these machines are totally innovative compared to conventional ones and their operating principle is strictly related to the presence of HTS materials.

Présentation orale; National audience; L'azote solide (SN2) est capable de fournir un environnement cryogénique uniforme et stable pour des systèmes à base de supraconducteurs à haute température (HTS) tels que des échantillons massifs durant leur aimantation et/ou pour leur caractérisation.Un système de refroidissement utilisant de l’azote solide a été dimensionné, conçu et testé.Un réseau de résistances thermiques non-linéaires a été développé afin d'estimer le temps de refroidissement de cette installation. Nous avons estimé que 50 L d'azote liquide (LN2) peuvent être refroidis à 30 K en 36 h à l’aide d’un cryoréfrigérateur Sumitomo CH-110 et d’un échangeur approprié dans le bain de LN2.Durant ce spot, nous détaillerons les différents éléments du système, les résultats expérimentaux issus du premier refroidissement ainsi qu’une comparaison avec les résultats obtenus par notre modélisation numérique.

International audience; Mg-B superconducting bulk materials are characterized as simple and uniform metallic compounds, and capable of trapping homogeneous field with beautiful conical shapes. Although pulsed-field magnetization technique (PFM) is expected as a cheap and an easy way to activate them, the heat generation due to magnetic flux motion in bulk magnets causes serious degradation of captured fields. It is important to suppress the flux motions during PFM to decrease the heat generation. In this study, authors precisely estimated the flux trapping to the bulk samples, found that the flux-shielding effect related to the sample dimensions. Moreover, the Ti-addition to the bulk sample affected the frequency of flux jump happenings. The magnetic field capturing of 5.0wt%Ti-added samples reached the highest value of 0.76 T, while the starting point of magnetic field invasion into the sample centre shifted from 1.0 T to 1.2 T with increasing sample thickness from 3.67 mm to 5.80 mm. The occurrence of flux jumps was suppressed in 5.0wt%Ti-dopped sample, which means that the heat capacity of the compounds shifted with varying Ti addition, effectively prevented the flux jumps.

International audience; The paper is based on idea realized under Erasmus+ Strategic Partnership Key Action 2 and focused on educational activities for attracting technical education to young people. The project based on exchange of good practices involves seven partners from three countries (Poland, Estonia and France). Three of them represents sector of higher education and four are schools located in the neighbour of the technical university. Unfortunately, in recent years number of candidates entering to technical universities (particularly in Europe) decreases. Partners exchange ideas how to increase number of students who starts education in technical area. Another project goal is to examine possibilities how the university can contribute in increasing quality of technical education at schools. Main project ideas and results of project activities are reported in the paper.

International audience; For possible applications as trapped field (TF) magnets, it is essential to fabricate large, polycrystalline bulk samples from the FeSe compound, the simplest high-Tc superconductor (HTSc) possible. FeSe is relatively cheap to prepare, and does not contain any rare-earth material. The grain boundaries in this compound are not acting as weak links as it is the case for the YBCO compound. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing the FeSe samples using solid-state sintering yields samples exhibiting strong magnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above Tc. Due to the complicated phase diagram [1], the samples are a mixture of several phases, α-FeSe, β-FeSe, δ-FeSe (Fe7Se8) and metallic α-Fe [2]. The amount of the latter two phases depends directly on the Se loss during the sintering process. The δ-FeSe is antiferromagnetic, and α-Fe is ferromagnetic [3]. In the present contribution, we show MHLs of a variety of samples measured up to ±7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T) measurements. We performed a thorough analysis of the microstructures using polarization microscopy, Kerr effect, MFM, SEM, EBSD and TEM in order to establish a relation between microstructure and sample properties. To prepare good superconducting samples, the presence of the (anti)ferromagnetic phases must be reduced by carefully adjusting the Se content using Ti foils as getter materials. Measuring magnetoresistance of these samples [4] implies that the samples are always cooled in the own local field, and thus, the analysis of the resistance data calculating the fluctuation-induced conductivity above Tc [5] is strongly affected by this local magnetic field. We demonstrate the importance of preparing phase-pure FeSe samples, which are essential for the various applications envisaged. This work is part of the SUPERFOAM international project funded by ANR and DFG under the references ANR-17-CE05-0030 and DFG-ANR Ko2323-10, respectively. References [1] H. Okamoto, J. Phase Equilibria 12, 383 (1991). [2] P. Diko et al., Physica C 476, 29 (2012). [3] A. J. Williams, T. M. McQueen, R. J. Cava, Solid State Commun.149, 1507 (2009). [4] T- Karwoth et al., J. Phys. Conf. Ser. 1054, 012018 (2018). [5] A. Almessiere et al., Mat. Chem. Phys. 243, 122665 (2020).

International audience; The development of the sintering process for the synthetization of superconducting Iron-selenide (FeSe) polycrystal enables a quick and cheap fabrication of superconducting bulks for trapped field applications without the use of Rare Earths or toxic elements. A relevant number of papers were published in the literature in order to determine the best way to produce a pure β-phase, responsible of the superconductivity without any other non-desirable phases (α-FeSe, unreacted Fe, FeSe – also called δ-FeSe). Using the collected information, we produced a pure β-phase by classic sintering process at temperatures above 685°Cfollowed by annealing below 450°C for optimizing the sample and improvement of the critical currents. Using the Spark Plasma Sintering method enables an improved homogeneity and a better grain coupling, which leads to a higher quality for FeSe superconducting bulks and an increased sample density. The consequence is another increase of the critical currents which makes the samples competitive for trapped bulks applications.

International audience; The use of High Temperature Superconducting (HTS) cables as a technology is considered as key to increasing the power of the electrical grid while reducing the volume of installation. In addi-tion, when transmission currents exceed a few kA, DC HTS ca-bles significantly reduce power losses, rights-of-way and total sys-tem mass. This article describes the various studies that need to be carried out in order to correctly size HTS DC cables for the new railway network envisaged by the French company SNCF, which must consider ultra-urban needs. The process used to de-sign DC cables for operating powers of 10, 20 and 30 MW at a nominal voltage of 1.5 kV using commercial (RE)BCO tapes is presented. In this sizing process, the critical current density Jc(B, θ, T) dependence of the superconducting tapes, the thermal prop-erties of the materials and the different cooling modes are taken into account. Several solutions are suggested to ensure that the cable can withstand fault currents and reduce recovery time in the initial state. Finally, with the help of analytical models and in-dustrial data, once the cables have been designed, the various sys-tem losses are reported as well as the power or size of the various auxiliary components.

Bulk discs (20 mm diameter and 4.3 mm thickness) of MgB2 added with Ge2C6H10O7 were obtained by Spark Plasma Sintering. Six samples with composition MgB2(Ge2C6H10O7)0.0014 and one undoped sample were fabricated under similar conditions and were magnetically characterized in order to determine the scattering of properties and reproducibility. The main source of the scattering of the properties is the decomposition of the additive due to elimination of the organic part in gas form, which occurs stepwise with intensive vacuum drops at around ~ 560 and ~ 740 °C. A third drop, which is sometimes not well resolved being part of the second peak at 740 °C, occurs at ~ 820 °C. The critical temperature at the midpoint of the transition, Tc, shows only a relatively small variation between 37.4 and 38 K, and the irreversibility field at a low temperature of 5 K takes values between 8 and 10 T. The pinning force and pinning force related parameters do not correlate with the carbon substituting for boron in MgB2 and suggest a synergetic influence of the microstructural details and carbon. Overall, despite the superconducting properties scattering, the samples are of high quality. Stacked into a column of six samples, they can trap at the center and on the surface of the column a magnetic field of 6.78 and 5.19 T at 12 K, 5.20 and 3.98 T at 20 K and 2.39, and 1.96 T at 30 K. These promising values, combined with facile fabrication of the samples with relatively high quality and reproducibility, show the feasibility of their use in building complex and large compound arrangements for bulk magnets and other applications.

International audience; The paper is focused on new educational activities focused on attracting young people for technical education. The idea is based on realization of the project under Erasmus+ Strategic Partnership (Key Action 2). The project based on exchange of good practices involves seven partners. Three of them represents sector of higher education and four are schools located in the neighbor of the technical university. Partners exchange ideas how to increase number of students who starts education in technical area, which seems a bit difficult, but engineers are very expected by the industry. The second project goal is to examine possibilities how the university can contribute in increasing quality of technical education at schools. Main project ideas and results of realized project activities are reported in the paper.

MgB2 bulk superconductors are expected to be utilized as rare-earth-free and lightweight trapped field magnets. However, the flux jumps frequently happen during the magnetizing processes, and heavily degrade the field-trapping performances. We have investigated the effect of additives to the MgB2 bulk samples prepared by spark plasma sintering process, and observed various flux jumps during the pulsed field magnetizing processes, which were carried out at 14 K which was obtained by the 2-stage GM cryocooler. The authors classified the flux motions as three categories as “no flux flow”, “fast flux flow”, and “flux jump” regions, and investigated the conditions where the flux jumps happen. We observed some drastic flux jumps in the pristine and clarified the effect of graphene addition to the flux jumps. The experimental results showed us a possible expansion of no flux jump region, and suggested us the improvement of field trapping capability.

International audience; Humanity is facing environmental issues that require the whole way in which we produce, distribute, and consume energy to be challenged. One technical solution that is widely considered is the smart grid, meaning a decentralized and multi-scale smart energy system. For more than a decade, various pilot projects have been implemented successfully all over the European Union. If such pilots are to be scaled up into effective roll-outs, the electrical engineering workforce needs to be prepared, as do the public authorities, engineering researchers and broader public. They need quick awareness-raising and training in the benefits, drawbacks, opportunities and skills associated with smart grids. Due to the complex nature of the situation and the related challenges for society, designing this training requires an innovative and participative approach. This article proposes a case study on applying a living lab approach to the design of innovative online courses on smart grids.

We would like to acknowledge the cryogenic centre of the IJL institute (Institut Jean Lamour), and more especially Thomas Hauet for allowing us to carry out these experiments in the research centre, and Luc Moreau for his help during experimental tests.; International audience; In practice, the voltage of a hydrogen-oxygen fuel cell is around 1 V at open circuit and from 0.6 V to 0.7 V at full rated load and it can be considered as a low-voltage energy source. Moreover, preliminary investigations undertaken on a single proton exchange membrane fuel cell (PEMFC) highlighted its behavior as a DC current source, that can be directly controlled by the H2 flow rate when the operating point is at very low voltage. In this paper, we present an innovative application of PEMFC that relies on taking advantage of both low voltage level and current source operating mode to feed a high inductive superconducting coil. Such a coil has no resistance and among others, is very sensitive to current ripples. Thus, specific power supplies are designed to feed them but they exhibit in most cases a huge volume and/or a low energy yield. Connecting a superconducting coil to a PEMFC implies to operate in short-circuit, which is an unusual use of PEMFC. To this end, requirements of such an application are defined, by making use of a PEMFC electrical model based on a 1D analog representation of mass transport phenomena. This model, that enables to take into account the influence of gas supply conditions, notably diffusion limit operation, is directly implemented in a standard simulation software used in electrical engineering. Then, simulation results and experimental results obtained by supplying a 10 H superconducting coil cooled by liquid helium by means of a single 100 cm2 PEMFC are compared and discussed.

International audience; When the MgB2 bulk body is magnetized by the pulse magnetizing method, the MgB2 bulk can take the role of a pseudo permanent magnet. However, it was confirmed that the temperature at which MgB2 is expected to be used is close to the critical temperature (TC) of MgB2 and the specific heat is small, so that the heat generated by the pulse magnetizing method causes flux jump and the trapped magnetic field performance deteriorates. In this study, pulse magnetizing was performed on different samples to verify and consider changes in the trapped magnetic field performance. As the sample, the sample obtained by changing the amount of titanium added was synthesized by uniaxial hot pressing and the sample synthesized by spark plasma sintering (SPS). Figure (a) shows a comparison of the trapped magnetic fields of the samples synthesized by the uniaxial hot press. From Fig. (A), it was shown that the capture magnetic field performance may be improved or deteriorated by the composition ratio of the material, the addition of impurities, and the method of synthesizing the bulk material.

International audience; With the objective to concurrence the well-known melt texturing YBCO samples in term of irreversible field and critical current, infiltration growth YBCO bulks (IG-processed) show a recent increase of interest due to their advantage of better controlling the shape of the final product and better mechanical stability. Two types of IG-processed YBCO samples were developed: standard bulk samples and superconducting foams which show the advantage of a huge cooling capacity and a low weight due to his low density, particularly appreciated for space applications. Intended for use as permanent superconducting magnets (trapped field (TF) magnets), precedents works were done to evaluate the pinning properties of such materials by pinning force scaling (Dew-Hughes and Kramer) but only under relatively low magnetic field (up to 9 T). Considering a very high irreversible field (Hirr), even at 77 K, a lot of assumptions and extrapolations were needed. Small pieces of both designs (standard bulk and foam) were cut and mechanically polished (1 x 1 x 0.1 mm3) according to the crystallographic orientation (H // c). To experimentally determine the pinning properties for magnetic field above 9 T, we were able to make measurements at high magnetic field (above 30 T) for different temperatures between 85 K and 40 K with the help of EMFL facilities using Cantilever Torque Magnetometry at the High Magnetic Field Laboratory of Nijmegen.

Poster; International audience; Pieces of bulk, superconducting FeSe samples prepared by solid-state sintering were irradiated with 2.5 MeV electrons (T = 23.5 K) at SIRIUS facility using two different fluences, 2×10^19 electrons/cm2 and 4×10^19 electrons/cm2. In contrast to previous work performed on FeSe single crystals, the bulk FeSe samples are polycrystalline without specific texture and exhibit a density of ~4 g/cm3 (= 78% of theoret. density) due to pores. Furthermore, the samples contain magnetic impurities which lead to a ferromagnetic signal of the sample. Such bulk FeSe samples are intended as base material for superconducting trapped field magnets operating at 5 K, with the benefit of a higher upper critical field, Hc2, and reduced flux jump probability as compared to MgB2. The electron irradiation introduced point defects to the FeSe grains. As result, the superconducting transition temperature, Tc, is slightly reduced, depending on the fluence, but the critical currents are increased by about 20-30%. We discuss the changes of Tc, of the magnetic background and the superconducting properties of the samples before and after the electron irradiation.

Poster; International audience; We suggest the use of superconducting trapped field magnets (TF) for Flux-Pinning Docking Interfaces (FPDI) in satellite systems. Previous work introducing the FPDI concept to cube-sat satellites was using bulk high-Tc superconductors and permanent magnets of Nd-Fe-B. This arrangement was proven to work both experimentally and theoretically. However, such FPDIs have limited magnetic field strength, consist of heavy-weight material, and can only be used with a single purpose. Replacing the magnetic material in the FPDI by a TF-magnet would enable the interface to operate for both purposes, i.e., generating a (stronger) magnetic field and trapping it. We show the requirements for such a system and discuss the possible gains when using a TF-FPDI in satellites. A further possibility to reduce the system weight would be the use of superconducting foams as superconducting material.

International audience; —An electromagnetic simulation of YBCO bulks was performed and the resulting trapped magnetic flux density was compared to Field Cooling experimental measurements for an applied magnetic flux density up to 3 T. The simulation relied on an axisymmetric problem implementing an A-formulation of the Maxwell’s equations solved by means of the Finite Element Method so that the time evolution of the magnetic flux density was computed over the cross section of the bulk. To derive its electrical conductivity, a classic power law was adopted that includes the dependence of the critical current density upon temperature and external magnetic field modelled on the basis of a Modified Kim-Anderson relation. It was found that this model tends asymptotically towards the critical state for which the n-value becomes a free parameter that should be then estimated experimentally. The experimental results could be fairly reproduced at different operating temperatures with the best trapping at temperatures below 77 K benefiting from an increasing critical current density.

The complete penetration magnetic field $B_{P}$ is the main feature of a superconducting pellet submitted to an axial applied magnetic field. The electric $E{-}J$ characteristics of HTS bulk is generally described by a power law $E(J) = E_{C}(J/J_{C})^{n}$ . The influence of the $n$ -value and applied magnetic field rise rate $V_{b}$ on the $B_{P}$ of a HTS cylindrical pellet has been presented in a previous paper. The numerical results presented come from a numerical resolution of a nonlinear diffusion problem. With the help of these simulations, a linear relationship between $B_{P}$ , ln $V_{b}$ , and $n$ -value has been deduced. This comparison allows determining the critical current density $J_{C}$ and the $n$ -value of the power law based on direct measurement of $B_{P}$ in the gap between two bulk HTS pellets. In this paper, an improvement of this method is presented. The influence of geometric parameters $R$ and $L$ is studied to give generality to the relationship between $B_{P}$ , $V_{b}$ , and $n$ -value. A previous $B_{P}$ formula is confirmed by these new simulations. To correctly connect simulation and experimental results, the influence of spacing $e$ between bulks is studied and presented. A relationship between $B_{P}$ and measured complete penetration magnetic field $B_{PM}$ is determined.

The low-temperature superconducting (LTS) coils are designed to produce strong magnetic fields at the working temperature of liquid helium. Some kinds of them have been created to produce strong and homogeneous magnetic fields, as in magnetic resonance imaging systems. However, the critical and mechanical parameters of superconducting material limit the performances of the coils. Most of the time, LTS coils are built to produce a homogeneous magnetic flux density in one single direction. Consequently, it is necessary to rotate the subject or the sample, when a study is needed depending on the magnetic field orientation. This is not suitable for special kinds of experiments involving high-temperature superconducting systems or biomedical applications, in which the subject must not be rotated. In order to satisfy these requirements, we have designed a new coil setup. The design was made using a 3-D electromagnetic software in order to calculate the performances of the coils system, to optimize the magnetic field homogeneity and to find a cost effective solution. The first results reported in this paper concern the design of a magnet of three Helmholtz coils, each of which generates a unidirectional magnetic field ( $x$ , $y$ , and $z$ ). Each Helmholtz coil can generate 1.73 T; thus, the flux density magnitude in the center of system could reach 3 T. The price of the all system, including the cryostat, is around 45 k $\geneuro$ .

International audience; The FeSe compound is the simplest high-temperature superconductor (HTSc) possible, and relatively cheap, not containing any rare-earth material. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing FeSe using solid-state sintering yields samples exhibiting strong ferromagnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above Tc. Due to the complicated phase diagram, the samples are a mixture of several phases, the superconducting beta-FeSe, and the non-superconducting delta-FeSe and gamma-FeSe. Furthermore, antiferromagnetic Fe7Se8 and ferromagnetic alpha-Fe may be contained, depending directly on the Se loss during the sintering process. Here, we show MHLs measured up to ±7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T ) measurements. We also performed a thorough analysis of the microstructures in order to establish a relation between microstructure and the resulting sample properties.

The 14th European Conference on Applied Superconductivity (EUCAS 2019) was held in Glasgow, UK from the 1st to the 5th September 2019. The conference was attended by over 1000 delegates from around the world, drawn from both academia and industry. The conference programme covered the latest research and industrial developments in all areas of applied superconductivity including superconducting electronics, large-scale applications and superconducting materials. Selected papers presented at EUCAS 2019 have been published separately in a Special Focus Issue of the journal Superconductor Science and Technology (SUST). This present volume in the Journal of Physics: Conference Series (JPCS) collects together the remainder of the contributed papers submitted to the EUCAS 2019 proceedings. Editors: Edward Romans (Editor-in-chief) Mark Ainslie Kévin Berger Nikolay Bykovskiy Oscar Kennedy Anna Palau Colin Pegrum Loïc Quéval Guilherme Gonçalves Sotelo Wescley Tiago Batista de Sousa Min Zhang

Oral; International audience; Toutes les méthodes d’aimantation des supraconducteurs sont basées sur la variation du champ magnétique appliqué qui résulte des caractéristiques de l’inducteur utilisé. Dans ce papier nous avons développé un modèle analytique pour montrer l’effet de la présence d’une pastille supraconductrice sur la valeur de l’inductance d’une bobine circulaire de section transversale rectangulaire placée dans un circuit ferromagnétique avec entrefer. Ce modèle est basé sur la détermination du potentiel vecteur magnétique à partir de la résolution les équations de Laplace et de poisson dans les différentes régions d’intérêt en utilisant la méthode de séparation des variables. Cette résolution analytique s’effectue à l’aide d’un code de calcul développé sous MATLAB. Les résultats obtenus sont comparés avec ceux obtenus par une simulation numérique basée sur la méthode des éléments finis implémentée sous COMSOL. Une concordance remarquable est observée entre les deux résultats.

Oral; International audience; In this paper we present a quick 3D model using reluctance network (RN) of a superconducting inductor dedicated to a synchronous machine. This inductor is made with two kinds of superconducting materials, which are: two coils of NbTi coaxial are fed by a currents in opposed sides and four superconducting rectangular pieces (YBCO) placed between coils for modulation of the magnetic field generated by NbTi coils. For the test, this inductor is fed by 260 A, and is cooled to 4.2 K. The results obtained by 3D RN are compared with FEM and an experimental prototype.

National audience; L’histoire des machines supraconductrices a débuté dans les années 1960. La première machine fabriquée en 1967 était un alternateur entièrement supraconducteur utilisant des fils NbTi. Ensuite, les supraconducteurs à haute température (HTS) ont été découverts et ce fut le point de départ de nouvelles études sur les machines supraconductrices. Cet article passe en revue diverses topologies de moteurs supraconducteurs et de générateurs utilisant les supraconducteurs à haute température critique et qui ont été étudiées et réalisées au GREEN.

International audience; In this paper, we propose to replace the usual dedicated electronic power supplies of superconducting coil, which are in most cases a huge volume and/or a low energy yield, by a 100 cm² single proton exchange membrane fuel cell (PEMFC). Indeed, fuel cells are electrochemical converters which can be considered as DC low voltage electrical source. Preliminary investigations led with a single proton exchange membrane fuel cell (PEMFC) could evidence the behavior as a DC current source, in which the current is directly controlled by the hydrogen flow rate. In a first step, we highlighted the feasibility by supplying a small superconducting coil, with an inductance value of 4 mH by means of a single PEMFC and proved that dedicated power source can be replaced by fuel cell. The second step is to feed a superconducting coil with higher inductance, 10 H, as presented in the present work.

International audience; High Temperature Superconductors (HTS) are promising for applications requiring high power densities, such as superconducting electrical motors. Various approaches have been developed to model HTS, in particular for AC losses evaluation in thin wires and tapes. Indeed, AC losses are one of the key factors to size properly the cryogenic systems. In some applications, where the HTS materials are used as magnetic screens or as permanent magnets, such as in electrical motors, it is relevant to estimate properly the penetration of the magnetic field in order to optimize the magnetization processes and the integration of these materials in such systems.In previous works [1]–[3], analytical tools in 2D have been successfully developed for calculating the magnetic field distribution in different devices integrating HTS bulks by considering them as perfect diamagnetic materials, e.g. a superconducting electrical machine or an inductor with an iron core used for the pulsed field magnetization of a bulk HTS. These methods provide continuous derivatives and are useful tools for the design and optimization of such systems. They lead to meaningful solutions with helpful physical insights. However, they are limited to simple geometries and do not take into account either eddy currents or the variation of the critical current density with the magnetic field, which is crucial in HTS. On the other hand, rapid modeling approaches are needed, and the use of classical numerical tools is often ineffective in the design and optimization process due to considerable calculation time. Specific numerical approaches ensuring a better compromise between precision and calculation time are still required.In this context, the present work presents a hybrid model in which the HTS behavior is represented by the power law E(J, B) = Ec (J /Jc (B)) ^ n(B), taking into account the variation of the critical current density and the power exponent with respect to the magnetic flux density. The magnetic vector potential, the magnetic field and the current density distributions are computed by means of analytical and integral equations implemented on MATLAB. The resolution of the obtained stiff ordinary differential equations are performed using available solvers in MATLAB with adaptive time steps. The integral equations used to calculate the induced currents in the HTS materials are based on the well-known “Brandt method” developed in 1996 for strips and slabs, and later for disks and cylinders in an axial magnetic field.One of the main advantage of the hybrid method proposed here is that only the active parts are discretized. Distributions of the magnetic field over time and other quantities such as losses are presented. Calculation times for the studied problems are given and compared with those obtained from classical FEM software.

The authors would like to thank Thierry Lubin and Alexandre Colle for their useful discussions on this topic.; International audience; The development of bulk magnets based on REBCO type superconductors that can generate up to 17 T requires the implementation of a magnetization system, sometimes pulsed. Therefore, at least one coil around the High Temperature Superconducting (HTS) bulk is required. In this paper, we suggest an analytical approach in order to assess the magnetic vector potential and to compute the inductance of a coil having a rectangular cross-section and concentric with a magnetic circuit including an air-gap. On a proposed geometry, analytical results are compared with those obtained by using Finite Element Analysis (FEA). The gained analytical expressions give accurate results with an error less than 1% on the inductance value of the coil. The induced current density and trapped magnetic field in a HTS bulk during a Pulsed Field Magnetization (PFM) are also calculated using the proposed analytical expressions of the magnetic vector potential combined with an integral method. The achieved results show a remarkable match with those obtained with COMSOL.

International audience; In this work, we propose to study and design a coil used to magnetize, by means of a Pulsed Field Magnetization (PFM) process, an inductor of a radial flux superconducting machine with one pair of poles. Each pole contains four similar HTS bulks of 30 mm diameter arranged in a square pattern. The cryostat already exists for this application and the temperature of the HTS bulks can vary from 4.2 K to their critical temperature, in transient state. For a given primary source of energy, here a capacitor bank of 10 kJ (5 mF, 2 kV) is available, the PFM process depends strongly on the value of the coil inductance used to generate pulsed field, because it defines the waveform of the current: peak value and time constant. Thus, 3D modeling of the coil is required in order to be sure that its inductance and the magnetic field produced will provide a full magnetization of HTS bulks. From the practical point of view, we would like to achieve an average magnetization of each pole around 3 T. In this paper, numerical modeling of coils with different number of turns coupled with circuit's equations is achieved. The maximum magnetic field obtained on the HTS bulks and estimated magnetization at the top center of each HTS bulk, are presented and discussed.

International audience; In this paper a new superconducting inductor topology intended for synchronous machine is presented. The studied machine has a standard 3-phase armature and a new kind of 2-poles inductor (claw-pole structure) excited by two coaxial superconducting coils. The air-gap spatial variation of the radial flux density is obtained by inserting a superconducting bulk which deviates the magnetic field due to the coils. The complex geometry of this inductor usually needs 3D finite elements (FEM) for its analysis. However, to avoid a long computational time inherent to 3D FEM, we propose in this work an alternative modeling which uses a 3D meshed reluctance network. The results obtained with the developed model are compared to 3D FEM computations as well as to measurements carried out on a laboratory prototype. Finally, a 3D FEM study of the shielding properties of the superconducting screen demonstrates the suitability of using a diamagnetic-like model of the superconducting screen.

International audience; The story of superconducting machines began in the 1960s. The first machine made in 1967 was a fully superconducting alternator using NbTi wires. Then, High Temperature Superconductors (HTS) were discovered and this was the starting point of many new studies on superconducting machines. This chapter reviews various topologies of superconducting motors and generators using HTS published in the literature. It begins with a brief presentation of the operating principle of electric motors. Then follows a striking description of the various realizations listed by machine typology. Some of these machines are totally innovative compared to conventional ones and their operating principle is strictly only related to the presence of superconducting materials.

Complete penetration magnetic field BP is a feature of a superconducting sample submitted to an applied magnetic field. It is very important to know this for applications such as an electrical motor or levitation. The electric E-J characteristics of a high-temperature superconductor (HTS) bulk is generally described by a power law. The main purpose of this paper is to investigate the influence of the n-value and the applied magnetic field rise rate Vb on the BP of a cylindrical HTS pellet. The numerical results presented come from the resolution of a nonlinear diffusion problem with commercial software. In this paper, cylindrical HTS pellets are submitted to an axial applied magnetic field. With the help of these simulations, a linear relationship between BP , Vb, and the n-value has been found. A comparison between measurements and simulations is done for the magnetization of cylindrical bulk superconducting samples. This comparison allows to determine the critical current density J C and n-value of the power law E(J) = EC(J/J C )n. The experiment is based on the direct measurement of the local magnetic field in the gap between two bulk HTS pellets. The field penetration measurements have been carried out on HTS pellets at 77 K by applying increasing magnetic fields with a quasi-constant sweep rate for the axial direction of the applied magnetic field. Two values of complete penetration magnetic field BP have been measured at two different rise rates Vb. The n-value of the real HTS pellet has been deduced.

International audience; Hybrid electric propulsion has been identified as a potential solu-tion to the ambitious environmental emissions and noise targets of the aerospace industry. Superconducting machines may be the key component of that topology enabling the high power densities and efficiencies needed in aerospace. Fully superconducting ma-chines, however, are not a mature technology. This paper looks at the different machine design configurations focusing on the stator magnetic circuit of a fully superconducting motor. The motor has been designed for an aerospace distributed fan propulsion motor with an aerospace benchmark specification of 1 MW. The AC ful-ly superconducting machine includes superconducting bulk magnets mounted on a conventional rotor core and an MgB2 su-perconducting wire wound stator. The AC losses in the stator winding are particularly sensitive to exposure to the main rotor field so different screening solutions were used to shield the su-perconducting windings from the rotor field. The effectiveness of the screening techniques for the stator coils and the impact on the machine performance and weight were evaluated for different stator designs, such as full stator core and air core with and without flux diverters. Various combinations of pole numbers, diverter geometries and magnetic materials have been checked. Results show that there is a trade-off between stator iron losses and superconducting losses.

Oral; International audience; Pour pouvoir dimensionner des dispositifs supraconducteurs, en particulier le système cryogénique associé, il est nécessaire d’estimer avec précision les pertes AC. Les méthodes de détermination analytiques ou expérimentales sont souvent difficiles à appliquer sur des systèmes réels, de sorte que ces pertes ne peuvent souvent être estimées que de manière numérique. Dans cet article, nous présentons les résultats de modélisations sur un cube supraconducteur. Après une étape de validation numérique entre plusieurs codes, cet article s’intéresse plus particulièrement à l’influence du maillage et au traitement de la loi de comportement non linéaire du supraconducteur par les codes de calcul utilisés. L’objectif final de ces travaux est de produire des modèles numériques 3-D des supraconducteurs plus rapides et plus précis.

Oral; International audience; In this work, we propose a numerical model for a rapid computation of the electromagnetic field in multifilamentary composite HTS tapes carrying sinusoidal AC currents or submitted to external sinusoidal time varying magnetic fields. The proposed model is based on an integrodifferential formulation in terms of the electric vector potential and allows taking into account the nonlinearity of the constitutive law, relating the electrical current density to the electrical field, in the frequency domain rather than in the time domain as it is commonly done. Only the active parts are discretized; the multiscale dimensions are thus easily treated.

Poster; International audience; Our basic method to characterize HTS pellets, which has been presented in earlier publications, is based on the measurement of their complete penetration magnetic flux density BP. The method allows assessing the critical current density JC and the n-value through the classical power law model modeling the magnetic behavior of pellets and the direct measurement of BP. A first method used two slightly axially separated HTS disks to determine BP in the gap between the disks. However, this first method exhibits two main disadvantages. The first disadvantage is that the magnitude of the derived values depends on the spacing e between the two pellets. The second one is that the two pellets ought to be strictly identical for the method to lead to representative values of a given pellet. To overcome those constrains and limitations, we have been looking for a new improved method that relies only on the measurement involving a single pellet. The following work introduces the principle of this new method. The main idea is thus to deduce BP from the magnetic flux density measured at the center of the top surface of the pellet referred to as BT when the applied magnetic flux density Ba equals BP. The influence of the applied magnetic flux density ramping rate and the geometric dimensions of the pellet on the ratio BP /BT is studied and used to assess BP from the measured BT. It is shown that the proposed method allows achieving a substantial improvement on the accuracy of the Jc and n-value measurements of a given superconducting disk over the past method based on two HTS pellets.

International audience; In this work, we present a numerical model for a rapid computation of eddy currents in composite materials, adapted for both carbon fiber reinforced polymers (CFRPs) for NDT applications and multifilamentary high temperature superconductive (HTS) tapes for coupling loss evaluation. The proposed model is based on an integro-differential formulation in terms of the electric vector potential in the frequency domain. It allows taking into account both the high anisotropy and the nonlinearity of the considered materials.