First tests of a 800 kJ HTS SMES

SMES using high critical temperature superconductors are interesting for high power pulsed sources. Operation at temperatures above 20 K makes cryogenics easier, enhances stability and improves operation as pulsed power source. In the context of a DGA (Delegation Generate pour l'Armement) project, we have designed and constructed a 800 kJ SMES. The coil is wound with Nexans conductors made of Bi-2212 PIT tapes soldered in parallel. The coil consists in 26 superposed simple pancakes wound and bonded on sliced copper plates coated with epoxy. The rated current is 315 A for an energy of 814 kJ. The external diameter of the coil is 814 mm and its height 222 mm. The cooling at 20 K is only performed by conduction from cryocoolers to make cryogenics very friendly and invisible for the SMES users. The cooling down has been successfully carried out and the thermal system works as designed. After a brief description of the SMES design and construction, some tests will be presented. From a current of 244 A, the SMES delivered 425 kJ to a resistance with a maximum power of 175 kW.Comment: 5 page

Analytical modeling of a coil in a ferromagnetic circuit including a superconductor pellet

PosterInternational audienceIn the design phase of the inductor required for the magnetization of High Temperature Superconductor (HTS) bulks, modeling and simulation are necessary for the design of this inductor. In this paper we have developed an analytical model to calculate the inductance for a coil of a ferromagnetic circuit with gap including a superconducting pellet. This model is based on the determination of the magnetic vector potential from solving the Laplace’s and Poisson’s equations in different regions of interest, using the separation of variables method in which the Cartesian coordinates are used. The boundary and continuity conditions between regions are considered to determine the global solution. This analytical resolution is carried out using a computation code developed under MATLAB. The results obtained are compared with those obtained by a numerical simulation based on the finite element method implemented under COMSOL. A remarkable concordance is observed between both approaches

Modélisation 3D par la MEF et couplage circuit du procédé d'aimantation impulsionnel d'un ensemble de pastilles supraconductrices massives

National audienceA superconductor can induce currents when exposed to a variation of an external magnetic field, created for example by a pulsed current flowing through a coil. As these induced currents circulate indefinitely, a trapped magnetic field is generated, which is the remaining magnetic field produced by the superconductor alone when the applied magnetic field has disappeared. This process is called Pulse Field Magnetization (PFM). In this article, we are interested in the modeling of the PFM of an array of several High Temperature Superconducting (HTS) pellets. A 3D model has been developed, based on the Finite Element Method (FEM), and coupled with an external electrical circuit. In this model, A and H formulations are coupled using appropriate boundary conditions to take advantage of each other in specific regions. The presented results are the current density during the magnetization process and the trapped magnetic field trapped in the HTS. These results highlight the need to model such a problem in 3D.Un supraconducteur peut induire des courants lorsqu’il est soumis à une variation de champ magnétique extérieur, créé par exemple par une impulsion courant circulant dans une bobine. Ces courants induits circulant indéfiniment, il en résulte un champ magnétique piégé, qui est le champ magnétique restant produit par le seul supraconducteur quand le champ magnétique appliqué a disparu. Ce procédé est appelé aimantation par un Champ Magnétique Pulsé (CMP). Dans cet article, nous nous sommes intéressés à la modélisation de l’aimantation par CMP d’un arrangement de plusieurs pastilles Supraconductrices à Haute Température Critique (SHTC). Nous avons développé un modèle de calcul 3D basé sur la Méthode des Eléments Finis (MEF) couplé avec un circuit électrique externe. Dans ce modèle, les formulations en A et H sont couplées à l’aide de conditions aux frontières appropriées afin de tirer avantages de chacune d’elles dans des régions spécifiques. Les résultats présentés sont la densité de courant durant le procédé d’aimantation et le champ magnétique piégé dans les SHTC. Ces résultats mettent en évidence la nécessité de modéliser un tel problème en 3D

Improved Method for Determining the n-Value of HTS Bulks

International audienceThe complete penetration magnetic field Bp 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) = Ec(J/Jc)^n. The influence of the n-value and applied magnetic field rise rate Vb on the Bp of a HTS cylindrical pellet has been presented in a previous paper. The numerical results presented come from numerical resolution of a non linear diffusion problem. With the help of these simulations a linear relationship between Bp, ln Vb and n-value has been deduced. This comparison allows determining the critical current density Jc and the n-value of the power law based on direct measurement of Bp 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 Bp, Vb and n-value. Previous Bp 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 Bp and measured complete penetration magnetic field Bpm is determined

What Formulation Should One Choose for Modeling a 3D HTS Magnet Motor Pole with a Ferromagnetic Material?

We discuss the relevance of several finite-element formulations for nonlinear systems containing high-temperature superconductors (HTS) and ferromagnetic materials (FM), in the context of a 3D motor pole model. The formulations are evaluated in terms of their numerical robustness and efficiency. We propose a coupled h-phi-a-formulation as an optimal choice. While maintaining a low number of degrees of freedom, this formulation guarantees a robust resolution and strongly reduces the number of iterations required for handling the nonlinearities of HTS and FM compared to standard formulations

What Formulation Should One Choose for Modeling a 3D HTS Motor Pole with Ferromagnetic Materials?

peer reviewedWe discuss the relevance of several finite-element formulations for nonlinear systems containing high-temperature superconductors (HTS) and ferromagnetic materials (FM), in the context of a 3D motor pole model. The formulations are evaluated in terms of their numerical robustness and efficiency. We propose a coupled h-phi-a-formulation as an optimal choice, modeling the problem with an a-formulation in the FM and an h-phi-formulation in the remaining domains. While maintaining a low number of degrees of freedom, the h-phi-a-formulation guarantees a robust resolution and strongly reduces the number of iterations required for handling the nonlinearities of HTS and FM compared to standard formulations

Exploring the flux pinning performance of bulk FeSe by electron irradiation

OralInternational audienceBulk, polycrystalline samples of FeSe are interesting candidates for trapped field applications as the material can be prepared by simple solid-state sintering, is free of toxic elements and exhibits magnetic properties like the copper-based high-Tc superconductors, i.e., high upper critical fields Hc2, and relatively small anisotropy. Polycrystalline, sintered FeSe material shows further promising features such as strong grain coupling. The FeSe material consists of well-coupled, but randomly oriented, platelet-like round grains with a diameter between 1 and 6 µm.However, the critical currents of the FeSe system are still low, but comparable to sintered MgB2. The goal of the present work is to explore the possibilities of the FeSe system in application-type samples (i.e., polycrystalline material, not single crystals) using electron irradiation. 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. The electron irradiation introduced point defects to the FeSe grains. The changes of Tc due to irradiation and the critical currents were measured using SQUID magnetometry. As result, the superconducting transition temperature, Tc, is slightly reduced, depending on the fluence, but the critical currents are increased by about 20-30%, which demonstrates that one can introduce additional disorder to FeSe to improve the flux pinning properties also in the polycrystalline material

Microstructural Parameters for Modelling of Superconducting Foams

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

Electron Irradiation of Polycrystalline Bulk FeSe Superconductors

PosterInternational audiencePieces 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

Non-hyperbolic ergodic measures and horseshoes in partially hyperbolic homoclinic classes

We study a rich family of robustly non-hyperbolic transitive diffeomorphisms and we show that each ergodic measure is approached by hyperbolic sets in weak$*$-topology and in entropy. For hyperbolic ergodic measures, it is a classical result of A. Katok. The novelty here is to deal with non-hyperbolic ergodic measures.Comment: 25 pages and 1 figur