Kévin Berger

Influence of Temperature and/or Field Dependences of the

Temperature, current density and magnetic field distributions in YBCO bulk superconductor during a pulsed-field magnetization (PFM) process are calculated using the finite difference method. Simulations are based on the heat diffusion equation with account of the heat produced by flux motion, and Maxwells equations. A power law with temperature and magnetic field dependent parameters is used to characterize the electromagnetic behavior of the superconducting material. We analyze how the stored magnetic energy depends on the temperature and field dependences of the power law.

E-J

The design of a twin coil 2 ? 200?kJ?1?MW pulse power high temperature superconductor (HTS) superconducting magnetic energy storage (SMES) demonstrator is presented. Its aim is to test at small scale various possibilities of electromagnetic launcher powering. The foreseen operation modes include high voltage discharge in power capacitors, sequential discharges of identical energies from two coupled coils, and XRAM current multiplication. Special attention was paid to the arrangement of the coils for the energies discharged to be equal. The coils are cooled by conduction from three cryocoolers; the thermal design was optimized in order to maintain the coils around 15?K in spite of the high number of current leads required for XRAM operation (eight). Preliminary tests of the demonstrator are also presented, showing that the thermal and electrical characteristics are in very good agreement with the design objectives.

Power Law on Trapped Magnetic Field in Bulk YBaCuO

When a superconductor is fed with an alternating current, the temperature rise created by the losses tends to reduce the current carrying capacity. If the amplitude of the current exceeds the value of the critical current, then the losses become particularly high and the thermal heating considerable. In this paper, a numerical and an analytical model which allow to estimate AC transport losses are presented. These models, which use the expression of I/sub c/(T) and n(T), are available for any applied current (below and above I/sub c/). The results are compared and the validity of the analytical model is considered. Then, the analytical formula allows to easily obtain the thermoelectric balance point of the system, when the losses and the temperature do not vary any more. Moreover, a maximum value of the current transport, beyond which the balance point does not exist, is detected. Indeed, when this maximum value is exceeded, the system is not stable and, say, superconducting current leads may quench.

Design and preliminary tests of a twin coil HTS SMES for pulse power operation

New sensors of magnetic induction in near field, dedicated to studies of electromagnetic compatibility, are proposed based on the Standard CISPR16-1 coils. The new coil shape allows the sensors to be sensitive only to specific components of the multipolar expansion, which is similar to a spatial filtering. In comparison with our previous approach in which we designed a complete set of coils, we aim to simplify the sensor coil geometry by rotating the spherical harmonics functions. After a description of the tools required, the design method is described.

AC transport losses calculation in a Bi-2223 current lead using thermal coupling with an analytical formula

This paper presents new semi-analytical expressions to calculate the self-inductance and the electromagnetic force for a ferromagnetic cylinder of flnite length placed inside a circular coil of rectangular cross section. The proposed analytical model is based on boundary value problems with Fourier analysis. Laplaces and Poissons equations are solved in each region by using the separation of variables method. The boundary and continuity conditions between the difierent regions yield to the global solution. Moreover, the iron cylinder is assumed to be inflnitely permeable. Magnetic fleld distribution, self-inductance and electromagnetic force obtained with the proposed analytical model are compared with those obtained from flnite-element.

Identification of Equivalent Multipolar Electromagnetic Sources by Spatial Filtering

A series of disk-shaped, bulk MgB2 superconductors (sample diameter 20 mm, reaction temperatures ranging between 750 °C and 950 °C) was prepared to improve the performance for superconducting supermagnets. These samples were characterized by magnetic and electric measurements in fields up to 7 T and at various temperatures 10 K <;T<; 35 K. The irreversibility lines, the current densities, the resistance, and the achievable trapped fields were determined. To analyze the data, a scaling of the flux pinning forces, Fp= jc× B, was performed. The different scaling behavior of the samples prepared at low- and high-reaction temperatures is discussed, considering the achieved microstructures.

Inductance and Force Calculation for Axisymmetric Coil Systems Including an Iron Core of Finite Length

The knowing the critical current density J_C is important to calculate AC losses in superconducting applications. Usually can be obtained from magnetization measurements or electric measurements based on global quantities as the magnetic flux or the voltage. In this paper a quick and direct magnetic method for determining J_C is proposed. It is based on direct measurements of local magnetic field in the gap between two bulk HTS pellets. Field penetration measurements were carried out on HTS pellets at 77 K by applying increasing axial magnetic fields with a quasi constant sweep rate. This determination of J_C is theoretically based on Bean model. J_C is deduced from the complete penetration magnetic field B_P. B_P is deduced from the delay T_P between the applied magnetic field B_a(t)and the magnetic field at the center between the two pellets B₀(t). Numerical calculations allow deducing J_C more precisely from theoretical calculations and measurements. The numerical calculations are made with the power law E = E_C(J/J_C)ⁿ. For the determination of J_C the influence of the gap due to Hall probe sensor and the applied magnetic field rise rate are taken into account. The influence of thermal is also studied.

Development of MgB 2 -Based Bulk Supermagnets

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 JC and n-value of the power law E(J) = EC(J/JC)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.

A New Direct Magnetic Method for Determining

We propose an original method for the characterization of superconducting tapes and wires from electrical measurements on a test coil. The principle is to measure the voltage-current characteristic of a coil for several values of the external applied flux density. A computer program then allows finding parameters of a model of the E(J, B) law that best fit the experimental curves. A numerical study on the algorithm convergence and on the accuracy of results was performed. This method has been employed experimentally to characterize a BiSCCO tape. Parameters of three different models have been obtained and compared. This process has several advantages compared to the conventional measurements on short samples: the voltage to noise ratio is higher and the self flux density of the coil is taken into account. Two models that fits well experimental curves were found.

{\rm J}_{\rm C}

It is well known that the critical current density Jc of a superconducting material depends on the magnetic flux density B. There exists an electric method to measure the Jc(B) deduced from the U(I) measurements. The problem with this method is the self field effect because the magnetic flux density is always the sum of the applied magnetic flux density and the self magnetic flux density. This paper presents a special experimental arrangement, compensating fully or partially the self magnetic flux density in an HTS tube. It allows characterizing the true zero magnetic flux density behaviour of the superconducting material. The experimental results of the compensation are discussed. A theoretical analysis based on Beans model is presented and gives results close to the experimental ones. The proposed compensation is not perfect but the experiments and the theoretical analysis allow validation of the compensation principle.