Bruno Douine

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

We have designed, constructed and tested an eight-pole superconducting rotating machine, based on an unconventional topology that could potentially lead to a significant increase in power density. Calculations have been carried out in two steps: estimation of the magnetic scalar potential from a Coulomb formulation using the Markov chain Monte Carlo (MCMC) method, and the calculation of the flux density by derivation of the potential using a regularization method. The use of the MCMC method enables the calculation of the magnetic scalar potential in selected regions of the discrete geometry, which is an important factor to minimize the computation time. The principle of the operation has been validated by a successful testing of the motor showing this novel configuration of an electrical motor as very promising

Power Law on Trapped Magnetic Field in Bulk YBaCuO

An original superconducting synchronous motor topology is presented. The inductor combines both bulk materials and wires to create an alternating magnetic field in the air gap of the motor. A prototype that uses NbTi superconducting wires and YBCO bulks has successfully been designed and constructed. Test results are presented. In the second part of this paper, the possibility of higher power motor is discussed. To do so, the active length needs to be increased. In this paper, a multistack structure is proposed, and computation results show the effectiveness of the suggested approach.

Design and Testing of a Superconducting Rotating Machine

This paper describes the design of a new kind HTS motor. Superconducting plates used as magnetic screen that cause a spatial variation of the flux density. It is theoretically possible to obtain an air gap flux density variation greater than 3 T. We perform a dimensionless study of the device showing that the torque depends on shape parameters and is limited by the maximum field on the superconducting wire. In the last section, we discuss the performance of the device in term of torque density.

Superconducting Multistack Inductor for Synchronous Motors Using the Diamagnetism Property of Bulk Material

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.

Theoretical study of a new kind HTS motor

This paper presents a new numerical method based on finite elements - finite volumes (FE-FV) for solving 2-D diffusion problems in high temperature superconductors (HTS). The approach does not involve directly the resistivity term (rho), generally used to model the <i>E</i>(<i>J</i>) characteristic as a power law, i.e <i>E</i>(<i>J</i>) = rho(<i>J</i>)<i>J</i>, with rho(<i>J</i>) prop <i>J</i> ^n-1. Instead, we use a <i>J</i>(<i>E</i>) constitutive law <i>J</i> prop <i>E</i> ^(1/n), with <i>E</i> ^rarr = <i>Ee</i> ^rarr _z (a single component), which leads to a scalar non-linear differential equation. After presenting in details the developments, the method is tested in the case of a superconducting cylinder submitted to a transverse magnetic field. The current density obtained is compared to another numerical technique (the semi-analytical method) in order to validate the results. Although not fully optimized yet, it appears that the proposed method is very stable, especially for large n-values (greater than 100).

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

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.

New Hybrid FE-FV Method for Computing Current Distribution in 2-D Superconductors: Application to an HTS Cylinder in Transverse Magnetic Field

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

A robust numerical method based on 2-D mixed finite-elements-finite volumes (FE-FV) allows the solution of diffusion problems in superconducting (SC) materials. The proposed approach handles the strong nonlinearity of the E(J) constitutive power law of high-temperature superconductors (HTS). The method is tested for a SC cylinder submitted to a sinusoidal transport current or to a transverse sinusoidal external field. The current density distributions as well as the AC losses are computed. Comparisons to a FE analyses that use the magnetic field as state variable show the validity of the proposed approach. It can be seen that the proposed method is very stable even for large n-values for which the FE method does not converge.

A New Direct Magnetic Method for Determining

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.