Denis Netter

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

Electrical motors need a spatial variation of the flux density created by their inductors to make torque. We propose to achieve this angular repartition of the induction with bulk high temperature superconducting plates and two low temperature superconducting solenoids. These solenoids have the same axis and are fed with opposite currents in order to create a radial magnetic induction. Four superconducting plates placed on the cylindrical surface, situated on the same axis and having the same radius as the solenoids, concentrate the magnetic field. This device provides a spatial variation of the flux density, and can be considered as an inductor for a synchronous motor with eight poles. This paper presents the experimental study of our prototype at 4.2 K. We compare the experimental results with calculation. Ten Hall probes permit us to reconstitute the angular variation of the flux density. The paper shows the feasibility of this new kind of superconducting inductor.

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

This paper presents an analysis of an original structure of induction heater using superconducting coils. With the proposed structure, two operating modes are possible to heat the aluminum billet. Either the billet is at standstill and placed within a strong rotating magnetic field (around 3.5 T) of low frequency (around 1 Hz), or the billet rotates at low angular speed (around 6 rad/s) within a strong dc magnetic field (around 3.5 T). The analysis is based on the analytical solution of two-dimensional diffusion equations for the magnetic vector potential. A closed-form expression is derived for the eddy-current power loss in the billet. This analytical expression can be used as a tool for design optimization of the induction heater. The analytical results are compared with those obtained by finite-element software. The study of the temperature field in the billet shows that a homogeneous temperature profile is obtained when a strong magnetic field of low frequency (rotating speed or ac current pulsation) is used.

Experimental study of a new kind of superconducting inductor

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.

Induction Heating of Aluminum Billets Subjected to a Strong Rotating Magnetic Field Produced by Superconducting Windings

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 deals with the calculation of transient eddy currents by a Markov chain Monte Carlo (MCMC) method. After we illustrate the principle in a one-dimensional calculation, we treat a two-dimensional problem. Then, we show simulation results and we discuss advantages and disadvantages of this method. An important advantage is that MCMC methods can be more efficient than finite-element methods in transient problems because they give an estimate of the solution at a point in space without calculation of the whole field distribution.

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

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).

Monte Carlo method for transient eddy-current calculations

A superconducting magnet energy storage (SMES) can be used as a pulsed power supply. A superconducting coil stores energy without electrical losses and this energy can be recovered through a second wire on which the charge (electromagnetic launcher, for example) is linked. The design of such an apparatus needs to solve simultaneously thermal, magnetic, and electric equations. We proposed a three-dimensional finite difference method to solve these coupled problems. This tool enables us to describe resistive zones of expansion in thick coils during a quench and to predict the duration and the efficiency of the discharge. Moreover, it indicates if the coil is prevented from an excessive temperature increase. Then, a probative device is described and experimental results are compared with theoretical ones.