Maria Ahoranta

Magnet design for superconducting open gradient magnetic separator

Abstract The use of superconductivity opens new applications for magnetic separation because very high magnetic fields become available. In this paper the magnet design for a laboratory scale superconducting open gradient magnetic separator is presented. The separator will be used to optimize the separation parameters for different kinds of applications, such as the foundry sand purification. Therefore, the goal of the magnet design is to obtain a constant magnetic force density distribution inside the working volume. The high magnitude of magnetic force density is required because the materials to be separated have low magnetic susceptibilities. The maximum achievable force density is determined by the critical current in superconducting magnets. The advantages and drawbacks of solenoid, racetrack and saddle coil geometries are compared. Ways for improving the performance of the system is discussed. Finally, the influence of the stray field on the slurry flow outside the working volume is studied.

Modelling of local strain and stress relaxation in bronze processed Nb3Sn wires

Recently, the need for mechanical modelling of Nb3Sn wires has increased, since these wires are used in large projects such as the International Thermonuclear Experimental Reactor (ITER). The finite element method makes it possible to solve multi-axial stress and strain distributions in complicated wire geometries. However, Nb3Sn conductors possess features that make building the mechanical model challenging and complicate the use of the finite element method. This paper focuses on two problems. First, detailed modelling of geometries with large filament amount can produce numerical problems that are too large for computers. A simple approach to circumvent this problem is presented, and its validity investigated. Second, the uncertainty in modelling thermal stress, caused by stress relaxation and other high-temperature phenomena, is treated. To investigate the credibility of the models, the computed results are compared with critical current and stress–strain data measured for a bronze processed Nb3Sn wire in an axial tension test. Computed results agree well with the measured ones, and show that the mechanical strain state of Nb3Sn in filaments is very sensitive to the degree of stress relaxation.

Finite element models for thermal stress, axial tension, bending, and transversal compression of filamentary Bi-2223 tapes

Most filamentary superconductors have soft matrix metal and brittle filaments, which make them very vulnerable during mechanical loading. Therefore, estimating the impacts of loading in the conductors is important in designing applications. Finite element models provide a way to study the stress and strain distributions in a tape, taking into account the details of the tape cross section. This paper presents finite element models for some basic loading conditions met, for example, in magnets. The cases investigated include thermal stresses, axial tension, transversal compression, and bending of a tape. The focus is on Bi-2223 tapes, although similar models could also be applied to other conductor types. To evaluate the validity of the models, the results are compared to experimental data. The results of the models showed the same trends as the experiments, but some inaccurately known properties of the modelled tapes caused uncertainty in the models. For example, the stiffness and fracture strength of the filament material are essential parameters to pay attention to when the initiation of critical current is predicted.

The effect of Fe-magnetization on Ic(B) and Ic(α) characteristics of iron-sheathed MgB2 composite wires

Single- and four-core MgB2 composites have been made by the powder-in-tube (PIT) method using commercial MgB2 powder in an Fe sheath. Transport currents of as-deformed wires were measured at 4.2 K and external magnetic fields B = 0?4 T. Different Ic(B) curves were observed for single- and four-core wires depending on the value of the self-field current. The behaviour Ic(B) and Ic(?) is influenced by Fe sheath magnetization by self-field and external field. Field distribution and Ic(B) curves were calculated using the finite element method (FEM) and they are used for an explanation of the experimental characteristics.

Cryogenic design of an isodynamic magnetic separator

A superconducting isodynamic open gradient magnetic separator has been designed in order to test the separation of various slurries at high magnetic field. The system will be used as a measuring device in order to determine the conditions for the separation process at different applications. The magnet system consists of two racetrack coils. The separator system will be tested with two types of magnets: a pair of NbTi coils and a pair of Nb/sub 3/Sn coils. Both magnets are operated at 4.2 K. The coils are designed to create a volume with a nearly constant magnetic force density distribution. The slurry flows through this volume at room temperature. The distance between the magnet and the working volume is 15 mm, which sets stringent constraints to the cryostat design. A thermal network model combined with numerical optimization is exploited in order to compute the heat load to the cryogenic environment. The computational model is used to determine the sensitivity of the total heat load with respect to the design parameters of the cryostat. Finally, the quench behavior and protection scheme of NbTi and Nb/sub 3/Sn magnets is discussed.

AC losses in non-homogeneous HTS conductors

Abstract The AC transport current losses in non-homogeneous and homogeneous HTS tapes have been calculated and compared. Both transverse and longitudinal inhomogeneities have been studied. The computation is based on Maxwells equations solved with finite element method. The results show considerable variation in the level and shape of loss power vs. current-curves between tapes having similar DC voltage–current characteristics.

The Ic(B) behavior of MgB2 composite with ferromagnetic sheath

Abstract Single core MgB 2 composites have been made by the powder-in-tube method using commercial MgB 2 powder (Alfa Aesar) and Fe or Cu sheath. Transport currents of as-deformed tapes were measured at 4.2 K and external magnetic fields B =0–6 T. A different I c ( B ) curves were measured for Fe and Cu sheathed tapes. This behavior is attributed to the Fe sheath magnetization by self-field and external field. Field distribution in the conductor cross section was analysed with the finite element method. The results were in agreement with the measurements. The presence of ferromagnetic layer in MgB 2 composite influence also the pinning curves and Kramer’s plots derived from measured data. This phenomenon has to be taken into account when the basic superconducting characteristics of the composite (wires/tapes) with magnetic sheaths are described and discussed.

Open gradient magnetic separation utilizing NbTi, Nb3Sn and Bi-2223 materials

Superconducting magnets enable the magnetic separation of particles with small magnetic susceptibility. In this paper, we compare superconducting separator magnets made of Nb3Sn, NbTi and Bi-2223 materials. The separator system is used to determine the optimal conditions for separation of various slurries. The magnet should provide a high and nearly constant magnetic force density. These requirements are met with racetrack coils. Geometries consisting of one or two racetracks have been examined. In order to keep the material costs at a reasonable level, the volume of the magnet has been minimized taking into account the constraints set by the force and current densities. Sequential quadratic programming (SQP) was used in the optimization procedure. The force density has been calculated using an analytical two-dimensional model. The critical current density of the coil was obtained by solving the magnetic flux density from a three-dimensional model using the finite element method. We have compared magnetic force densities and wire lengths in magnets made of different materials. For magnets made of low-temperature superconductors, the optimized geometry consisted of two coils. For magnets made of high-temperature superconductors, the minimum volume was achieved by using only one coil.