V. Cavaliere

Robust design of high field magnets through Monte Carlo analysis

An effective approach to the optimal design of electromagnetic devices should take into account the effect of mechanical tolerances on the actual devices performance. A possible approach could be to match a Pareto optimality study with a Monte Carlo analysis by randomly varying the constructive parameters. In this paper it is shown how such an analysis can be used to allow an expert designer to select among different Pareto optimal designs.

A genetic algorithm approach to the design of split coil magnets for MRI

Innovative, open magnet designs for Magnetic Resonance Imaging (MRI) of parts of human body have been recently proposed. These open configurations require notable skill in the design phase to obtain satisfactory performance in terms of compactness and field homogeneity; optimization techniques thus seems adequate to assist the designer of such devices. A novel optimization scheme, based on a genetic algorithm (GA) has been implemented for the resolution of this problem. The GA seems well suited for this applications thanks to its ability in exploring the whole design parameters space, not being trapped by local minima, but some improvements have been implemented, namely floating point parameter representation, introduction of a directional mutation operator and adaptive probability of operators action.

Magnetic Field Trapping in MgB

In order to realize superconductive permanent magnets to be used in power applications like the magnetic levitation or electrical motors, we have studied the magnetic field trapping capability of MgB2 discs of different shapes, at temperatures >;10 K. In particular we have compared MgB2 bulk discs with MgB2 Superconductive Inserts in Metallic Substrates (SIMS) of diameters of 70 mm. Both superconductive devices have been produced by the Reactive Mg Liquid Infiltration (Mg-RLI) process. The magnetization was performed by the application and removal of an external magnetic field up to 2 T, produced by a superconducting magnet, or by Field Cooling of the superconductive devices nearby NdFeB permanent magnets. The SIMS devices showed a higher stability of the trapped fields with respect to the bulk discs. Typical trapped fields, measured at 1 mm from the surface of the device, are of the order of 1 T. The density distribution of the supercurrents has been estimated by measuring the trapped field at various temperatures up to Tc and performing magnetic levitation forces measurements.

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Superconducting magnets for MRI are designed to provide high level of magnetic flux density in a wide testing volume with the greatest level of field homogeneity. The design of such magnets is usually performed using optimization techniques able to tune the geometrical parameters of the magnets, taking also into account constructional issues and dimensions and packaging factors of the wires for each coil and, in addition, technical and physical constraints, such as the critical current of the superconductor to prevent the quench phenomenon. Unfortunately due to manufacturing tolerances, the actual geometrical parameters of the magnet differ from the design ones, affecting the field homogeneity. In this paper the effects of the manufacturing tolerances on the field homogeneity are investigated for an MRI magnet by means of a statistical Monte Carlo analysis.

Bulks and Inserts

A novel method for the design of open configuration magnets for Magnetic Resonance Imaging (MRI) has been recently set up. The method is based on an innovative approach to the genetic optimization algorithms, described in some detail in an accompanying paper. The method has been applied to the design of open magnet configurations for MRI of parts of the human body. The configurations are optimally designed not only by taking into account field strength and homogeneity, but also looking for the most effective current density distribution in terms of superconductor performance, and looking for the most compact assembly. The need for optimal design in terms of superconductor performance is due to the severe working conditions in split coil configurations because of the field map in the windings, which presents values much higher than the central field.

Improvement of MRI magnet design through sensitivity analysis

The behavior of high temperature superconductor (HTS) coils is ruled by thermal and electromagnetic interacting phenomena. In this paper, a three-dimensional modeling of the quench propagation in HTS coils is discussed and numerically modeled. As a matter of fact, in spite of the axially symmetric design, a simple two-dimensional modelization may reveal too rough, particularly from the thermal point of view. In addition, the possibility of representing the complex structure of the coil composite material by means of a simplified homogeneous and anisotropic model is discussed and the advantages in terms of computational burden are evaluated

Design of split coil magnets for magnetic resonance imaging

Magnetic resonance is becoming a routine analysis for many applications. Present day devices include active shielding systems instead of passive ferromagnetic shields. This calls for sophisticated design techniques able to provide satisfactory performance in terms of central field homogeneity, reduced stray field and minimal superconductor’s volume. In the paper some of the possible techniques to deal with such a problem are presented and discussed.

Three-dimensional quench propagation in HTS coils

DC magnetic fields have been applied to a superconducting hollow MgB2 cylinder of relatively large dimensions (dext/int = 48/44 mm, h = 31 mm), produced by reactive Mg liquid infiltration (RLI) process. Such a kind of full dense cylinder has no discontinuity in its shape, at difference with other HTS textured cylindrical objects, and it represents a prototype of a larger one that can be applied in many electromechanical applications, like bearing, or pure magnetic application, like shielding. The aim of the experiments is to verify its shielding ability at low fields and the maximum trapped fields, when higher field are applied. The measurements are performed at variable temperatures from 13 K up to Tc . The magnetic field values, measured by Hall probe located at the center of the cylinder, dependent on the various cooling regimes (either zero field cooling (ZFC) and field cooling (FC)) and on the presence of a background external magnetic field. The trapped fields dependence from the temperature and background field is reproduced by a simple model of the current distribution and with an analytical expression of Jc (B,T). It was possible also to describe the transition of the superconducting cylinder to the normal state, induced by flux jumping, on a base of a phenomenological model with two process having different time scale.

Optimised design of actively shielded NMR magnets

The bulk superconductors can be used as permanent magnets of very high magnetic induction, provided that a feasible way of magnetization is identified and that the superconducting material can withstand the magnetic forces induced by high trapped fields. Here it has been considered as superconducting permanent magnet a bulk MgB2 ring, obtained by the Reactive Liquid Infiltration (RLI) process. This material offers several advantages with respect to the well known YBCO textured material, even if its application temperature will be far lower, around 20-30 K. First of all, MgB2 does not require grain orientation to reach enough current density, it can be produced in a quite straight forward way, it is lightweight and it is mechanically robust. To magnetize the MgB2 ring, we have applied a DC magnetic field with a superconducting magnet, using a step wise, field cooling type, procedure. The induced magnetization of the bulk MgB2 ring, at 4 K, is around 2 T in its vicinity. It has been verified that the magnetization is very stable during the time and it can withstand also AC disturbances of several Hz in frequency.

The Magnetic Characterization of a

In order to investigate the quench behavior of conduction cooled MgB2 magnets, a 100 mm inner bore diameter, 0.84 T at 20 K magnet has been built and tested. The test magnet consists of 4 double pancakes wound with the MgB2/Ni -Cu-Fe tape produced by Columbus. Each coil is cooled by means of a 2 mm thick copper disk placed during winding at the middle of the double pancake; after winding the double pancakes have been separately epoxy impregnated. Several experiments have been performed inducing a local transition in the coil by means of a controlled heater placed on the double pancake surface. The propagation of the normal zone has been monitored by means of 8 voltage probes positioned along the tape during coil manufacturing. Each voltage probe detects the voltage drop across a tape length of 30 mm. The experiments have been performed at different temperatures while keeping the magnet current constant during quench. In the paper we report an analysis of the quench propagation velocity measurements and a numerical investigation of the thermal and electrical behavior of the magnet.