A. Matrone

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.

_{2}

We have analyzed the behavior of a 10 cm long straight piece of copper stabilized multifilamentary tape, manufactured by Columbus Superconductors, in a nearly adiabatic condition. The tape was biased at different current values as a function of either the temperature or the induced heat disturbances at fixed temperatures. From the analyses of the voltage increase, recorded from voltage taps placed at known positions, we estimated the highest temperatures which give rise to a quench of the tape at constant values of the current bias. We also measured the minimum energy triggering the quench at fixed values of current bias and temperature. The voltage taps have also been used for homogeneity check of the physical properties which rule the thermal runaway in the tape. The voltage-versus-time curves in different sections of the tape also provide information about the quench propagation speed as a function of the hot spot energy and the temperature. Due to the composite nature of the tape, made of nickel, iron, copper, and , to perform simple FEA computation we modeled the tape with a one dimensional single wire with averaged value of the physical quantities involved in the Joule heat generation and thermal conductivity.

Bulks and Inserts

Within the Italian project for the development of 1 MJ/1 MW SMES, Ansaldo-CRIS and CESI have been involved in the design and manufacturing of 1100 A resistive/HTS current leads conduction cooled by a closed cycle refrigerator. Main design and technological problems concern the achievement of electrical insulation between cryocooler and current leads at the voltage level of 6 kV together with a thermal contact good enough to keep the upper part of the HTS material at a temperature below 70 K, when the cryocooler temperature is 60 K and about 100 W is the thermal load. Thermal transport properties of electrical insulating materials as thin sheet placed between copper massive plates have been measured as a function of temperature and hence, according to the measured thermal performances, the area for thermal contact with the cryocooler has been suitably dimensioned. Mechanical structure for the whole current leads has been designed and manufactured. Final assembly of resistive section has been completed. Results about the achieved lead performances are presented and discussed.

Stability Measurements in Multifilamentary

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.

{\rm MgB}_{2}

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.

Tapes

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.

Cryocooler cooled HTS current leads for a 1 MJ/1 MW-Class SMES system

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.

Design of split coil magnets for magnetic resonance imaging

The adoption of High Temperature Superconductors (HTS) tapes is becoming a suitable and interesting alternative to copper for windings in transformers, thanks to the improvement in performance and the lowering of costs. Of course, optimized designs, different with respect to usual layouts, must be considered, due to the additional requirements of HTS tapes, such as minimization of orthogonal magnetic fields and consideration of additional losses in the HTS. In the framework of a scientific cooperation among some Italian Universities and private companies, a test model for a 10 KVA transformer with HTS secondary windings has been developed, and validated against a demonstrative prototype, manufactured during the project. In the paper, the device model performance is assessed, with particular care to the HTS losses modeling, and some comparisons to the experimental results are presented

The Magnetic Characterization of a

Power transformers based on High Temperature Superconductors (HTS) technology are an appealing promise for several practical applications. The present designs still leave wide margins of possible improvement in terms of both layout optimisation and introduction of new technologies. In the framework of a technical-scientific cooperation among scientific and industrial subjects, a 10 kVA single-phase transformer was designed and manufactured, using copper for primary windings and BSCCO-2223 HTS tape for secondary windings. The layout has been optimized taking into account the particular characteristics of BSCCO tapes, in particular their AC losses, and the usual figures (stray flux, Joule and iron losses, weight and overall footprint) considered in transformers design. The prototype has then been realized and characterized, using general as well as specific tests. The performance of the device has been evaluated and compared with numerical calculation. In the paper, an overview of the device design and manufacturing will be presented, together with a critical comparison between computed and measured performance.

{\rm MgB}_{2}

The hybrid permanent magnet undulator for the LISA-FEL (free electron laser) experiment (INFN, Frascati, Italy) has been designed and manufactured. A computerized driving system has been developed in order to lighten the mechanical structure and gain accuracy in positioning of the jaws during gap variation. The NdFeB permanent magnet blocks have been measured and sorted in order to reduce magnetic field errors along the undulator axis. The field integrals are minimized by the electronic control, which feeds the correction coils with a gap-dependent current. The first results of mechanical tests and magnetic field measurements confirm the good performance and reliability of the device.<<ETX>>