G. Masullo

Stability Measurements in Multifilamentary

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.

{\rm MgB}_{2}

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.

Tapes

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

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.

Three-dimensional quench propagation in HTS coils

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

Normal Zone Propagation in 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 numerical simulation of complex, nonlinear and coupled, electromagnetic and thermal dynamics involved in the quench phenomena of magnesium diboride (MgB2) superconducting magnets requires a suitable modeling of the interaction among the different aspects. As a matter of fact, magnets wound using MgB2 tapes are characterized by material parameters and working temperatures leading to different behaviors with respect to either low T c superconductors (LTS) and high T c (HTS) superconductors, and thus requiring special care in the modeling of quench diffusion phenomena. In this paper, a 3-D finite-element method (FEM) model able to cope with such phenomena is presented and model predictions are compared to experimental data.

Conduction Cooled Test Magnet

In order to investigate the possibility of using superconducting magnets for fast field cycling (FFC) NMR relaxometry, a NbTi magnet able to achieve high field variation rates in a constant background field has been designed and tested. The design layout consists in an outer superconducting magnet working in persistent mode and an inner superconducting magnet working in pulsed mode. The layout of the inner magnet has been optimized taking into account field strength, field homogeneity, stray field and inductive coupling with outer magnet. A demonstration pulsed magnet has been tested in a background field, achieving the main desired working features.

Performance Evaluation for a HTS Transformer

The design process of superconductor coils must include the optimization of the quench protection system. A reliable model for quench analysis of superconductor coils would consequently be highly beneficial. A numerical tool able to cope with the simulation of the quench phenomena, involving electromagnetic, thermal and circuital dynamics coupled each to the others, has been recently proposed. In this paper the mathematical model is briefly revised and extended to take into account the presence of metallic support structures, needed for the coil assembly and cryogenic purposes

New Perspectives in HTS Transformer Design

A high homogeneity magnet, up to 14 Tesla at 4.2 K with magnet bore of 74 mm was developed. This magnet is the starting point in order to develop a 600 MHz/52 mm magnet system for NMR spectroscopy. The field homogeneity is better than 5 ppm over 1 cm diameter sphere volume without any shim-coils. The magnet can be operated in persistent mode and driven mode. The required magnetic field decay rate is 0.1 ppm/hour. A quench analysis has been studied. Magnetic and structural analysis has been performed. Main data of the magnet system, phases of technological development and test results are reported in this paper.