Wolfgang Stautner

Test Coil for the Development of a Compact 3 T

An test coil was manufactured and tested as the first step in the development of a 3 T MgB2 magnet system. Due to the fact that MgB2 has a higher critical temperature, replacing conventional NbTi superconductor with MgB2, higher temperature operation will be possible. It will make the cryogenic design much simpler and less expensive. Furthermore, operating the magnet at higher temperature results in larger heat capacity of the materials and surrounding structures. Higher heat capacity, therefore, results in increased thermal stability of the magnet against quench initiation. The 3 T magnet design consists of several coils. One of the center coils was manufactured for testing the performance at higher temperatures. The test coil was conduction cooled and the quench performance of the coil was good, which means there were no critical issues during the coil manufacturing process. However, AC loss heating, as well as a small resistance of the coil was found, both of which might result from wire design, manufacture, and quality.

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The authors have reported the results of low n -value from a MgB2 test coil developed a year ago. A second test coil has been developed with wire of different structure and manufacturing process. Although the n-value related voltage of the second test coil was lower than the first test coil at designed current, it still showed low n-value. A third test coil has been wound with reduced mechanical stress. It also showed very similar n-value related voltage and n-value. Investigation of voltage distribution over the coil indicated that magnetic field was the major factor causing degradation of the n-value and resulting in n -value related voltages. Since the n-value related coil voltages were on the order of 0.1 μV/cm, the usual short sample Ic test (1 μV/cm was the definition of Ic ) might not detect the n-value related voltage and might not be able to investigate the cause of low n -value. Therefore, the medium length ( ~ 10 m) samples were tested and they showed the wires lengthwise nonuniformity both on n-value and Ic, which might be another potential cause of the low n-value of the coil. Along with the electrical investigation, the manufacturing process of the wire was carefully inspected for longitudinal uniformity. Some wire segment samples from the same batch exhibited nonuniformity in the particle size distribution resulting in nonuniform filaments. This might have occurred in the wire for the second and third test coils.

Magnet

Typical coils with BSCCO tape are wound in a pancake or double-pancake style to minimize the strain in the tape by reducing or eliminating the edge-wise bending. Stainless steel reinforced tapes are frequently used in the winding process to increase the strength and reduce the strain due to winding tension and handling. However, an MRI magnet requires high current density in the winding pack. This high current density in the winding pack gives a higher field in the imaging volume and also allows for a reduction in the overall magnet size. Layer winding was preferred for a better tolerance control and for a reduction in the number of joints, which are known sources of resistance and therefore locations of instability in the coil. A mock-up coil was wound using a high-current-density type of BSCCO tape without the typical stainless steel reinforcement. The coil was layer-wound which involved a few inline lap joints embedded in the winding pack. The test of the coil reveals a few issues that need to be addressed. Investigations and analysis lead to a better understanding of the issues. This paper discusses the lessons learned and solutions for using non-reinforced tape in a layer-wound coil, while controlling insulation dimensions within the build.

Second Test Coil for the Development of a Compact 3 T

The authors have reported results of MgB2 test coils that exhibited anomalously low n-value. It was discovered that the major cause of the n-value related voltage was nonuniformity of the wire along its length. Based on this finding, the development of a compact 3 T magnet has been started. The magnet consists of six coils of 30 cm bore each. The fields will be 3 T at 4 K and 1.5 T at 20 K, respectively. The coils will be cooled by thermal conduction. One of the center coils was manufactured with the refined wire of improved lengthwise uniformity. Results of tests on this coil showed no measurable n-value related voltage. Superconducting joint development has been ongoing. Current peak multifilament joints show superconductivity up to 120 A at 14 K. Further trials to achieve the full short sample operating current at each temperature are ongoing. The cryogenic design for the magnet is based on the use of dual coolants either with hydrogen or helium and consists of two distinct and separate primary and secondary cooling circuits.

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We present the engineering and design aspects of a cryo-free 10 T high-field magnet system for a completely new type of superconducting application. Design criteria and specifications for the racetrack shaped conduction-cooled magnet and the cryostat are discussed and disclosed.

Magnet

The team at GE Global Research presents cryo assembly and component test results of a high-temperature superconducting (HTS) limb size magnetic resonance imaging (MRI) scanner using Sumitomos DI-BSCCO tape conductor, under an NIH research grant. The goal is to investigate the thermosiphon behavior for different MRI operating modes, validating the cryogenic robustness of this cooling approach and its performance limits. The magnet is indirectly cooled using cooling tubes with liquid neon filling and a single-stage cryocooler for reliquefying.

Experimental Layer-Wound Mock-Up Coil for HTS MRI Magnet Using BSCCO Tape

We introduce an advanced and optimized cryogenic cooling concept featuring minimum coolant inventory requirements for small high temperature superconducting (HTS) magnets based on results obtained with an experimental model. Experience gained from these experiments led to a new design that will be experimentally verified by the end of this year. Winding of the HTS magnet has already begun and will be completed shortly. New components, current status and cryogenic scope of this new engineering model are described.

Development of a Compact 3 T

A limb size MRI magnet coldmass has been constructed using DI-BSCCO tapes from Sumitomo. The coils were wound with epoxy pre-impregnated fiberglass cloth (pre-preg) between layers to bond the wires. For radial dimensional control, temperature was elevated during the winding to thin out the epoxy and to adjust the pre-preg cloth layer thickness in order to control the coil build up. The wire tension was controlled within 1 kg with a set of moving pulleys. While the coils appeared solid after winding and curing, issues were found in the leads between coils. When the coldmass was cooled down to liquid nitrogen temperature, breaks in wire leads were found. Thermal expansion and contraction mismatch between the coil bobbin and the BSCCO tape was attributed to the leads break. The thermal stress was induced both in the oven curing and the cooling processes. Preliminary testing results at temperature are discussed. The magnet was designed to have a center field of 1.5 T operating at a liquid neon (LNe) temperature of 27 K.

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To build and evaluate a small‐footprint, lightweight, high‐performance 3T MRI scanner for advanced brain imaging with image quality that is equal to or better than conventional whole‐body clinical 3T MRI scanners, while achieving substantial reductions in installation costs.

Magnet

The experimental results of a cryo-free high-field magnet system for a novel magnetizer are presented. The magnet for the application is a superconducting racetrack coil of 0.7 m length using low AC-loss wire from Luvata. The coil selection, reaction, epoxy selection, winding trials and vacuum impregnation are reported in a previous paper. Here the experimental testing of the prototype is presented. The magnet was cooled down with a 4 K GM cryocooler to its operating temperature and ramped. The behavior during cooldown, ramping and training are presented, demonstrating the viable operation of this low-cost engineering prototype.