Kathleen Melanie Amm

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.

{\rm MgB}_{2}

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

Long lengths of metal/MgB2 composite conductors with high critical current density (Jc), fabricated by the power-in-tube (PIT) process, have recently become commercially available. Owing to its electromagnetic performance in the 20 K - 30 K range and relatively low cost, MgB2 may be attractive for a variety of applications. One of the key issues for magnet design is stability and quench protection, so the behavior of MgB2 wires and magnets must be understood before large systems can emerge. In this work, the stability and quench behavior of several conduction-cooled MgB2 wires are studied. Measurements of the minimum quench energy and normal zone propagation velocity are performed on short samples in a background magnetic field up to 3 T and on coils in self-field and the results are explained in terms of variations in the conductor architecture, electrical transport behavior, operating conditions (transport current and background magnetic field) and experimental setup (short sample vs small coil). Furthermore, one coil is quenched repeatedly with increasing hot-spot temperature until Jc is decreased. It is found that degradation during quenching correlates directly with temperature and not with peak voltage; a safe operating temperature limit of 260 K at the surface is identified.

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.

\hbox{MgB}_{2}

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

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.

Magnetic field dependent stability and quench behavior and degradation limits in conduction-cooled MgB2 wires and coils.

Open MRI magnets are generally designed with ferromagnetic poles to contain and shape the magnetic flux and to reduce conductor cost. Permanent magnet MR magnets have blocks of PM and bulk ferromagnetic materials on or close to the pole face. These electrically conducting regions are sources of eddy currents that affect the image quality because of their relatively long time constants and close proximity to the imaging volume. The impact on image quality can be minimized by appropriate segmentation and/or lamination of these components. Detailed eddy current diffusion models are necessary to quantify the field distortion and time constants of the resulting field to perform design studies. The three dimensional frequency or time domain models required to accurately predict effects of eddy currents due to gradient fields are not computationally economical. This paper describes modeling of a PM imaging system using simplified 2D models with appropriate assumptions to evaluate the impact of these eddy currents. Experimental validation of some of the results with a prototype magnet is provided

Development of a Compact 3 T

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.

\hbox{MgB}_{2}

A steady-state test rig for measuring the thermal conductivity of thin carbon fiber composites at cryogenic temperatures has been constructed and calibrated using a 304 stainless steel sample. Thermal conductivity measurements were carried out on both unidirectional and multi-directional 34-700 carbon fiber-NCT 301 epoxy composite samples from 8 K up to 150 K using the test rig. Thermal diffusivity measurements were made at 295 K and were then used to calculate the thermal conductivity. Unidirectional samples with fibers oriented at 0, 90, and 45 degrees to the direction of heat flow have been evaluated. The thermal conductivity of a sample with laminate plys in the configuration ±45/90/0(3)/90/±45 has also been measured. At room temperature, the thermal conductivity in the 0° direction is 7 times that in the 90° direction. For the multidirectional sample, the thermal conductivity ranges from 0.22 W/(mK) at 5.7 K to 2.98 W/(mK). The general trend of the data is similar to that of the T-300 carbon fiber samples.