Paul Shadforth Thompson

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

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.

A dual refrigerator assembly for cryogen-free superconducting magnet applications

Reliable refrigeration is critical to the viability of cryogen-free superconducting magnets. The servicing of a refrigerator coldhead must not interrupt the magnet operation. A dual refrigerator coldhead assembly is described in this paper which enables the disconnection of a coldhead for servicing while a second coldhead provides continuous cooling. The two-stage Balzers UCH-130 coldheads are thermally connected to the cryostat thermal shield at the first stage and to the superconducting magnet at the second stage. The assembly includes high-T/sub c/ superconducting current leads heat stationed between the first and second stages of the coldheads in order to minimize the heat leak to the magnet. Vibration isolation components minimize the transmission of the coldhead vibration to the magnet and the thermal shield. This paper describes the design, fabrication, and testing of the dual refrigerator assembly.<<ETX>>

A Cryogen-Free 0.5 TESLA MRI Magnet for Head Imaging

A compact, cryogen-free 0.5 T superconducting Magnetic Resonance Imager (MRI) magnet for heads and limbs has been developed using conductively-cooled NbTi coils in conjunction with a 4.2 K Gifford-McMahon (GM) refrigerator. The bell-shaped magnet configuration fits over the shoulders of the patient and provides a 20 cm spherical volume to image the head. The magnet is also suitable to image human limbs.

A cryogen-free superconducting magnet for Maglev applications: design and test results

A novel cryogen-free superconducting magnet for use in levitating and propelling Maglev vehicles has been developed and tested. The magnet is conduction-cooled by a two-stage CTI Cryodyne-1020 Gifford-McMahon cryogenic refrigerator. YBCO high-Tc superconducting current leads are incorporated in order to minimize the heat leak to the magnet. A low eddy-current cryostat has been fabricated using fiberglass-reinforced epoxy structures with subdivided stainless steel permeation barriers. The magnet has been cooled to approximately 10 K and ramped to the design current of 100 amps DC. The magnet has also been operated with a 20 Hz, +/-15 amp sinusoidal excitation. Test results for the thermal and magnetic performance of the magnet are presented.<<ETX>>

The Cryogenics of an MRI Demonstrator Based on HTS Technology With Minimum Coolant Inventory Technology

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 Large Bore Long Pulse Magnet at GE Global Research Center

A 3.5 Tesla pulsed magnet with a 1.35 meter bore wound with copper strips has been developed, successfully tested and used at GE Global Research. The magnet has energy of 14 MJ at the peak field. The magnet is composed of 4 pancake coils in series pre-cooled with liquid Nitrogen and powered by a 750 V, 8000 A power supply. This paper describes the design of the magnet, power supply, control circuit and the test results of the system

A two-stage pulse tube cryo-cooled MRI magnet

A compact, cryogen-free 0.5 Tesla superconducting Magnetic Resonance Imager (MRI) magnet has been conductively-cooled using a two-stage Pulse Tube cryo-refrigerator. With the absence of cold moving parts and seals, Pulse Tube cryo-refrigerators offer lower transmitted vibrations to a MRI magnet and potentially longer MTBF rates. A 7 kW input power compressor package was connected to the Pulse Tube. The Pulse Tube system provided an estimated 1.3 Watts of cooling power at 4.2 K. Vibration measurements made on the vacuum enclosure could not detect vibration or noise due to cryo-refrigerator operation even in the presence of very low levels of background noise. The cryo-refrigerator has been in continuous operation for >4000 hours without degradation of thermal performance.

Thermal Conductivity of 34-700 Carbon Fiber Composites at Cryogenic Temperatures

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.

A Gaseous-Helium Cooling System for a High-Tc Superconducting Coil

A gaseous-helium cooling system for a high-Tc coil operating at 20K is described. The system uses a two-stage Gifford-McMahon (GM) coldhead and counterflow heat exchangers to cool a pressurized flow of helium gas from room temperature down to approximately 19K. The cold helium gas circulates through a vacuum-insulated transfer line to a single-pass heat exchanger in contact with an epoxy-impregnated high-Tc coil The coil was wound using 2000 meters of Bi-2223 tape produced by Intermagnetics General Corporation (IGC). A water-cooled rotary compressor supplies pressurized helium gas to both the GM coldhead and the helium gas cooling circuit, in parallel. Descriptions of the coldbox configuration, the flow control and temperature monitoring systems, the high-Tc coil, and the test dewar are given. Test results, including the cooldown and energization of the Bi-2223 coil, are included.