Evangelos Trifon Laskaris

Vibration isolation of superconducting magnets

In order to improve image resolution and magnetic field uniformity in a magnetic resonance imaging device, the vibrations and loads produced by the cryocooler are isolated from the superconducting magnet and cryostat. The vibrations and loads are isolated through the use of flexible, laminated copper connectors and rubber mounts.

Development of a High Speed HTS Generator for Airborne Applications

General Electric, under contract with the Air Force Research Labs (AFRL), has successfully developed and tested a high speed, multimegawatt superconducting generator. The generator was built to demonstrate high temperature superconducting (HTS) generator technology for application in a high power density Multimegawatt Electric Power System (MEPS) for the Air Force. The demonstration tested the generator under load conditions up to 1.3 MW at over 10,000 rpm. The new MEPS generator achieved 97% efficiency including cryocooler losses. All test results indicate that the generator has a significant margin over the test points and that its performance is consistent with program specifications. This demonstration is the first successful full-load test of a superconducting generator for the Air Force. In this paper we describe the development of the generator and present some key test results used to validate the design. Extrapolation to a higher power density generator is also discussed.

A cryogen-free open superconducting magnet for interventional MRI applications

A 0.5 T cryogen-free superconducting magnet with an open geometry has been designed for use in MRI-guided minimally invasive surgery and other interventional procedures. The magnet consists of a split pair of coils to allow surgeons direct access to the patient within the field-of-view of the imaging system. The magnet is wound using Nb/sub 3/Sn tape conductor and is maintained at approximately 10 K using a pair of Gifford-McMahon refrigerators. High-Tc superconducting current leads have been used in order to minimize the heat leak to the magnet. A passive quench protection system was developed for this unique magnet geometry. High quality MR images have been produced and used for interventional procedures. Descriptions of the design, fabrication, and test results of the magnet are presented.<<ETX>>

Performance of an HTS Generator Field Coil Under System Fault Conditions

High-temperature superconducting (HTS) coils are generally stable against transient thermal disturbances. Protection against spontaneous quenches is not a main design issue for an HTS coil. However, HTS coils used in many electric devices such as motors, generators, transformers, and current limiters will operate under over-current fault conditions, which may result in a coil quench and thermal runaway. Those electric devices should be able to ride through some grid fault conditions and remain functional. This requires a certain over-current capability of the HTS coils. This paper discusses the over-current requirements from grid faults and the thermal transient responses of a BSCCO coil. It presents the analysis results of the coil subjected to over-current pulses at different operating conditions. The study also investigates the thermal runaway current and its relationship with the over-current pulse

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}

A cryostat for use in magnets for NMR medical diagnostic imaging includes inner and outer vessels suspended by sets of at least three low thermal conductivity ties. The positioning of the tie anchoring points, particularly on the interior vessel, are selected in accordance with the present invention to preclude increased stresses in the supporting ties that occur as a result of thermal contraction. This configuration is particularly advantageous in that it permits the utilization of ties having smaller cross-sectional areas than would otherwise be required. The reduced cross-sectional area requirement is also therefore seen to increase the thermal isolation of the interior cryostat vessel. Cryostat vessels may be nested within one another.

Magnet

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.

Supporting tie configuration for cryostat for cold shipment of NMR magnet

This paper describes a high power density high-temperature superconducting (HTS) electric machine topology that is scalable for marine propulsion and power generation. The design, currently being pursued for airborne applications, is based on homopolar inductor alternator (HIA) technology, which is new within HTS applications. The basic machine design configuration of the HTS HIA is based on a stationary HTS field excitation coil, a solid rotor, and an advanced but conventional stator comprising liquid-cooled air-gap armature winding and an advanced iron core. High power density is obtained by the enhanced magneto-motive force capability of the HTS coil, the increased airgap flux density and armature current loading, and the high tip velocity of the rotor. Preliminary scaled up designs look attractive for three marine applications: propulsion drive, primary ship power generation, and power generation modules. The generators are driven directly by the turbines without the additional complexity of a clutch and gear system. A conceptual design study of a 36-MW 3600-r/min generator, a 4-MW 7000-r/min auxiliary generator, and a 36-MW 120-r/min and 4-MW 132-r/min propulsion motor are summarized.

Second Test Coil for the Development of a Compact 3 T

The development and fabrication of a layer-wound, epoxy-impregnated Bi-2223 high-temperature superconducting (HTS) racetrack coil which generates 40000 ampere-turns of magnetomotive force (MMF) at 25 K is described. The coil was wound using Ag-sheathed Bi-2223 tape conductor laminated with copper foils for strength enhancement and insulated using a paper-wrap method. After epoxy impregnation, the coil was tested over a range of 16-25 K in a vacuum dewar using a closed-cycle helium refrigeration system. Descriptions of the tape lamination and insulation processing, the coil winding and impregnation, and the experimental test setup are given.

\hbox{MgB}_{2}

Grain-aligned YBCO superconductors have been developed for use in a conduction-cooled, cryogen-free superconducting magnet application. A magnetic alignment technique has been used to orient the c-axis of the YBCO grains. Current leads 0.3*1.4*6.4 cm were fabricated and thermally connected between the first and second stage of a Gifford-McMahon cryocooler. The heat conduction down the leads was determined from a load map of the cryocooler. With operating temperatures of 40 K on the first stage and 10 K on the second stage, the measured heat leak was 0.15 W/lead. The YBCO current leads were used to power a conduction-cooled Nb/sub 3/Sn magnet at 10 K to 250 A. Test results for the YBCO leads are reported.<<ETX>>