In early 1980th magnetic resonance imaging (MRI) became utilized practically as a medical diagnostic instrument. MRI has several advantages over computed tomography (CT) in the evaluation of anatomy and pathology of human body. First, contrast resolution obtained by MRI surpasses that of CT, because, with MRI, several parameters such as T1 and T2 relaxation times, mobile proton density, and blood flow affect signal intensity and manipulation of tissue cotrast is achieved by highlighting differences in these parameters, and differences in density on CT is obtained only by attenuation differences of the X-ray beam. Other advantages of MRI include the ability to depict cross-sectional anatomy in any planes without patient manipulation, the lack of artifacts from bony structures and air, and the absence of ionizing-irradiation. Because of these capabilities, MRI has quickly evolved into the premier imaging tool for central nervous system including brain and spinal cord. In addition, usefulness of MRI has been recognized in the spine, heart and great vessels, pelvis and extremities and it will become one of the important diagnostic imaging methods in various organs and parts.
A brief review was made of the recent development of the high critical temperature oxide superconductors from the pioneering work by Bednorz and Muller to the liquid nitrogen temperature superconductor and the magnetic superconductors. The electronic structures and the superconducting properties were discussed from the experimental point of views. The problems associated with the critical current and the microstructures of the oxides were described.
A method of flow visualization in He II was developed by the use of H2-D2 solid particles and hollow glass spheres as tracers which were prepared to be neutral buoyant in He II. Streaks of tracers which follow the normal fluid flow enable us to see flow structure. It is of great importance in fluid dynamics to grasp a flow field as a pattern as well as to take a quantitative measurement. The thermal counterflow jet was investigated by means of this method. A motor-driven 35-mm camera was used to photograph each flow event. Some pictures are examined to recognize flow patterns and others to measure the flow velocity from the lengths of particle streaks. The jet evolution appeared depending on the Reynolds number quite similarly to usual viscous fluids. The traceability was found to be satisfactory at least up to about 6cm/s for both kinds of tracers. The existence of large scale vortical structure in fully developed jets and the occurrence of wide-spread entrainment into main jet from the surroundings were evidently pointed out in the pictures. The calculation of the total momentum flux in jet indicates that the zero-relative-velocity situation is reached in the fully developed stage of turbulent jet.
In succession of part I of this paper, the stability margin of a tape superconductor is analysed theoretically in order to prove scientific feasibility of high current density, large-scale superconductors. As a typical example, Cu-Nb3Sn-Cu conductor is investigated: The thickness of Cu and Nb3Sn are 1mm and 50μm, respectively and the strip width is 50mm. If the critical current density of limiting stability Jc=1.29×1010A/m2 is assumed, the stability margin of pulsive field applied perpendicular to the broad face of the conductor becomes 4.9T. The thermal margin is shown to be about 5×104J/m3 when disturbances are point-like, transient nature. These results indicate that the tape conductor is stable enough to be wound into superconducting magnets.
Electromagnetic properties of powder-metallurgy processed superconducting Nb3Sn wires are usually similar to those of single core wires due to strong coupling among filaments. In this study, it has been demonstrated that the effective filament diameter of the powder-metallurgy processed wire can be decreased by twisting and the wire can be practically used for magnets. Firstly, Cu-22.5wt% Nb powder-metallurgy processed Nb3Sn wires with a diameter of 0.25mm were fabricated in a semi-industrial scale, and electromagnetic properties were measured. With theoretical consideration about twist effect and magnetic instability based on the experimental results, a magnet with an inner diameter of 34mm, an outer diameter of 147mm and a length of 160mm was designed and wound by a 21-strand cable, of which the effective filament diameter was decreased to 53μm by twisting with the pitch of 11mm. In a d.c. operation, the magnet generated a field of 4.1T at the current of 450A, though this value corresponded to only 60 percent of the critical current of a short sample, which might resulted from the ununiformity of critical current in the wires. The magnet could be operated to the current of this value also in a pulse mode at the maximum sweep rate of 7.8T/sec. Therefore, the wire fabricated in this way was confirmed enough stable.
Experimental study was carried out on superconducting ac magnets with a high terminal voltage immersed in liquid helium. As a preliminary experiment, we measured ac breakdown voltage of liquid helium boiling at 4.2K under normal atmospheric pressure. Based on these data including the effects of the magnetic field or the bubbles boiling from the electrode surface on the breakdown voltage, we designed and made the 50kVA ac magnet wound by a multifilamentary wire with fine Nb-Ti filaments. The magnet has cooling channels in each layer which act as an electrical insulator as well as a conduit of coolant. This test magnet of a high voltage type was successively charged up by means of the resonance circuit with static condensers for various frequencies. The observed ac losses and quench currents of the magnet were in a good agreement with theoretical ones.
Several kinds of mechanical strength of high density GFRP for cryogenic use are studied in order to check the combinations of glass fiber materials and matrix resins. Glass materials such as unidirectional woven cloth, plain woven cloth and roving cloth are selected from the view point of good mechanical characteristics. Resin content are established more less than value of 22 weight percent concerning low thermal contraction and GFRP are constituted in isotropy. It is found that the order of excellence in mechanical strength are unidirectional woven cloth GFRP, plain woven cloth GFRP and roving cloth GFRP at both room and cryogenic temperatures. The delamination strength of roving cloth GFRP is found to be extremely small. In order to decrease thermal contraction, the combinations of glass materials and fillers in GFRP are also studied. It is clear that the addition of fillers is effective in decrease the thermal contraction, but the addition of fillers make a deterioration of mechanical strength such as delamination strength, bending strength and tensile strength of GFRP.
Relationship between absorbed energy and chemical composition for 308 and 316 type deposited metals of austenitic stainless steels at 4K was investigated by a multiple regression analysis of experimental data. The multiple correlation coefficient of regression equation was 0.917 and the equation is described asvE=390-19.9×(%Cr)+6.00×(%Ni)-35.3×(%Nb)-784×(%0)-1890×(%P)+98.8×(%Si)-3.34×(%Mn)-9.16×(%Mo) where vE is absorbed energy at 4K (J) and %Cr, %Ni etc. are mass % (wt%) of each element.
The object of this work is to develop a Stirling cycle refrigerator which can be cooled to 4-5K range without the use of a Joule-Thomson loop. Rare earth compounds were used in the regenerator matrix and metal bellows was used as the compressor and the expanders for the purpose of improving the cryogenic performance. According to the test results obtained up to now, the refrigeration capacity of more than 2W was attained below 5K, starting from 80K. Furthermore, recondensation of GHe in a seal off cryostat a 1.9W heat load was realized starting from room temperature.