A high field superconducting magnet, 4-5T in central magnetic filed, is required for magnetic resonance spectroscopic imaging (MRSI) on 31P, essential nuclei for energy metabolism of human body. This paper reviews superconducting magnets for high field MRSI systems. Examples of the cross-sectional image and the spectrum of living animals are shown in the paper.
A brief review is presented of the recent space cryogenic applications. There are many potential space missions requiring cryogenic cooling, some of which are in progress and others are still under consideration. Techniques for cryogenic cooling in space are discussed with emphasis on the temperature range and the refrigeration load. A He II cooled onboard infrared telescope which is indeed the highlight of the application of current space cryogenic technology is described by taking the IRTS as an example. Several new techniques required for the utilization of He II in space have been developed prior to the practical design studies of onboard IR telescopes. Some other projects, in addition to IR telescopes, requiring He II cooling are also briefly described. Finally, a prospect of future space cryogenics is discussed for further development of space cryogenics.
The strain effect on the critical current density, Jc, of NbN tapes has been investigated in 4.2K and 9T. Superconducting films of NbN have been prepared on metal tapes of Hastelloy B and 304 Stainless steel by reactive sputtering in an argon-nitrogen atmosphere. The strain dependences of Jc are influenced by the substrate material and NbN film thickness. The NbN tapes with Hastelloy B substrate maintains constant values of Jc up to strain values of above 1%. However, those with 304 Stainless steel substrate show a gradual decrease in Jc from 0.3 to 0.9% and exhibit a marked decrease in the range of above -0.9%. The value of the irreversible strain limit εirrev. where each NbN tape is permanently damaged, is in good agreement with the ratio between the proportional limit and Young's modulus in an investigated stress-strain curve. The maximum value of εirrev. reaches -1.4% and has been obtained in the tape consisting of the Hastelloy B substrate and a 3, 000Å NbN film. In tapes thickly coated with NbN, which have been measured up to above their εirrev. values, some cracks are found and films of NbN are frequently peeling off their substrates.
The magnetic field dependence of the time constant has been studied of coupling currents among filaments in a multifilamentary superconducting NbTi wire. The time constant was measured with resolution of 0.1T in the background fields from zero T to 10T by a newly developed direct method. The measured time constant values showed a peak at around 1T with a maximum time constant of 43ms. This peak effect could be theoretically explained by the effective transverse resistivity and effective permeability of the composite wire with the magnetic field dependence. The variance of the permeability of the NbTi filaments in a magnetic field is adequately accounted for by the critical state model.
The magneto-resistance of three newly introduced Japanese Industrial Standard (JIS) platinum resistance thermometers, GR 0705, was studied. They were manufactured by a Japanese company, Hayashi Denko, and found to possess an excellent long term stability by our recent work. Their magneto-resistance was analyzed by the equation D(T)B2/(1+E(T)B) and its field dependence and temperature dependence were found to agree with each other. Following this observation, the universal functions for D(T) and E(T) were derived as a function of temperature between 20K and 240K. The resistance of the thermometer under magnetic field now can be represented by a simple analytic function of temperature and field strength of up to 8T. The error in the correction function was estimated by an independent experimental run. It reaches its maximum of 30mK at 20K and decreases as temperature rises. Above 40K the error is estimated to be as small as 10mK and dominated by an individuality of the sensors. The parameters, D(T) and E(T), are tabulated as a function of temperature.
To improve the cryocooler efficiency, the regenerator performance should be well understand. As one of the evaluation methods to know the regenerator performance, in general, the regenerator efficiency which is defined as the ratio of actual heat transfer rate to the maximum possible heat transfer rate has been used. In the case of the regenerator for the cryocooler, however, the definition of the “maximum possible heat transfer rate” does not clear because of the significant effect of the working gas within the void volume in the regenerator. In this study, the basic equations of the regenerator having the void volume has been introduced. These equations are applicable for the boundary condition of the time dependent mass flow rate and pressure. A set of the numerical solutions of these equations for the ideal gas indicates that the mass flow at the low temperature side of the regenerator is always greater than that of the high temperature side, which leads the uncertainty of the maximum possible heat transfer rate. An alternative expression of the regenerator efficiency has been proposed.
Cracking and debonding of epoxy filler materials generates heat by releasing elastic energy, and the heat output may result in premature quenching of a superconducting magnet because of the low heat capacity of the winding materials of the magnet at low temperatures. This paper considers dissipated energy during the low temperature fracture in a notched glass-epoxy composite material (G-10). The dissipated energy as a function of crack velocity follows from a dynamic strain energy release rate. The dissipated energy is compared with the measured heat output at 77K for partial fractures, and the heat conversion factor, that is the conversion rate of dissipative energy into heat, is obtained. Then, from the average value for the heat conversion factor, the heat outputs at 77K for total fracture and at 4K for partial fracture are predicted and shown graphically.
A prototype superconductive levitational system by Meissner effect is successfully developed. The system is made up of a magnetic guide and a superconductive slider with no additional mechanisms for levitation. The superconductor used is made of YBaCuO with high critical temperature. Experimental results show that an optimal structure for the levitational system is obtained if the distance between the guide and the slider is given. This levitational mechanism is also applicable to the superconductive radial bearing.