“Safety” in this paper indicates not only injury inflicted by magnetic attractive force to ferromagnetic substances but also to chronic health problems through prolonged exposure to ELF (extremely low frequency)-electromagnetic fields. Safety issues of electromagnetic fields nowadays include those of super strong static magnetic field (MF), pulse MF and ELF time-varying MF. A strong magnetic field is used widely for MRI (magnetic resonance imaging) systems, for medical diagnosis, thus exposing the general public to such strong MF. The author describes the limit on application of strong MF for resolution improvement of MRI. Pulse MF is also applied in medical diagnosis. As stimulation of muscles and nerves depends on the level of dB/dt and on the duration of density alteration, direct stimulation of cardiac muscle by high dB/dt fields is one of the major concerns regarding the safe use of pulse MF in the medical and industrial fields. Regarding the safety of ELF time-varying MF many papers were published that reported a high incidence of leukemia or certain types of cancer in people living under high-power transmission lines and in people working in industrial fields in which ELF electromagnets are used. Typical epidemiological studies are practiced in Nordic countries where they use a computer registration system to analyze the demographics of the population. In particular, we know of the Swedish practice of strictly regulating the level of MF to 0.2-0.5μT to protect the residents under power lines. Personnel related to power facilities all over the world would pay attention to the movement of regulation of the fields strength under power lines.
A Bi-2212 superconducting magnet with a practical size room temperature (R. T.) bore of 45mm in diameter been fabricated. The magnet has been cooled by a GM-cryocooler which generated a magnetic field up to 0.68T at 20K. Oxide superconducting magnet with such a large R. T. bore requires high homogeneity in critical current density of silver-sheathed Bi-2212 tape in a long range such as 1500m. This problem has been solved by optimizing the time for calcination of Bi-2212 powder and employing a new heat-process.
To establish guidelines for the molecular design of epoxy for cryogenic use, mechanical and thermal properties of epoxies were measured at cryogenic temperatures. The fracture toughness at cryogenic temperatures was thought to be one of the most important factors as the selection standard of epoxies. Resins with larger epoxide equivalents and that with flexible molecular structures exhibited higher fracture toughness at cryogenic temperatures. It is thought that the fracture toughness of epoxies is dependent on the intermolecular force and the stress relaxation at the crack tip. This can be used as a guideline for molecular design of epoxies with high fracture toughness at cryogenic temperatures.
FOM, which is an index of the efficiency of refrigerator, is closely related to entropy production in the refrigerator. Use of entropy amplification coefficient (EAC) is proposed as an index of inefficiency of refrigerators. The entropy production of a Stirling refrigerator is divided into those in the heat exchangers and that in the regenerator. The entropy production in the heat exchanger is due to dissipation of work flow. and that in the regenerator is due to enthalpy flow Hr in addition to dissipation of work flow. Thermoacoustic discussion makes clear the mechanism of entropy production. The dissipation of work flow in the heat exchanger consists of the pressure induced dissipation Wp and the viscous dissipation Wv. The dissipation of work flow in the regenerator includes the standing component of the energy conversion Wstand in addition to Wp and Wv. The enthalpy flow Hr in a good regenerator predominantly consists of the standing component of heat flux Qstand, the displacement induced heat flux QD and simple heat conduction Qκ.
The tensile tests of experimental SUS 304 series stainless steels which have nitrogen contents of 0.003, 0.132 and 0.230 were carried out at liquid helium temperature after fast neutron irradiation at 5K, without any intermediate warm-up. The 0.2% yield strength at liqiud helium temperature after the irradiation increased for the no-additional-nitrogen stainless steel and, however, the yield strength became independent of the irradiation with nitrogen content. The tensile strength and martensite volume after the irradiation were not affected by the nitrogen content. A high manganese steel (25Mn-Cr) was tested at liquid helium temperature after the irradiation at 5K, warm-up to liquid nitrogen temperature. The tensile strength and the 0.2% yield strength have a tendency to increase with irradiation. The effects of irradiation on tensile properties are discussed.
The mechanical properties and the axial tensile strain/transverse compressive stress dependence of the critical current, Ic, in Ag sheathed Bi (2212) 7-core superconducting wires with different construction were evaluated at a temperature of 4.2K and a magnetic field of 14T. Increase of the superconductor volume fraction shifted the stress-strain curves upward. The tensile strain for Ic degradation onset was in the range of 0.14-0.27%. It depended on the residual compressive strain and the thickness of the superconductor layer. On the other hand, the transverse compressive stress for Ic degradation onset was in the range of 9-20MPa. The wires with thin Ag sheath layer showed a remarkably large degradation in the onset stress. There was no correlation between the onset stress for degradation of Ic caused by the tensile stress and that caused by the transverse compressive stress.