Recent developments of multichannel SQUID magnetometer systems enabled us to perform magnetoencephalography (MEG) as a means of non-invasive functional brain mapping in patients. In this review, clinical applications of somatosensory, auditory and visual evoked magnetic fields are described. For the source estimation, current dipole models were used in a sphere approximated from the head shape of each subject as determined by MRI. Localization errors of sources in the multimodal evoked responses were evaluated as low as 2 to 3mm when related to cerebral structures. Further developments of clinical MEG systems are discussed.
The thermal conductivity of STYCAST reinforced Bi(2223) superconducting materials has been studied between 10 and 150K using a steady-state heat flow method. The value of heat leakage per pair of bulk leads from 4.2K to 77.3K for the composite materials has been estimated from the thermal conductivity data. The measured thermal conductivity of the composite materials below 100K is in rough agreement with the value obtained from the mixtures rule of Bi(2223) and STYCAST. It is found that the heat intrusion of the reinforced materials is one-fourth as small as that of 800 A-class conventional gas-cooled current leads. The thermal conductivity of Bi(2223) superconducting material has been studied from 30K to 130K in constant magnetic fields up to 13T. The thermal conductivity value of the Bi(2223) material is strongly suppressed as the applied field is increased along the c-axis of the sample. The contribution of the magnetic field to the value of heat leakage for the current leads of the high-Tc materials is negligible even in high fields. Therefore, STYCAST reinforced Bi(2223) superconducting materials are suitable for current leads to design the high-field and/or large-scale superconducting magnet with a cryocooler-cooled type using no liquid helium.
High Tc melt-processed YBaCuO bulk superconductors have been utilized for the development of high Tc superconducting magnetic flywheel and bearing. In such systems, the superconductors suffer an A.C. magnetic field with frequencies of from tens Hz through several kHz during the rotation of a rotor where permanent magnet rings are installed. The A.C. magnetic field is caused by the inhomogeneity of the magnetic field generated by the magnet in the azimuthal direction. Here, the decay of a rotational speed induced by the A.C. magnetic field, termed rotational loss, becomes one of the most serious technical problems. We analyzed the A.C. magnetic properties of high Tc superconductors in the above frequency range using a fundamental experiment and numerical simulation based on the flux flow·creep model and the vortex-liquid model. Finally, we elucidate that the motion of the fluxoids and the resultant energy dissipation is dominant in the rotational loss.
Insulators for the superconducting magnets of fusion reactor are classified as electrical and thermal insulators for which tough organic materials will be used. When the magnet is exposed by fast neutrons and gamma-rays from plasma in a fusion reactor, the fusion reactor systems will cause fatal damage by the degradation of insulators. Therefore, it is necessary to select materials resistant irradiation damage for use as insulators. Electrical and mechanical tests were carried out at 4.2K without warmup after the reactor irradiation at 5K. The effects of reactor irradiation at the dose of 107 Gy on epoxy resins (bisphenol-A), G-10 CR, VL-E 200 and G-11 CR caused large decreases in mechanical strength. Polyetheretherketone (PEEK), polyimide and phenol novolac resins, which were used to laminate reinforced plastics with glass-cloth against irradiation, showed good resistance. Effects of cryogenic reactor irradiation on several organic materials and epoxy laminate-reinforced plastics with glass-cloth and kevlar-cloth were also discussed.