History and theoretical design equations for the ferromagnetic shielding which has been generally used for biomagnetic measurement are described. Methods of demagnetization are reviewed, which are needed for the improvement of shielding performance. The main features of the large magnetically shielded room for operation with SQUID systems in medical research and diagnosis in the near future are also described. Finally, high performance shielded room which is under construction at SSL is presented.
Immersion cooling of a coated metallic object is discussed. Paradoxically, a coated object cools quicker than when the same object is bare. Through this phenomena the reader is introduced to various related topics. The main subject in the first issue is about the stability of the interface between liquid and vapor around the object and how this instability causes an early transition from film boiling to nucleate boiling state. In the second issue, the reader is introduced in more detail to film boiling by taking up the issue of film boiling on a long flat heater. A model for film boiling on such a heater is cited and effects of heater orientation and temperature on film boiling are qualitatively described based on this model. Finally, heat transfer in subcooled forced-flow helium is compared with that of supercritical helium, and ranges of heat flux are discussed for effective use of these two types of helium heat transfer.
A 20T large bore superconducting magnet has been developed at the National Research Institute for Metals. In this paper, design for its coils is mainly discussed. Design policy for this magnet system is to meet two contradicting requirements; large bore diameters and generation of high magnetic fields. The magnet consists of four coils, and should be operated at 1.8K in saturated superfluid helium to generate its highest field. Magnetic fields of 15T in a 314mmφ thermally insulated chamber, of 18T in a 160mmφ bore and of over 20T in a 44mmφ bore are designed to be generated with combinations of these four coils.
A 20T large bore superconducting magnet has been developed at the National Research Institute for Metals. In this paper, design for its cooling system is mainly discussed. As a coolant for this magnet, saturated superfluid helium has been applied. Two separated chambers where two coils are placed, respectively, are evacuated with rotary pumps. A liquid helium tank is equipped with each chamber over the coils. This tank functions as a reservoir of liquid helium and source of cooling gas for power leads. Two outer coils are designed to be cryogenically stabilized in saturated superfluid helium.
A 20T large bore superconducting magnet has been developed at the National Research Institute for Metals. In this paper, operation results of this magnet are reported. Cooling down to 1.8K was successfully carried out. Magnetic fields of 15T in a 314mmφ chamber, of 18.06T in a 160mmφ bore and of 20.33T in a 44mmφ bore could be generated. Even after the quenches of two inner coils, they could be re-energized immediately. This fact indicates that this magnet provides an ideal environment for high-field coil experiments. Operation results also proved that saturated superfluid helium can be applied as a coolant for large scale superconducting magnets.
The critical temperatures, Tc, have been resistively measured as functions of axial tensile strain and transverse compressive stress for bronze processed Nb3Sn single-core wires or tapes with various volume ratios of bronze matrix to Nb core. For the case of axial tensile strain, Tc's first increase with externally applied strain, reach their maxima and then decrease with further increasing strain, as reported for the critical current, Ic, under applied strain. Increases in Tc due to applied strain and strains where Tc's reach their maxima become larger with increasing the volume ratio of the bronze matrix to the Nb core. These results confirm that Tc measurements under applied strain are very effective to predict Ic under applied strain. The strain dependence of Tc is quantitatively analyzed based on phenomenological theory by Welch, and it is shown that the difference in the Tc-degradation before and after the maximum in Tc may be interpreted in terms of the relative significance of hydrostatic and nonhydrostatic components of strain in altering Tc. For the case of transverse compressive stress, Tc's decrease monotonically with externally applied stress. It is shown that there is no large difference in the Tc-degradation with stress between transverse compressive stress and axial tensile stress in contrast with the results on Ic reported.