Japan Atomic Energy Research Institute (JAERI) successfully developed high field and large coils in Cluster Test Program and Large Coil Task, respectively. When these coils are designed and constructed, there were few data of ordinary 304, 316 stainless steels and no design standard. Therefore, JAERI studied effect of carbon and nitrogen on strength and toughness of 304, 316 stainless steels to determine LCT coil structural materials and adopted strength at 4K as design stress. From these experience, JAERI concluded that it was indispensable to develop high strength, high fracture toughness and high rigidity materials and to establish design standard in order to realize superconducting coils for fusion reactor. JAERI began to develop new structural steels which had yield strength over 1, 200MPa and fracture toughness over 200MPa √m in collaboration with steel industries in 1982 and has successfully developed 6 stainless steels. AC losses which generated in metallic materials due to pulsed magnetic field is serious problem in superconducting coils. Therefore, non-metallic structural materials which have high strength and rigidity must be developed as soon as possible. In addition, material test standards at 4K have to be established to generate data base which is used in superconducting coil design. On the other hand, design standards of superconducting coil for fusion reactor is not yet established such as ASME code in commercial fission reactor at present. However, this establishment is indispensable to design and construct high field and large superconducting coils for fusion reactor. Development of cryogenic structural materials, evaluation technique of materials at 4K and key items to establish design standard are described here from viewpoint of superconducting coils development for fusion reactor.
The magnetic shielding with superconducting materials have been studied aiming at the practical application of shieldings. The leakage of the magnetic field should be reduced in order not to induce undesirable effects on the measuring instruments. The shieldings were made by lead, niobium or NbTi wire and the shielding efficieny were compared experimentally in the field direction of parallel to the face of plate or cylinder axis (made by wire). The shield efficiency of the wire was best as expected. Furthermore from the engineering view point, the superconducting wires were decided to use the magnetic shieldings. The several types of network were constructed and the efficiency of shielding were examined with changing the density and structure of meshes. The magnetic field as high as 2T could be reduced to several handreds gauss with the superconducting networks. It was, therefore, concluded that the superconducting networks are applicable to the practical magnet shieldings.
The necessity of magnetic shielding has been increased accompanied with the development of superconducting magnets. The shielding networks which were made of superconducting wire have been studied aiming at the practical application for the high field shielding. Cylindrical shieldings were made using superconducting networks. In the fields parallel to the shielding axis the external field of 1T could be reduced below 0.02T. In the field vertical to the axis 0.6T was decreased below 0.2T. The shielding efficiency depends on the change rate of the external field. The shielding network has shown the effective shield capability even in the graded magnetic field, because the shield current is induced corresponding to the gradient of the external field. The magnetic shielding with the superconducting networks is confirmed to be applicable to actual use.
As an important technology of developing a large scale superconducting magnet, we are working at power leads of a supercritical helium (SHE) force-cooled superconducting coil. For evaporated gas cooled mode one has only been required to find an optimum dimensional ratio of lead, but for SHE force-cooled one must design a power lead with both an optimum flow rate and the optimum dimensional ratio. The optimum condition of the lead is evaluated by energy consumption of a refrigerator at room temperature. We have found the optimum gas flow rate through the lead to obtain the minimum energy consumption. Comparing evaporated gas cooled with SHE force-cooled power lead, we concluded the latter is superior to the former from the energy cost.