A superconducting magnet system is also one of the important components in an advanced magnetic confinement fusion reactor. Then it is required to have a higher magnetic field property to confine and maintain the steady state burning deuterium (D) tritium (T) fusion plasma in the large interspace during the long term operation. Burning plasma is sure to generate 14 MeV fusion neutrons during deuterium tritium reaction, and fusion neutrons will be streamed and penetrated to superconducting magnet through large ports with damping neutron energy. Therefore, it is necessary to consider carefully not only superconducting property but also neutron irradiation property in superconducting materials for use in a future fusion reactor, and a``low activation and high field superconducting magnet'' will be required to realize the fusion power plant beyond International Thermonuclear Experimental Reactor (ITER). V based superconducting material has a much shorter decay time of induced radioactivity compared with the Nb based materials. We thought that the V 3 Ga compound was one of the most promising materials for the``low activation and higher field superconductors'' for an advanced fusion reactor. However, the present critical current density ( J c ) property of V 3 Ga compound wire is insufficient for apply to fusion magnet applications. We investigated a new route PIT process using a high Ga content Cu Ga compound in order to improve the superconducting property of the V 3 Ga compound wire.

Vanadium-gallium (V 3 Ga) superconducting wire is an "old superconducting material", and was one of the original materials famed for the "Cu additive effect" that was extended to the "Bronze route process". The "Cu additive effect" in A15 phase compounds promotes A15 phase formation via diffusion reaction. The V 3 Ga compound has interesting properties for an advanced magnetic confinement fusion reactor beyond ITER. The decay time of induced radio activity for V 3 Ga is within 1 month and is much shorter than that of Nb-based superconductors such as Nb-Ti, Nb 3 Sn and Nb 3 Al. We thought that V 3 Ga wire was one of the candidate materials for "Low activation superconducting wires" to operate under a neutron irradiation environment such as in a fusion reactor. However, the J c and H c2 properties of V 3 Ga wire are insufficient to realize this feature in fusion application. In previous studies, V 3 Ga wire was mainly investigated in term of the "Diffusion process" between Cu-Ga within a 20 at% Ga composition and V filament. For further J c and H c2 enhancements, we investigated the fabrication of V 3 Ga compound multi-filamentary wires using a high Ga content Cu-Ga compound applying the powder-in-tube process. Thicker V 3 Ga layers formed along the boundary between the Cu-Ga powder filaments and V matrix, and the volume fraction of V 3 Ga increased compared to previous diffusion processed samples. We also found that the new route PIT process using a high Ga content Cu-Ga compound is effective for improving the superconducting properties of the V 3 Ga compound wire.

Synopsis:The advantages of using a MgB 2 compound in fusion applications are not only higher T c (39 K) and low cost but also lower induced radioactivity than Nb-based superconductors. However, the J c value of MgB 2 wire is lower than that of Nbbased superconducting wire, and further improvement in J c is required for use in advanced fusion reactor applications.Furthermore, the selection of low-activation sheath material is also required to keep the low induced radioactivity property. We prepared a Cu-added MgB 2 /Ta/Cu mono-cored wire using a Mg 2 Cu compound as the source material via a low-temperature diffusion process, and the transport J c -B performances in the 4.2 K to 20 K temperature region were measured in order to investigate the possibility of the high-temperature operation.

In the magnetic confinement type fusion reactor, we thought that the high critical current density ( J c ) performance Nb 3 Sn wire will become the first candidate material for the future fusion magnet system due to its established industrial mass production and its technical achievements. However, the degradation of transport J c property due to the high mechanical and thermal strains on the practical Nb 3 Sn wire is a serious problem to apply for the future fusion magnet to be operated under higher electromagnetic force environment. We developed the new bronze processed Nb 3 Sn wire using the Zinc (Zn) solid solution high Sn content bronze (Cu Sn Zn) alloy matrix for improvement of the mechanical strength by the solid solution strengthening. We also confirmed that the Zn promoted to diffuse between Nb and Sn elements, and that Zn homogeneously remained in the matrix after the diffusion reaction. In the Nb 3 Sn wires with various Cu Sn Zn (Ti) alloy matrices, the non Cu J c value was increased by the two stage heat treatment, and the maximum non Cu J c values under the external magnetic fields of 15 T and 18 T at 4.2 K were obtained about 420 and 160 A/mm 2 . The degree of J c enhancement due to the two stage heat treatment was estimated 30 higher compared with one stage heat treatment. Change in the microstructure and the non Cu J c enhancement due to the two stage heat treatment on the Nb 3 Sn multifilamenatry wire with various Cu Sn Zn alloy matrices were reported.