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.

Fig. 1 The reel to reel rapid heating and quenching apparatus for making a long length of multifilamentary bcc supersaturated solid solution wire. The RHQT (rapid heating, quenching and transformation) processed Nb 3 Al superconducting wire has a larger critical current density J c in high magnetic fields and better strain tolerance than Nb 3 Sn does. Thus, the Nb 3 Al wire is one of the most promising candidate superconductors for the large scale and high field applications. Although the RHQ optimization is a key to a high J c through controlling the microchemstry and grain structure of bcc and hence A15 phases, investigated have been only the RHQ parameters of heating current I RHQ and wire speed v wire for a given electrode spacing (100 mm). From a view point of apparatus design a smaller I RHQ is favorable, but a heating time which it takes for a wire to move between electrodes is instead needed to become longer for heating the wire up to 2273 K. Thus v wire must become slower. However, such a slow v wire also causes a slower cooling rate of wire that would partly form undesirable A15 phase at the RHQ treatment. In the present study, an attempt has been made to extend the electrode spacing from 100 mm to 300 mm. This enables a relatively large v wire (high cooling rate) but a small I RHQ , which ensures a heating time enough to react the precursor. The Nb/Al precursor used has been prepared by the double stacked rod in tube process, where the starting material is a 7 core Nb/Al composite of which Al was alloyed with 5 at Mg. The final size of Al alloy core calculated is 0.56 mm, several times larger than the thickness of Al layer of jelly roll Nb/Al precursor. The extension of electrode spacing from 100 mm to 300 mm did not degrade T c of the Nb 3 Al transformed from bcc supersaturated solid solution, which was 17.6 K, and allowed the I RHQ to be reduced to ～1/ 3 in comparison to the conventional I RHQ condition with the same v wire . The newly optimized RHQ condition of 0.33 m/s, 48 A for the electrode spacing of 300 mm eventually doubled the J c (15 T, 4.2 K), at least, for the rod in tube processed Nb 3 Al that has not been mechanically deformed between RHQ and transformation annealing.