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.

Synopsis:The Japan Atomic Energy Agency (JAEA) and National Institute for Material Science (NIMS) have been collaborating in the development of high-performance (i.e., more than 16 T and 80 kA) Nb 3 Al cable-in-conduit (CIC) conductor prepared using the rapid-heating, quenching and transformation (RHQT) process, and aiming for the application of this conductor to a demonstration plant. The technical issue relating to application of the RHQT Nb 3 Al strands to a fusion magnet is stabilization against perturbation. NIMS has developed a technique to attach a copper stabilizer using electroplating, and a sub-scale CIC conductor is developed using this conductor. JAEA performed a stability test of the developed CIC conductor to demonstrate the efficiency of this copper stabilization technique. The initial perturbation was applied via inductive heating, whose energy was calibrated using a calorimetric method. The measured stability margin is sufficiently high compared to that of a similar NbTi CIC conductor previously tested by the authors. In addition, a heat transfer coefficient to define the so-called limiting current is estimated to be about 1 kW/m 2 K, a sufficiently high value that is almost the same as that of a CIC conductor consisting of normal Cu stabilized strands. From these experimental results, it can be concluded that the copper stabilizer works efficiently from the viewpoint of stability, thus offering a solution to the remaining technical issues relating to the RHQT CIC conductor. On this basis, we can say that the RHQT Nb 3 Al CIC conductor is the most promising candidate for application to a magnet in the demonstration plant.