Search

In Japan, 9 ITER TF coils were manufactured for the ITER project. Each TF coil consists of seven Nb3Sn conductor double pancakes (DPs). In DP manufacturing, heat treatment is one of the most important processes, because the insertion of a heat treated conductor into the radial plate groove (i.e., transfer) is complicated owing to the elongation of the conductor under heat treatment. A critical current distribution over the DP may also occur owing to the temperature distribution and disturbance in the heat treatment process. To address these issues, this study developed a procedure wherein the conductor length elongation caused by heat treatment is precisely considered, and transfer can be successfully achieved for all the DPs. In addition, strand witness samples were simultaneously heat treated with the DPs, and their critical current (Ic) was measured. Subsequently, it was confirmed that the average Ic is equivalent to those in the strand acceptance test while the standard deviation of Ic for the witness samples is approximately 4 % larger than that for the strand acceptance test. It was also confirmed that the disturbance of the heat treatment was negligible. Related results are described herein.

JAEA is fabricating the toroidal field insert coil (TFIC) in cooperation with Hitachi, Ltd. This solenoidal coil has about 50 m of ITER TF conductor wound in a 1.44-m diameter. In preparation for fabricating the TFIC, fabrication trials of windings, removal of Cr plating of the cable and welding of the terminal sleeve were performed for process qualification. The winding trials were accomplished without breaking any of the superconducting strands. In the trials to remove the Cr plating from the cable, HCl-soaked unwoven cloth was used, and the surface of a strand selected from the cable was confirmed by magnification to be free of Cr plating. In trials to fabricate termination, using electron beam welding (of OFHC copper and SS316LN) and fillet-welding (of SS316LN and SS316LN using JK2LB weld wire), tensile tests of the welds were conducted at room temperature and 4 K, and ultimate tensile strength values equivalent to that of the base metal of the weld were obtained. Also these welds passed test items of JIS Z3040 code, so the weld procedures were qualified. During fillet-welding the maximum temperature of the cable under the weld was 120 °C, not high enough to damage the cable. From the results obtained, the main processes involved in TFIC fabrication were established and qualified.

Under the International Thermonuclear Experimental Reactor (ITER) project, the Japan Atomic Energy Agency (JAEA) is procuring all of the Nb3Sn conductors for the Central Solenoid (CS). The CS consists of six vertically stacked modules. The height and outer diameter of the CS are approximately 13 m and 4 m, respectively. The CS has a circular five stage cable. All of approximately 43 km of Nb3Sn CS cables will be manufactured in Japan. Before mass-production start, the jacketed cable conductors should be tested in the SULTAN facility in Switzerland to confirm their superconducting performance. The original cabling design had relatively long twist pitches and is referred to as the normal twist pitch (NTP) conductor. The NTP conductor test results revealed decreasing the current sharing temperature (Tcs) with increasing number of electro-magnetic (EM) load cycles. Therefore, a short twist pitch (STP) design was proposed and the STP conductors were also tested. The STP conductor results showed that the Tcs is stable during EM cyclic load tests. Because the conductors with STP have a smaller void fraction in the cable area than those with NTP, a higher compaction ratio during cabling is required and the possibility of damage on strands increases. The STP cable technology was developed in collaboration among Japanese cabling suppliers and JAEA. Several key technologies will be described in this paper.

JAEA successfully completed the manufacture of the toroidal field (TF) insert coil (TFIC) for a performance test of the ITER TF conductor in the final design in cooperation with Hitachi, Ltd. The TFIC is a single-layer 8.875-turn solenoid coil with 1.44-m diameter. This will be tested for 68-kA current application in a 13-T external magnetic field. TFIC was manufactured in the following order: winding of the TF conductor, lead bending, fabrication of the electrical termination, heat treatment, turn insulation, installation of the coil into the support mandrel structure, vacuum pressure impregnation (VPI), structure assembly, and instrumentation. In this presentation, manufacture process and quality control status for the TFIC manufacturing are reported.