Superconducting magnet design and construction of Large Helical Device

The Large Helical Device (LHD) project is now successfully executing the seventh year programme of the eight year construction period. Superconducting (SC) research and development tasks for the LHD SC coil construction have already been completed, and now more than 75% of the whole construction schedule has been accomplished. In the paper, the recent progress of SC R and D and the fabrication technology developed for a huge and reliable SC coil system are reported. These results are applicable to a future experimental reactor in the next decade with a much larger SC coil system. The LHD is called heliotron and has l/m = 2/10 SC helical coils and three sets of SC poloidal coils, whose coil currents are 7.8, 5.0, -4.5 and -4.5 MA, respectively. Major radius, minor helical coil radius, minor plasma radius and plasma volume are 3.9 m, 0.975 m, 0.5-0.65 m, and 20-30 m3, respectively. In addition to an SC coil system, the main body of the LHD consists of huge supporting structures for the electromagnetic forces, a vacuum chamber, an outer cryostat and a machine base. The total weight is about 1500 t, of which the LHe cooled mass amounts to 850 t. The LHD has a maximum stored energy of 1.6 GJ (4 T at the plasma centre). The major goal of the LHD project is to demonstrate the high potentiality of the helical type device producing currentless steady state plasmas with a sufficiently large Lawson parameter in the absence of any danger of plasma current disruption. It provides a useful and reliable database making it possible to predict fusion reactor conditions. The realization of steady state operation requires a large extent of engineering innovation, primarily in the areas of superconducting technology, plasma facing materials, cooling systems and heating systems. It also contributes to building up the engineering scenario required for long pulse, regulated plasma operation necessary for fusion research. (author)