Progress summary of LHD engineering design and construction

In March 1998, the LHD project finally completed its eight year constructio n schedule. LHD is a superconducting (SC) heliotron type device with R = 3. 9 m, a(p) = 0.6 m and B = 3 T, which has simple and continuous large helica l coils. The major mission of LHD is to demonstrate the high potential of c urrentless helical-toroidal plasmas, which are free from current disruption and have an intrinsic potential for steady state operation. After intensiv e physics design studies in the 1980s, the necessary programmes of SC engin eering R&D was carried out, and as a result, LHD fabrication technologies w ere successfully developed. In this process, a significant database on fusi on engineering has been established. Achievements have been made in various areas, such as the technologies of SC conductor development, SC coil fabri cation, liquid He and supercritical He cryogenics, development of low tempe rature structural materials and welding, operation and control, and power s upply systems and related SC coil protection schemes. They are integrated, and nowadays comprise a major part of the LHD relevant fusion technology ar ea. These issues correspond to the technological database necessary for the next step of future reactor designs. In addition, this database could be i ncreased with successful commissioning tests just after the completion of t he LHD machine assembly phase, which consisted of a vacuum leak test, an LH e cooldown test and a coil current excitation test. These LHD relevant engi neering developments are recapitulated and highlighted. To summarize the co nstruction of LED as an SC device, the critical design with NbTi SC materia l has been successfully accomplished by these R&D activities, which enable a new regime of fusion experiments to be entered.