Progress Summary of LHD Engineering Design and Construction

In March 1998, the large helical device (LHD) project finally completed its 8 years construction 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 simplex and continuous large helical coils. The major mission of LHD is to demonstrate the high potential of currentless helical-toroidal plasmas, which are free from current disruption and have an intrinsic potential for steady state operation. After the intensive physics design studies in the 1980\u27s, the necessary programs of SC engineering R&D were made and carried out, and as a result, LHD fabrication technologies were successfully developed. In this process, a significant database on fusion engineering has been established. These achievements have been made in various areas, such as the technologies of SC conductor development. SC coil fabrication, liquid helium (LHe) and supercritical helium (SHe) cryogenics, development of low temperature structural materials and welding, operation and control, and power supply systems and related SC coil protection schemes. They are integrated, and nowadays comprise a major part of the LHD relevant fusion technology area. These Issues correspond to a necessary technological duta base for the next step of future reactor designs. In addition, we could increase this with successful commissioning tests just after the completion of LHD machine assembly phase, which consisted of vacuum leak test, LHe cooldown test, and coil current excitation test. We recapitulate and highlight these LHD relevant engineering developments in this paper. To summarize our construction of LHD as an SC device, the critical design with NbTi SC material has been successfully accomplished by our R&D activities, which enables us to move into a new regime of fusion experiments