In March 1998, the LHD project finally completed its eight year construction
schedule. LHD is a superconducting (SC) heliotron type device with R = 3.9 m, ap = 0.6 m and B = 3 T,
which has simple 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 intensive physics design studies in the 1980s,
the necessary programmes of SC engineering R&D was carried out, and as a result, LHD fabrication
technologies were successfully developed. In this process, a significant database on fusion engineering
has been established. Achievements have been made in various areas, such as the technologies of SC conductor
development, SC coil fabrication, liquid He and supercritical He 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 the
technological database necessary for the next step of future reactor designs. In addition, this database
could be increased with successful commissioning tests just after the completion of the LHD machine assembly phase,
which consisted of a vacuum leak test, an LHe cooldown test and a coil current excitation test.
These LHD relevant engineering developments are recapitulated and highlighted. To summarize the construction of LHD
as an SC device, the critical design with NbTi SC material has been successfully accomplished by these R&D
activities, which enable a new regime of fusion experiments to be entered.
Abstract-Transient normal-transitions have been observed in the superconducting helical coils of the Large Helical Device (LHD). Stability tests have been performed for an R&D coil as an upgrading program of LHD, and we observed asymmetrical propagation of an initiated normal-zone. In some conditions, a normal-zone propagates only in one direction along the conductor and it hence forms a traveling normal-zone. The Hall electric field generated in the longitudinal direction in the aluminum stabilizer is a plausible candidate to explain the observed asymmetrical normal-zone propagation.
The Large Helical Device (LHD) that has been demonstrating high performance of heliotron plasma is the world's largest superconducting system. Availability higher than 98% has been achieved in a long-term continuous operation both in the cryogenic system and in the power supply system. It will be owing not only to the robustness of the systems but also to efforts of maintenance and operation. One big problem is shortage of cryogenic stability of a pair of pool-cooled helical coils. Composite conductors had been developed to attain the sufficient stability at high current density. However, it was revealed that a normal-zone could propagate below the cold-end recovery current by additional heat generation due to the slow current diffusion into a thick pure aluminum stabilizer. Besides, a novel detection system with pick-up coils along the helical coils revealed that normal-zones were initiated near the bottom of the coil where the field is not the highest. Therefore, the cooling condition around the innermost layers, the high field area, will be deteriorated at the bottom of the coil by bubbles gathered by buoyancy. In order to raise the operating currents, methods for improving the cryogenic stability have been examined, and stability tests have been carried out with a model coil and small coil samples. The coil temperature is planned to be lowered from 4.4 K to 3.5 K, and the operating current is expected to be increased from 11.0 kA to 12.0 kA that corresponds to 3.0 T at the major radius of 3.6 m.
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