S. Imagawa

Physics and engineering design studies on the Large Helical Device

Abstract The construction of the Large Helical Device (LHD) is progressing as a seven year project in Japan, which began in 1990. This year, necessary research and development programs are nearly reaching the final goal of the original schedule and we have started the construction of the basic parts of LHD. We report on the results of the physics and engineering design studies, and the recent status of the construction of LHD.

Blanket and divertor design for force free helical reactor (FFHR)

Abstract Conceptual design of blanket and divertor for a force free helical reactor (FFHR) is presented. The demonstration-relevant FFHR is a heliotron-type helical reactor having superconducting helical and poloidal coils based on the large helical device (LHD) which is now under construction in the National Institute for Fusion Science. The main feature of FFHR is force free configuration of helical coils, which allows us to simplify the coil supporting structure and to use high magnetic field instead of high plasma β. For the goal of a self-ignited D—T reactor of 3 GW thermal output, the design parameters for FFHR are investigated under the LHD scaling for energy confinement and density limit. In particular, to satisfy the reactor lifetime of 30 years, the engineering issues in FFHR are discussed by focusing on selection of structrual materials for 500 dpa, optimization of tritium breeding system with neutron multiplier, cooling with molten-salt Flibe and operation temperature in the blanket, radiation shielding to achieve a reduction of more than 5 orders of magnitude at superconducting coils, and steady state helium ash removal with an efficiency of around 30%.

Goal and Achievements of Large Helical Device Project

Abstract The Large Helical Device (LHD) is a heliotron-type device employing large-scale superconducting magnets to enable advanced studies of net-current-free plasmas. The major goal of the LHD experiment is to demonstrate the high performance of helical plasmas in a reactor-relevant plasma regime. Engineering achievements and operational experience greatly contribute to the technological basis for a fusion energy reactor. Thorough exploration for scientific and systematic understanding of the physics in the LHD is an important step to a helical fusion reactor. In the 12 years since the initial operation, the physics database as well as operational experience has been accumulated, and the advantages of stable and steady-state features have been demonstrated by the combination of advanced engineering and the intrinsic physical advantages of helical systems in the LHD. The cryogenic system has been operated for 56 000 h in total without any serious trouble and routinely provides a confining magnetic field up to 2.96 T in steady state. The heating capability to date is 23 MW of neutral beam injection, 3 MW of ion cyclotron resonance frequency, and 2.5 MW of electron cyclotron resonance heating. Highlighted physical achievements are high beta (5.1%), high density (1.2 × 1021 m−3), and steady-state operation (3200 s with 490 kW).

Large Helical Device (LHD) program

The largest superconducting fusion machine, Large Helical Device (LHD), is now under construction in Japan and will begin operation in 1997. Design and construction of related R&D programs are now being carried out. The major radius of this machine is 3.9 m and the magnetic field on the plasma center is 3 T. The NbTi superconducting conductors are used in both helical coils and poloidal coils to produce this field. This will be upgraded in the second phase a using superfluid coil cooling technique. A negative ion source is being successfully developed for the NBI heating of LHD. This paper describes the present status and progress in its experimental planning and theoretical analysis on LHD, and the design and construction of LHD torus, heating, and diagnostics equipments.

Progress summary of LHD engineering design and construction

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.

Experimental Observation of Anomalous Magneto-Resistivity in 10–20 kA Class Aluminum-Stabilized Superconductors for the Large Helical Device

Degradation of recovery current due to the unexpected enhancement of resistivity of aluminum stabilizers has been observed in pool-boiling-type superconductors that have been developed for the helical coils of Large Helical Device. Dependence of the measured resistivity on the magnetic field suggests that this is a kind of anomalous magnetoresistivity. The Hall effect in metal-metal composites is considered to be the most plausible candidate to explain this observation. We compared our data with the calculated values based on this model and confirmed that this model explains the experimental results well.

Present status of design and manufacture of the superconducting magnets for the Large Helical Device

The Large Helical Device (LHD) is a nuclear fusion experimental device with superconducting magnets. Manufacture of the cryostat, the inner vertical coils, and the helical-coil winding machine are now being carried out. Designs for constructing two helical coils and two other pairs of poloidal coils are in progress. The outside diameter of the torus-shaped cryostat is 13.5 m. There are two operational stages for the LHD. Phase I and Phase II. The helical coils will have a magnetic energy of 1.6 GJ and an overall current density of 53 A/mm/sup 2/ in Phase II. The rated current is 13.0 kA in Phase I, and the maximum magnetic field in the helical coil winding in Phase I was calculated to be 6.9 T. Three pairs of poloidal coils are cooled by forced-flow supercritical helium because of the necessity of having no metal coil vessel. The rated current of one inner vertical (IV) poloidal coil is 20.8 kA, and its stored energy is 80 MJ. The maximum magnetic field of the two IV coils was calculated to be 5.8 T. The type of superconductor for the IV coils is a cable-in-conduit conductor.<<ETX>>

Asymmetrical normal-zone propagation observed in the aluminum-stabilized superconductor for the LHD helical coils

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.

Concept of magnet systems for LHD-type reactor

Heliotron reactors have attractive features for fusion power plants such as having no need for current drive and a wide space between the helical coils for the maintenance of in-vessel components. Their main disadvantage was considered to be the necessarily large size of their magnet systems. According to the recent reactor studies based on the experimental results in the Large Helical Device, a major radius of plasma of 14?17?m with a central toroidal field of 6?4?T is needed to attain the self-ignition condition with a blanket space thicker than 1.1?m. The stored magnetic energy is estimated at 120?140?GJ. Although both the major radius and the magnetic energy are about three times as large as ITER, the maximum magnetic field and mechanical stress are comparable. In the preliminary structural analysis, the maximum stress intensity including the peak stress is less than the 1000?MPa that is allowed for strengthened stainless steel. Although the length of the helical coil is more than 150?m, that is about five times as long as the ITER TF coil, cable-in-conduit conductors can be adopted with a parallel winding method of five-in-hand. The concept of the parallel winding is proposed. Consequently, the magnet systems for helical reactors can be realized with a small extension of the ITER technology.

Development and quality control of the superconductors for the helical coils of LHD

Abstract A composite-type superconductor with NbTi/Cu compacted strands and aluminum/copper stabilizers has been developed for the pool-cooled helical coils of the Large Helical Device. The internal configuration of the conductor was determined from the viewpoint of high stability performance and fabrication reliability, by incorporating some newly developed techniques, such as a Cu–2%Ni-clad pure aluminum stabilizer and electron beam welding of the half-hard copper sheath. The conductor has been fabricated over the total length of 36 km under careful inspections both on the component materials and on the final full-conductors from the viewpoint of quality control. Short sample tests of the conductors showed that the measured critical currents and the recovery currents satisfied the specified values for the Phase I operation condition of LHD, although they showed some scattering around their mean values.