N. Yanagi

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.

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.

Design Progress on the High-Temperature Superconducting Coil Option for the Heliotron-Type Fusion Energy Reactor FFHR

Abstract Feasibility studies on applying high-temperature superconductors (HTS) to the heliotron-type fusion energy reactor FFHR are being carried out. Using HTS, we consider that the three-dimensional helical coils with a ~40 m diameter can be constructed without preparing a huge winding machine. A practical method for realizing this concept is proposed. The electromagnetic stress inside the helical coil packs is examined using an FEM analysis for double-pancake windings. The effect of error magnetic field generated by the shielding currents in HTS tapes is also examined.

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.

Extra AC losses for a CICC coil due to the non-uniform current distribution in the cable

Extra AC losses were observed during the Experiments on a Single Inner Vertical coil (EXISV). The Inner Vertical (IV) coils are the smallest poloidal coils for the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with cable-in-conduit conductor (CICC) whose strands are NbTi/Cu without any surface coating. Many causes for the extra AC losses were considered, such as the decrease of a contact resistance between strands due to the large electromagnetic force in the conductor or due to the stress during the coil winding process, etc. and possibilities were investigated from the experimental data. Finally, we found that a coupling current with a very long time constant of 124 s caused the AC loss increase. The coupling current with such a long time constant cannot be explained from the symmetric twisting configuration of the CICC but can be explained as a local loop current corresponding to a cyclic change of the non-uniform current distributions in the cable. The non-uniform current distribution could be induced by an asymmetry of the strand transposition in the cable. To verify the above reasoning, we did fundamental experiments on a two-strands-cable, which has an intended asymmetry in the cable twisting. Extra AC losses were also observed for an asymmetric two-strands-cable, and it was demonstrated that the non-uniform current distribution causes an increase of AC losses.

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.

Stability of cable-in-conduit superconductors for Large Helical Device

The stability of cable-in-conduit superconductors has been experimentally investigated as part of a poloidal field coil program for the Large Helical Device (LHD) project. A new conductor was designed and fabricated, focusing on the stability. As a result of a zero-dimensional stability analysis, it was found that the conductor had a high stability, 5*10/sup 5/ J/m/sup 3/, at the design condition of 20.8 kA and 6.5 T. Current transfer performance after partial quenching has been investigated by using a short sample of the conductor for the poloidal field coil. The effects of the current transfer among the strands on the conductor stability are discussed.<<ETX>>

Superconducting test facility of NIFS for the Large Helical Device

Abstract For development of superconducting conductors and coils, the National Institute for Fusion Science prepared a new superconducting test facility which is a combination of an electro-mechanical testing device and a large cryogenic cooling system at the new Cryogenics and Superconductivity Building at the Toki site. The cryogenic cooling system has a capacity of 250 liters of liquid helium per hour or 500 W at 4.5 K, and can also supply supercritical helium up to 50 g/s. A dc power supply generates 75 kA current at 21 V. A split-type superconducting coil generates a magnetic field of 9 T of 100 mm in width. The mechanical testing machine has a capacity of 10 MN at liquid-helium temperature. The following R & D experiments for the Large Helical Device have been done using the facility: critical current and stability measurement of helical coil conductors of which the nominal current is 21 kA at 7 T, an elasticity measurement of a helical coil, and stability measurement of a cable-in-conduit conductor.

Stability tests of the Nb-Ti cable-in-conduit superconductor with bare strands for demonstration of the Large Helical Device poloidal field coils

Stability and quench experiments of a Nb-Ti cable-in-conduit superconductor with bare strands have been carried out to determine the performance of the poloidal coil for the Large Helical Device (LHD). The conductor was formed into a double-pancake coil named IV-S and was mounted on TOKI-PF-a previously tested R&D coil. In excitation tests at 7.4-8.3 K, the IV-S coil reached the critical currents without premature quenches. Stability tests at 7.5 K indicated that the limiting current exceeds 20 kA, which is the nominal operating current of the LHD poloidal coils. These results demonstrated that the stability of the chosen conductor with bare strands is high enough for the LHD. >