The Large Helical Device (LHD) is a heliotron-type toroidal fusion experimental device which will provide useful and reliable datasets for high-temperature helical plasmas with an equivalent Q value of 0.1 to 0.35. One of the crucial tasks of LHD is to demonstrate steady-state operations by taking the advantage of its magnetic configuration with currentless plasmas as well as built-in divertors. In this respect, the coil systems are fully superconducting (SC); consisting of a pair of helical coils, three pairs of poloidal coils and nine bus lines. All the SC coils have been successfully fabricated with high accuracy and they are presently in the final stage of assembly in a cryostat vessel along with outside preparations of plasma heating devices, diagnostic equipment and control systems. The first plasma operation is scheduled for the end of March 1998. The technological development of SC magnets through this project played a key role in LHD, and will also be useful for constructing future fusion reactors.
The Large Helical Device (LHD) under construction at the NIFS is a plasma physics experimental device consisting of two helical coils wound from aluminum-stabilized composite superconductors, three pairs of poloidal coils wound from cable-in-conduit conductors, a cryostat, and a plasma vacuum vessel. The design concepts of the helical and poloidal coils are especially described. Successful IV-L coil tests showed the validity of the poloidal coils. The LHD superconducting coils, cryostat, and plasma vacuum vessel will be assembled at the end of 1997, and trial tests will be carried out in March 1998.
A pair of helical coils for LHD, the largest pool-cooled superconducting coil system with a magnetic energy of 1.6GJ. In order to produce a fine magnetic surface, highly accurate manufacture within ±2mm and small deformation below 3.4mm against electromagnetic forces at 4T operation are required. Besides high current density, more than 53A/mm2 is necessary to keep sufficient distance between the coil and the plasma vacuum vessel. The helical coils are designed to satisfy the cryostable criterion by optimizing the wetted surface fraction of each conductor. Against large electromagnetic forces, the conductors were packed into thick cases supported by an outer shell structure. In order to suppress the maximum tensile stress in the conductors below the yield stress, the average compressive Young's modulus of the coils should be high. We used an insulator with a high compressive modulus of 30GPa, and we planned to maintain the average gap between layers within 65μm while winding. Whole conductors 36km long had been wound on-site by day-and nighttime work over 16 months. We have successfully managed average gap and position error within 50μm and ±1.5mm, respectively. After winding, the top cover of the coil cases were set up on the coils. By inserting machined spacers and controlling the shrinkage of welding, the top covers were successfully installed without any additional gaps and without deforming the accurate shape.
Composite-type superconductors with NbTi/Cu compacted strands and aluminum/copper stabilizers have been developed for use in the pool-cooled helical coils of the Large Helical Device (LHD). Topics related to the development, fabrication, short sample tests and soldered lap joint of these superconductors are summarized. Evaluation of AC loss has been made based on two measurement techniques. Heat transfer coefficients from the conductor surface to liquid helium have also been measured for the purpose of stability analysis.
Poloidal coils for the Large Helical Device (LHD) were fabricated and finished in September 1996. The poloidal coil system consists of three pairs of circular solenoids: inner vertical (IV), inner shaping (IS) and outer vertical (OV) coils which have a center diameter of 11.1m. Conductors are of the NbTi cable-in-conduit type and cooled with supercritical helium. The conductors were designed to ensure high stability and safety. In addition, the conductors were strictly inspected during fabrication. A coil consists of eight double-pancake coils. To minimize the error magnetic field, the following three items were considered. First, the shapes and sizes of the feeder, layer transitions and conductor joints were optimized. Second, the coils were made to a tolerances of 2mm. The tolerance corresponds to an extreme accuracy of about 5×10-4 for the diameter. Lastly, a compact superconducting joint was applied for the electric joints between pancakes.
We conducted cool-down and excitation tests of an IV coil using the test facilities at the cryogenics and superconductivity laboratories of the NIFS. The main purpose of this experiment was to confirm the designed coils parameters before installing them in the Large Herical Device (LHD) cryostat. The other purpose was to demonstrate operations of a large SHe forced-flow coil, as an example of a total superconducting system including a cryogenic system, a superconducting bus line, a power supply and a protection system. The first experiment was carried out from the end of January to the beginning of March 1995. The coil could be cooled down to the superconducting state for 23 days including cooling suspension times. An inlet filter of the coil was, however, repeatedly blocked with impurities in helium gas during cool-down and steady operation. Due to the unstable operation of the cooling system, the excitation test was limited to 2.2kA. The second experiment was conducted from November to early December 1995. We readjusted the test facilities by increasing the cooling ability of LHe storage and adapting a new inlet-filter unit. The test facilities could be operated steadily, and the coil was successfully energized up to the LHD nominal current, 20.8kA.
This paper describes a short history of material selection for the cryogenic support structures for the Large Helical Device (LHD) which has superconducting coils. Since the support structures are cooled down to 4.4K together with the coils, SUS316 was chosen because of its stable austenitic phase, sufficient mechanical properties at cryogenic temperature and good weldability. Also, outlines of the design and fabrication processes of the support structures are summarized. On the design of the support structures, a deformation analysis was carried out to maintain the proper magnetic field during operation. Afterwards, a stress analysis was performed. During machining and assembling, tolerance was noticed to keep coil positions accurate. Special welding grooves and fabrication processes were considered and achieved successfully. Finally, a cryogenic supporting post which sustains the cryogenic structures and superconducting coils is presented. CFRP was used in this specially developed supporting post to reduce the heat conduction from ambient 300K structures.
A flexible superconducting (SC) bus line was developed as a current feeder system for an experimental fusion device, the Large Helical Device (LHD). An aluminum stabilized NbTi/Cu compacted strand cable was developed to satisfy the cryostable condition at a rated current of 32kA. A pair of SC cables was electrically insulated and installed in a cryogenic transfer line. Nine sets of SC bus lines with a total length of 497m have been installed in LHD. The SC current feeder system, including the stuctures of the SC bus line and both peripheral terminals, their cooling methods and their construction status are presented in this report.
The LHD has 12 superconducting coils that generate 3T of magnetic field at the plasma center, and their stored energy becomes 1GJ. These coils require large current DC power supplies that have a 17-30kA class output current, small control error less than 0.04% and high reliability for quench protection. This section introduces a DC power system to drive these coils. First, we give a description of the specifications and structures of power supplies, that are thyristor rectifiers, a LC passive filter and a current dump circuit. Next, the quench protection system which uses a novel DC circuit breaker, is described.
The cryogenic system and cooling schemes for the Large Helical Device (LHD) are described. The cryogenic system for the LHD consists of a helium refrigerator/liquefier, the LHD cryostat in which the superconducting helical coils, poloidal coils and coil supporting structures are installed, and the peripheral equipment such as superconducting bus lines, control-valve boxes and cryogenic transfer lines. The helium refrigerator/liquefier has cooling capacities of 5.65kW at 4.4K and 20.6kW from 40 to 80K, and 650l/h liquefaction. Three different cooling schemes are utilized for each cooling object: a pool boiling for the helical coils (cold mass of 240t), a forced flow of supercritical helium for the poloidal coils (182t), a forced flow of two-phase helium for the coil supporting structure (390t) and the superconducting bus lines (total length of 463m). The heat in-leaks calculated from the final design of the cryogenic system become about half of the initial design values because of the extensive efforts of designing and developing each component. Many technical data were gathered during research and development, and were utilized to modify the configurations and operational conditions of the cryogenic system. To operate the complicated LHD cryogenic system automatically, we developed a new cryogenic control system which is highly flexible and functional in terms of both hardware and software.
A large-scale helium refrigerator/liquefier has been developed to provide reliable and safe operation for the Large Helical Device (LHD). The refrigerator is required to satisfy four different types of cooling methods: forced-flow supercritical helium, a pool boiling method, two-phase helium flow and forced-flow low-temperature (40-80K) helium gas. The forced-flow supercritical helium is widely used in modern large-scale superconducting magnets. This method requires a much more complex refrigeration system than does pool boiling because of the circulation of low-temperature helium within a very long cooling path. The overall refrigeration system is fairly complicated because of these multi-refrigeration requirements. As a matter of fact, it is not likely to find this type of refrigeration plant in the world. The helium refrigerator has a total refrigeration capacity of 5.65kW at 4.4K and 20.6kW at 80K and 650l/h liquefaction. The refrigerator was designed to have high processing efficiency since the construction expense is much less than the operating cost. In order to achieve this, the refrigerator has two precooling cycles (300 to 80K and 80 to 20K) and has two turboexpanders running in parallel with different temperature levels at the cold end. To achieve a high mass flow rate in a low-temperature regime, eight screw-type compressors are operated at room temperature. There are two compressor groups, group A and group B, to reduce the overall work load. Each group consists of 1st and 2nd stage compression processes. The total mass flow rate becomes 960g/s at 1.864MPa. This article reviews the basic chracteristics of a 10kW class helium refrigerator/liquefier and a simple refrigeration cycle.
As the concluding chapter of this special issue regarding Large Helical Devices (LHD), the history of establishing the Superconducting coil concept for LHD is reviewed, and it is pointed that the present LHD coil designs are strongly guided and affected by the discussions and investigations made by the scientists and engineers from universities, institutes and industries of the country. A brief chronological description about finished construction is made, and the outlook of paths for remaining work before the first ignition of LHD were illustrated. Finally, the past and future research activities of the NIFS Superconducting group are introduced.