Reliable long-term operation of superconducting bus lines for the LHD

The Large Helical Device (LHD) is an experimental device for helical type fusion plasma in National Institute for Fusion Science and plasma experiments over 150,000 shots have been successfully conducted during twenty long-term plasma experimental campaigns. The LHD has two kinds of superconducting magnets and nine flexible superconducting bus lines with an average length of 55 m, which are utilized as a part of the current feeder system between the coils and the power sources. The superconducting bus lines consist of a pair of aluminum stabilized NbTi/Cu compacted stranded cable insulated electrically and coaxial five corrugated stainless steel tubes with two layers of vacuum insulations. The nominal current is 32 kA and the withstand voltage is 5 kV in 77 K gas helium. From the first experimental campaign, the superconducting bus lines have been stably operated at steady state by using automatic control. It is also confirmed that the status of the superconducting bus lines are kept good thanks to appropriate maintenances. As the results, the reliable operation of the superconducting bus lines has been achieved during the plasma experimental campaigns without any serious failure and the total operational time of the steady state cooling is approximately 58,000 hours

Effects of Subcooling on Lengths of Propagating Normal Zones in the LHD Helical Coils

Propagation of a short normal zone was observed in a helical coil of the Large Helical Device, when the coil was cooled with subcooled helium, of which the inlet and outlet temperatures are 3.2 K and below 4.0 K, respectively. The normal zone was induced at the bottom position of the coil. It propagated to only the downstream side of the current with recovery from the opposite side, and stopped after passing the outer equator of the torus. The induced balance voltage is obviously lower and the propagating time is shorter than those of propagating normal zones observed in the helical coil cooled with saturated helium at 4.4 K. According to the simulation of the propagation of a normal zone, it is considered that such a short normal zone at the current close to the minimum propagating current propagates without full transition to film boiling

Engineering research and development of magnetically levitated high-temperature superconducting coil system for mini-RT project

A magnetically levitated superconducting coil system is being developed using high temperature superconductors for examining a new magnetic confinement of high-beta plasmas. A miniature double-pancake coil was fabricated with a Bi-2223 Ag-sheathed tape for the purpose of developing a floating control using laser displacement gauges. The coil was inductively excited with liquid nitrogen cooling and successfully levitated in the air. A persistent current switch is also being developed with a Bi-2223 Ag-0.3wt%Mn-sheathed tape, and a prototype model was successfully tested

Highly Efficient Liquid Hydrogen Storage System by Magnetic Levitation Using HTS Coils

Highly efficient liquid hydrogen storage system is studied with magnetic levitation using high-temperature superconducting (HTS) coils. The system also has high safety in case of emergency, such as an earthquake, with a seismic isolation to absorb vibrations provided by HTS levitation coils setup on the ground side. In such an emergency case, it is considered that a large amount of ac losses are generated in HTS coils, and the winding temperature may rise to lead to a coil quench. In this study, the self-oscillation-type heat pipe (OHP), whose thermal transport property is much greater than that of solid thermal conduction, is used to cool the coil windings. As a result, an HTS coil equipped with an OHP cooling system can be realized, supporting both low heat loads in the usual operation and high heat loads in an emergency

Analysis on the cryogenic stability and mechanical properties of the LHD helical coils

Transient normal-transitions have been observed in the superconducting helical coils of LHD. Propagation of a normal-zone is analyzed with a numerical simulation code that deals with the magnetic diffusion process in a pure aluminum stabilizer. During excitation tests, a number of spike signals are observed in the balance voltage of the helical coils, which seem to be caused by mechanical disturbances. The spike signals are analyzed by applying pulse height analysis and the mechanical properties of the coil windings are investigated

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

Analysis of the normal transition event of the LHD helical coils

Normal transitions and a subsequent quench were experienced with the pool-cooled helical coils of the Large Helical Device (LHD) during its excitation test. Although the initiated normal zone once started to recover, a disruptive transverse propagation followed and triggered an emergency discharging program. The cryogenic stability of the composite-type superconductor has been studied by sample experiments as well as by numerical calculations. Due to the rather long magnetic diffusion time constant in the pure Al stabilizer, transient stability of the conductor seems to play an important role for driving finite propagation of a normal zone. The cause of the final quench is also discussed from the viewpoint of cooling deterioration due to a possible accumulation of He bubble

Effect of Direction of External Magnetic Field on Minimum Propagation Current of a Composite Conductor for LHD Helical Coils

The conductor for helical coils of the Large Helical Device consists of a Rutherford-type NbTi/Cu cable, a pure aluminum stabilizer, and a copper sheath. The dimensions of the conductor and the stabilizer cross-sections are 18.0 mm × 12.5 mm and 12.4 mm × 5.2 mm, respectively. The measured cold-end recovery current in the magnetic field parallel to the shorter side (B//12.5) is clearly lower than that in the field parallel to the longer side (B//18.0) because of the difference in magnetoresistance by Hall currents. Since the minimum propagation current Imp is important to determine the upper limit of operation current, Imp has been measured for two types of one-turn coil samples, which were bent flatwise (B//18.0) and edgewise (B//12.5) with the inner radius of 0.14 m to extend the length in the uniform background field of the test facility. The measured Imp at B//12.5 is almost the same as that at B//18.0 in spite of the large difference in the steady-state resistance. Imp is considered to be determined by the heat balance before the current diffuses deeply into the stabilizer

Commissioning Test Results of Variable Temperature Helium Refrigerator/Liquefier for NIFS Superconducting Magnet Test Facility

The superconducting magnet test facility in the National Institute for Fusion Science has been upgraded for excitation tests at a wide temperature range and a higher magnetic field of 13 T. As part of the upgrade, the helium refrigerator/liquefier that operated for 24 years was replaced with a variable-temperature helium refrigerator/liquefier. The required liquefaction rate is 250 L/h, and the required refrigeration capacity is 600 W at 4.5 K, same as the previous one. In addition, it has a new feature that can supply helium gas of a wide temperature range. The typical design cooling capacity is 1 kW under the condition of 20-K supply/30-K return and 1.5 kW under the condition of 40-K supply/50-K return. After the replacement, a series of commissioning tests were performed under the various operational conditions. From the results, the satisfactory thermodynamic performance was confirmed. In the future, it is expected that the substantial progress will be made in the development of large-scale superconducting magnets with advanced superconductors such as high-temperature superconductors and MgB2. The design of the variable-temperature helium refrigerator/liquefier and the results of the commissioning tests are reported in detail