Yoshiro Terazaki

Design and development of high-temperature superconducting magnet system with joint-winding for the helical fusion reactor

An innovative winding method is developed by connecting high-temperature superconducting (HTS) conductors to enable efficient construction of a magnet system for the helical fusion reactor FFHR-d1. A large-current capacity HTS conductor, referred to as STARS, is being developed by the incorporation of several innovative ideas, such as the simple stacking of state-of-the-art yttrium barium copper oxide tapes embedded in a copper jacket, surrounded by electrical insulation inside a conductor, and an outer stainless-steel jacket cooled by helium gas. A prototype conductor sample was fabricated and reached a current of 100 kA at a bias magnetic field of 5.3 T with the temperature at 20 K. At 4.2 K, the maximum current reached was 120 kA, and a current of 100 kA was successfully sustained for 1 h. A low-resistance bridge-type mechanical lap joint was developed and a joint resistance of 2 nΩ was experimentally confirmed for the conductor sample.

Progress of the Design of HTS Magnet Option and R&D Activities for the Helical Fusion Reactor

The high-temperature superconducting magnet option is being explored in the conceptual design studies of the LHD-type helical fusion reactor FFHR-d1. A 100 kA-class conductor is being developed by simply stacking REBCO tapes in a copper and stainless-steel jacket. One of the design options of the HTS conductor includes internal insulation so that the windings do not require vacuum pressure impregnation process. Innovative winding method of the huge helical coils is being investigated based on the segment fabrication of half-helical-pitch conductors by developing a bridge-type mechanical lap joint. A “30 kA-class” prototype conductor sample was fabricated using GdBCO tapes and successfully tested. The critical current was measured at various temperatures at 4.2-40 K and magnetic field <; 8 T. The joint resistance was evaluated by changing the applied stress. These experimental results are boosting the HTS magnet design of FFHR-d1.

Performance of a Mechanical Bridge Joint for 30-kA-Class High-Temperature Superconducting Conductors

In this report, we propose segment-fabricated high-temperature superconducting (HTS) magnets as candidates for the FFHR-d1 heliotron-type fusion reactor. The FFHR-d1 requires 100-kA-class superconducting conductors used at 12 T for a pair of helical coils. We fabricated and tested two 30-kA-class GdBCO conductors with bridge-type mechanical lap joints (mechanical bridge joints). This report details the design of the joint section and the experimental results of those samples, especially, those of their joints. We improved the geometry of the joint region in a second sample, based on our results from the first. The second sample has sufficiently low joint resistance (less than 5 nΩ), and we could apply 70 kA to it without causing quenching at the joint. Its joint resistance was also acceptable for providing the electric power required to run the cryoplant for the segmented HTS helical coils.

Measurement and Analysis of Critical Current of 100-kA Class Simply-Stacked HTS Conductors

Based on the successful plasma experiments in the Large Helical Device (LHD), design activities of the LHD-type helical fusion reactor FFHR-d1 are progressing at National Institute for Fusion Science (NIFS). A 100 kA current capacity is required for the winding conductor under the maximum magnetic field of ~12 T. The high-temperature superconductor (HTS) is a promising option for the helical coil conductor. For the development of such a HTS conductor suitable for the helical fusion reactor, we fabricated 30 kA-class HTS conductor samples, and the excitation tests were successfully carried out. We then fabricated and tested a 100-kA class HTS conductor. The conductor sample is a one-turn short-circuit coil with a race-track shape having a bridge-type mechanical lap joint. The transport current of the sample was induced by changing the external magnetic field, then the critical current of the sample was measured. A numerical analysis of the critical current is being performed by self-consistently solving the spatial distributions of the current density and magnetic field among the simply-stacked HTS tapes to verify the measured critical current of the samples. The critical current characteristics of a single HTS tape is evaluated by the percolation model in the precise analysis.

Critical Current Measurement of 30 kA-Class HTS Conductor Samples

Design activities on the helical-type fusion DEMO reactor, FFHR-d1, are progressing at NIFS. A 100 kA current-capacity is required for the helical coil conductors under the maximum magnetic field of ~ 13 T. High-temperature superconducting conductor has been proposed as one of the conductor options for the FFHR-d1 magnet. In this study, a 30 kA class HTS conductor sample has been fabricated and tested. The sample had no current feeders and the current was induced by changing the background magnetic field generated by the 9 T split coils in the cryostat. Rogowski coils and Hall probes were used for the measurement of the transport current of the sample. The critical current of the sample was measured at various temperatures and bias magnetic fields. To verify the self-field effect of the sample, a numerical analysis was performed by considering the current and magnetic field distribution among the tapes self-consistently. The analysis result was compared with the experimental observation.

Enhancement of Thermal Properties of HTS Magnets Using Built-in Cryogenic Oscillating Heat Pipes

Enhancement of the thermal properties of high-

Experimental Results of the HTS Floating Coil Using REBCO Tapes for the Mini-RT Upgrading

T_{\rm c}

Current-carrying capability of the 100 kA-class HTS STARS conductor for the helical fusion reactor FFHR-d1

superconducting (HTS) magnets has been investigated using built-in cryogenic oscillating heat pipes (OHPs). A cryogenic OHP is built into windings of an HTS magnet to improve the thermal properties of windings and to protect them from damage caused by a large temperature gradient. It is rather difficult for an HTS magnet to quickly remove the heat generated in windings, especially, in a protection operation when a magnet quenches, because the thermal diffusivities of component materials of windings decrease with an increase of temperature. Therefore, a local hot spot can be formed in a magnet, and there are possibilities of having degradation of superconducting properties and/or mechanical damages by thermal stresses. A flat-plate cryogenic OHP has been developed that is suitable for imbedding in magnet windings as a high-performance heat transportation device in order to increase the thermal conductivity and the thermal diffusivity at the same time. By using hydrogen, neon, and nitrogen as working fluid, its excellent thermal transport properties have been proved in the operating temperature range of 18–84 K.

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

The upgrading plan of the Mini-RT which is an experimental device for plasma physics was initiated to replace the magnetically-levitated high temperature superconducting (HTS) coil by the new one wound with the latest REBCO tapes. The performance of the new HTS coil has been examined at the National Institute for Fusion Science. The coil was indirectly cooled to 35 K by forced flow of cold helium gas. The coil was successfully excited up to 100 A with a proper persistent current switch (PCS) operation. The central field given by the Hall probe was 0.21 T, which was the same with that expected by a numerical calculation. The time constant of the current decay during the persistent current mode was evaluated at the coil temperature of 36 and 41 K to be 306 and 228 h, respectively. These values are consistent with the estimation based on the measured joint resistances and they are much longer than that of the previous floating coil of Mini-RT wound 10 years ago (41 h before aged deterioration). By the test results, it has been confirmed that the promising performance of the manufactured REBCO floating coil has been achieved to further promote the Mini-RT project.

Bridge-Type Mechanical Lap Joint of a 100 kA-Class HTS Conductor having Stacks of GdBCO Tapes

A 100-kA-class conductor using REBCO high-temperature superconductor (HTS) is proposed as one of the conductor options for the helical coils of the LHD-type helical fusion reactor FFHR-d1. We named it STARS (Stacked Tapes Assembled in Rigid Structure) conductor. Current carrying capability of such a simply-stacked tape conductor is an important issue. We fabricated and tested a 100-kA class STARS prototype conductor sample to verify whether a premature quench occurs. The conductor sample is a one-turn short-circuit coil with a race-track shape having a bridge-type mechanical lap joint. The transport current of the sample was induced by changing the external magnetic field, and the critical current of the sample was measured. A numerical analysis of the critical current is being performed by self-consistently solving the spatial distributions of the current density and magnetic field among the tapes to verify the measured critical current of the samples.