Kyohei Natsume

Development of Highly Effective Cooling Technology for a Superconducting Magnet Using Cryogenic OHP

A highly effective cooling technique for a superconducting magnet is proposed by incorporating the cryogenic oscillating heat pipes (OHP) as cooling panels in the coil windings. The OHP is a high performance two-phase heat transfer device, which can transport several orders of magnitude larger heat loads than heat conduction of solids. The cryogenic OHP using , Ne, and as working fluids have been developed and tested at the operating temperature ranges of 17-25 K (H2), 26-32 K (Ne), and 67-80 K (N2). The measured effective thermal conductivities were reached to 500-3,000 W/m · K (H2), 1,000-8,000 W/m · (Ne) and 10,000-18,000 W/m · K (N2). The high thermal transport properties of the cryogenic OHP and its application as the cooling components of superconducting magnets are also discussed.

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.

Development of Cryogenic Oscillating Heat Pipe as a New Device for Indirect/Conduction Cooled Superconducting Magnets

Cryogenic oscillating heat pipes (OHPs) have been proposed as a new heat transfer device for conduction/indirect cooling of high-temperature superconducting (HTS) magnets. OHP is a highly effective two-phase heat transfer device which can transport several orders of magnitude greater heat flux than the heat conduction of solids. The performance of cryogenic OHPs has been intensively examined and the results indicate the ability of dramatically improving the performance of HTS magnets. Semi-empirical correlations stating thermo-physical properties of cryogenic OHPs are introduced based on those of room temperature OHPs. The modeling with non-dimensional quantities is useful for the design of cryogenic OHPs.

Achievement of High Heat Removal Characteristics of Superconducting Magnets With Imbedded Oscillating Heat Pipes

Oscillating heat pipes (OHP) for cryogenic use are being developed to improve the heat removal characteristics of high-temperature superconducting (HTS) magnets. It is generally difficult to remove the heat generated in HTS windings, because the thermal diffusivities of component materials decrease with an increase of the operating temperature. Therefore, a local hot-spot can be rather easily generated in HTS magnets, and there are possibilities of observing degradation of superconducting properties and/or mechanical damages by thermal stresses. As a new cooling technology to enhance the heat removal characteristics in HTS magnets, the cryogenic OHP is proposed to be imbedded in magnet windings. The feasibility of cryogenic OHP has been confirmed by fabricating proto-types and by observing stable operations using hydrogen, neon and nitrogen as the working fluid. A high thermal conductivity was achieved that surpasses those of high-purity metals. We also propose a modified-type OHP to mitigate the orientation dependence.

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.

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-

Influence of Resonance Phenomenon on Voltage Distribution in Central Solenoid of JT-60SA

T_{\rm c}

Commissioning of the JT-60SA helium refrigerator

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.

Performance of the JT-60SA cryogenic system under pulsed heat loads during acceptance tests

In the large-scale superconducting magnet, a nonuniform voltage distribution is present due to the resonance phenomenon resulting from inductance and capacitance. To investigate this phenomenon, a central solenoid (CS) model coil (four-layer) was prepared and the frequency dependence of the voltage in different coil turns was measured. Simulation was also performed for a CS model coil in order to determine the effect of factors such as the relative permittivity and the electrical conductivity of the insulation between the conductors. The results indicated that both the maximum voltage in the turns and the frequency at which resonance occurs strongly depend on the relative permittivity, whereas only the maximum voltage depends on the conductivity. Based on this simulation, we investigated the resonance phenomenon of JT-60SA CS. Consequently, the maximum voltage between layers at the resonance frequency is about 2 times higher than that at 5 kHz, and the maximum voltage between turns is about 20 times higher, so there is a possibility that the insulation between the turns is damaged at the resonance frequency. These results, therefore, represent important information for the safe operation of the JT-60SA.