A. Sagara

H. Hashizume

H. Noguchi

K. Natsume

N. Yanagi

S. Ito

S. Tomida

T. Mito

Y. Terazaki

National Institute for Fusion Science (NIFS-Repository)

Conceptual design studies of the LHD-type heliotron fusion DEMO reactor FFHR-d1 are being carried out at NIFS through domestic and international collaborations.The FFHR-d1 is four times bigger than LHD and the major radius of the helical coils is 15.6 m [1]. As a counter option to the Nb3Al cable-in-conduit conductor (CICC) that is supposed to be the primary selection for the FFHR magnet based on the ITER technology, the segmented-fabrication method using the High-Temperature Superconducting(HTS) conductors has been proposed [2, 3]. In this study, 10-kA class YBCO conductors having the joint section were fabricated. Then, the measurements of joint resistances have been carried out to verify the feasibility of the segmented fabrication of the helical coils of FFHR with the HTS option.To make joints for the FFHR helical coils, two methods are proposed: a soldered lap joint and a mechanical lap joint. The soldered lap joint method joins conductors by applying solder and it is considered that a low joint resistance can be obtained. Whereas, the mechanical lap joint method joins conductors by applying compressive forces on the joint section temporarily and then the welding of the outer stainless-steel jacket is done. In this method, though the joint resistance may become higher, the construction process of the FFHR helical coils will be further eased.For the 10-kA class sample with a soldered lap joint, Fig. 1 shows the measured joint voltage as a function of the sample current. In Fig. 1, the joint resistance is evaluated to be 20.2 n� and the contact resistivity is 109.5 n�cm2. Using this value, the overall joint resistance of a 100-kA classconductor is expected to be 1.1 n�, consisting of 2 connections at one joint location with 40 HTS tapes, each having a 50 mm joint length and the width of the tape 10 mm. As the entire helical coils will have ~8000 joints (~400 turns of windings in each coil, 10 segmented conductors for a toroidal turn and 2 coils), this requires ~5.2 MW increase of electrical power for the refrigeration system at room temperature, assuming the coil operation temperature at 20 K. This seems to be acceptable since the required electrical power for the entire FFHR refrigeration system is supposed to be ~30 MW in case of the CICC options. For the 10-kA class sample with a mechanical lap joint, Fig. 2 shows the contact resistivity as a function of the compressive load which is expressed as the conductor thickness in the present experiment. It is found that the contact resistivity decreases as the load increases, however,the minimum value so far is observed at 4.9 ��cm2(resistance: 406 n�). According to the previous study by A.J. Dietz et al. [4], the contact resistivity for the YBCO single tape is 200-40 n�cm2 by applying a pressure at 40-200 MPa. In another study by S. Ito et al. [5], the contact resistivity of a two-layer joint of BSCCO tapes is twice or three times higher than that of a single layer joint because of the misalignment of HTS tapes. Therefore, the joint resistivity measured in the present experiment can be explained by the misalignment of 14 tapes under insufficient loads. Presently, the soldered lap joint seems more feasible than the mechanical lap joint.Fig. 1 Joint voltage of the 10-kA class soldered lap joint as a function of current.Fig. 2 Contact resistivity obtained by the 10-kA class sample in mechanical lap joint as a function of theconductor thickness.[1] A. Sagara et al., to be published in Fusion Eng. Des. (2012).[2] G. Bansal et al., Plasma Fus. Res. 3, S1049 (2008).[3] N. Yanagi et al., Fusion Sci. Tech. 60 (2011) 648.[4] A.J. Dietz et al., IEEE Trans. Appl. Supercond. 18, 1171 (2008).[5] S. Ito et al., to be published in IEEE Trans. Appl. Supercond.  Improvement of superconducting magnets for high magnetic field, heat load and electromagnetic force is an important subject to achieve the early realization of nuclear fusion power generation. HTS (high temperature super- conducting) magnets may achieve higher magnetic fields and higher stability against a coil quench with less operation cost compared to low temperature superconducting magnets. The conduction cooling method, which is a promising candidate for a fusion magnet, can build the magnet more rigid and simplify the system and the operation However there is a difficulty on removing heat generated in the deep portion of large scale HTS magnets operated by conduction cooling method. The thermal diffusivity of each component materials such as copper or aluminum alloy decreases as the operating temperature increases. Hence, when a part of the windings turns into the normal-conducting state, large temperature gradients are easily produced in magnets, which could cause degradation of superconducting properties and mechanical damages by excess thermal stresses. A new method which thin cryogenic oscillating heat pipes (OHPs) are embedded into the inter-space of coil windings as heat transfer devices has been suggested in order to improve the efficiency of indirect/conduction cooling of HTS magnets[1]. OHPs are highly effective heat transfer devices which can transport several orders of magnitude greater heat flux than the heat conduction of solids and be formed in a thin plate structure. Fig. 1 shows the schematic detail of the experimental set-up. The OHP assembly is placed in a vacuum chamber of a cryostat. The installation angle of the OHP can be changed. The pressure gauges are installed in the buffer tank and the filling pipe, which are used to control the volumetric filling ratio of the working fluid in the OHP and to monitor the pressure oscillation during the operation of the OHP. The performance of cryogenic OHPs has been intensively examined by this measurement apparatus[2][3]. The typical effective thermal conductivity has been observed to be > 10 kWm-1K-1 at 20 K. As a reference, the thermal conductivity of Cu with RRR: Residual Resistivity Ratios, = 100 at the magnetic field of 1T and 20 K is about 2,000 Wm-1K-1. A modeling using the Karman number, the Prandtl number and the Jacob number has been attempted as the semi-empirical correlations[4]. Using a modified model and the data from cryogenic OHPs experiments, a correlation has been formulated for the heat flux in OHP, which is stated as follows:  q = 2.61Ka0.05Pr0.77Ja-0.97(exp(β))0.42            (1)  Eq. (1) is used to fit a total of 30 data sets by means of multi-regression analysis. It is considered that this modelling with non-dimensional quantities is useful for the design of cryogenic OHPs. In Fig. 2, the resulting given by Eq. (1) and the experimental data are compared. The cryogenic OHPs used by being imbedded in superconducting magnets as a heat transfer device has been demonstrated. The high heat transport properties of the cryogenic OHPs have been experimentally confirmed. We consider that it is possible to dramatically improve the performance of HTS magnets by using cryogenic OHPs.  Fig. 1  Experimental set-up of a series of performance evaluation experiments of cryogenic OHPs. 1: cryogenic OHP, 2: Cu bus bar, 3: GM cryocooler, 4: filling pipe, 5: isolation valve, 6: buffer tank, 7: pressure gauges 8: gas cylinders 9: vacuum pump, 10: radiation shield, 11: cryostat. Fig. 2  Comparison of Eq. (1) with experimental results. In legends, 1/8in. and 1/16 in. indicate the outer diameter of pipes and a=+90 and a=+45 mean the inclination angle of OHP orientations from horizontal direction.  1) T. Mito, K. Natsume et al: IEEE Transactions on Applied Superconductivity 20 (2010) 2023-2026. 2) T. Mito, K. Natsume et al: IEEE Transactions on Applied Superconductivity 21 (2011) 2470-2473. 3) K. Natsume, T. Mito et al: Cryogenics 51 (2011) 309-314. 4) K. Natsume, T. Mito et al: IEEE Transactions on Applied Superconductivity (to be published) P1P23214567891011N Ne HEvaporator2 2Condenser249§17. Measurement of the Joint Resistance of Large-Current YBCO ConductorsTerazaki, Y. (Grad. Univ. Advanced Studies),  Yanagi, N., Noguchi, H., Natsume, K., Mito, T., Tomida, S. (Kagawa National College of Technol.),  Ito, S., Hashizume, H. (Tohoku Univ.),  Sagara, A.

Measurement of the Joint Resistance of Large-Current YBCO Conductors

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Measurement of the Joint Resistance of Large-Current YBCO Conductors

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