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Large-current-capacity high-temperature superconducting (HTS) conductors using YBCO tapes are being considered as an option for the LHD-type fusion energy reactor FFHR. The typical operating current, magnetic field, and temperature of such conductors in FFHR are 100 kA, 13 T, and 20 K, respectively. A preliminary design of the HTS conductor has been proposed for the FFHR helical coils. Analyses have been performed on the proposed HTS conductor regarding thermal properties, mechanical structures, AC losses, and quench detection and protection. It is suggested that stainless steel might be a better choice for the outer jacket of the HTS conductor compared to aluminum alloy. Due to increased specific heats of conductor materials at 20 K, HTS magnets are supposed to be operated more stably compared to low-temperature superconducting (LTS) magnets operated at ∼4 K. The required refrigeration power is also reduced. Therefore, using HTS conductors, it is considered to be viable to assemble the continuous helical coils in segments with joints of conductors, as additional heat generation at the joints can be taken care by utilizing the surplus refrigeration power. According to these analyses, HTS conductors seem to be promising for the FFHR coils.

It is important to evaluate the local strain exerted on superconducting filaments in Nb3Sn strands, because it influences both superconducting and mechanical properties, in particular for the ITER (International Thermonuclear Experimental Reactor) project. The local strain in the twisted and untwisted Nb3Sn strands was directly measured at room temperature as well as at low temperatures by means of quantum beam techniques. The local strain consists of thermal strain and lattice strain. The latter changes as a function of external strain. The interrelation between the force-free strain and the intrinsic strain showing a maximum critical current was considered on the basis of the present experimental data as well as the recent theory. The thermal strains along both directions parallel and transverse to the strand axis were numerically evaluated. Their evaluated results could explain well the observed values, when To is the recovery temperature of pure Cu. The force-free strain along the axial direction is deduced to be distributed among grains with different crystal orientation with respect to the axial direction. It is suggested that this fact affects the definition of intrinsic strain.

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Several conductor samples were fabricated and tested in the SULTAN facility at CRPP for ITER Central Solenoid (CS) conductor qualification. From the result of the cyclic testing on the first and second conductor samples named CSJA01 and CSJA02, continuous linear degradation of the current sharing temperature (T cs ) was found. From the result of the visual inspection, a large deflection on the lower loading side (LLS) in the high field zone (HFZ) was observed. The bending strain of the strands cannot be evaluated from only the deflection obtained visually. To evaluate the strain of strands in CSJA01 quantitatively, a neutron diffraction measurement of the CSJA01 left leg was performed using the engineering materials diffractometer 'Takumi' in J-PARC. From the result, the large bending strain at the LLS in the HFZ was found. Therefore, the T cs degraded position in the conductor sample due to the cyclic testing can be determined.