Hideki Kajitani

Test Results and Investigation of Tcs Degradation in Japanese ITER CS Conductor Samples

Japan Atomic Energy Agency (JAEA) has fabricated and tested the four conductor samples composed of high performance strands manufactured by the bronze-route process for the ITER Central Solenoid (CS) conductor. The current sharing temperature (Tcs) electrically assessed at 45.1 K and 10.85 T along the cycling loading at 48.8 kA and 10.85 T initially were 6.0 K and 6.1 K, and then 5.3 K and 5.5 K after 6000 cycles for the first SULTAN sample named JACS01, respectively. As results of second SULTAN sample named JACS02, the Tcs values initially were 7.2 K and 6.8 K, and then 6.6 K and 6.1 K after 10000 cycles for each conductor, respectively. The Tcs degradation was not saturated at the end of the test campaign. From the destructive observation, the large bending at the low transverse loading side in the high field zone was observed. The strand buckling and accumulating by slipping between the cable and the jacket are considered.

Examination of Japanese Mass-Produced

The performances of six Nb3Sn conductors for the ITER Toroidal Field coils were tested. Four of them showed similar degradation rates of their current sharing temperatures Tcs over 1,000 electromagnetic cycles. By contrast, two of them showed sharp Tcs degradations at 50 cycles, after which their slopes became similar to those of the other four conductors. These two cables seemed to shrink under high magnetic fields during the first 50 cycles, which caused the sharp Tcs degradation. This shrinkage might arise from a decline in cable rigidity due to, for example, the deformation of strands or the breakage of the Nb3Sn filaments. The four mass-produced conductors had roughly the same AC loss before cycling. After 1,000 cycles, the AC losses of all the conductors decreased markedly to less than half of those before cycling, and the values became approximately the same. After the test campaign, the destructive inspection of two of the conductors made it clear that the conductor had shrunk by about 520 ppm under the high magnetic field during the test. It was also clarified that some strands were visibly deformed under the high magnetic field, whereas those under the low magnetic field did not look distorted. This plastic deformation of the strands could be one of the major reasons for the Tcs degradation with cyclic operation.

{\rm Nb}_{3}{\rm Sn}

The performance of four Nb3Sn conductors for the ITER central solenoids was tested. The current sharing temperatures (Tcs) were measured over approximately 9000 electromagnetic cycles, including two or three thermal cycles between 4.2 K and room temperature. Tcs increased and became almost constant through the cycling. The gradient of the electric field against the temperature gradually decreased against cycling. The degradations caused by the electromagnetic force of the short twist pitch conductors were smaller than that of the original twist pitch conductor. The ac losses of short twist pitch conductors were several times higher than that of original twist pitch conductor. The dents and the removals of the Cr plating on the strands, which were formed during cabling, decreased the electric resistance between strands, which may cause the observed high ac loss. Inspection of the cable showed neither a clear bias of cable in the cross-sectional surface nor distorted strands in the lateral face. The high rigidity of the short twist pitch cable could prevent these plastic deformations, caused by the Lorentz force.

Conductors for ITER Toroidal Field Coils

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 (Tcs) 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 Tcs degraded position in the conductor sample due to the cyclic testing can be determined.

Impact of Cable Twist Pitch on

We have fabricated (Sr,K)Fe2As2 superconducting wire through a powder-in-tube method that is combined with a hot isostatic pressing technique. The transport J c in the wire at 4.2 K has reached 1.0 × 105 A cm−2 under self-field, and 9.4 × 103 A cm−2 at 100 kOe. This is the first report that the transport Jc reaches 1 × 105 A cm−2 in a (Sr,K)Fe2As2 round wire. Magneto-optical imaging of the wire has confirmed the large intergranular critical current density in the wire core. The microstructure and compositional analyses imply that there is room for improvement of the Jc value through the optimization of the wire fabrication process.

T_{cs}

The Japan Atomic Energy Agency (JAEA) and Mitsubishi Electric Corporation are fabricating the insert coil needed to evaluate the performance of final design conductor for the International Thermonuclear Experimental Reactor (ITER) Central Solenoid (CS). The insert, designed by the US ITER Project Office, is a nine-turn single-layer solenoid of 1.5-m diameter. Major operations, such as winding, terminal fabrication, heat treatment, and turn insulation, have thus far been successfully completed. Fabrication will be completed in September 2014, and testing will begin at JAEAs CS Model Coil test facility in early 2015. The results of qualification and fabrication of the insert are reported.

-Degradation and AC Loss in

The evolution of the superconducting properties of round wires of (Ba,K)Fe2As2 fabricated by the powder-in-tube (PIT) method is systematically studied. After establishing the method to obtain the largest transport critical current density (J c) in round wires using the hot isostatic press technique, we investigated how the transition temperature (T c), J c, and microstructures change at each step of the wire fabrication. Unexpectedly, we find that superconducting properties of the wire core are significantly damaged by the drawing process. Systematic measurements of J c and T c of the core superconductor after each drawing and sintering process clarified the evolution of degradation by the drawing process and recovery by heat treatment.

\hbox{Nb}_{3}\hbox{Sn}

The performance of two conductors for the ITER central solenoids was tested. The current sharing temperatures were measured over 17 050 electromagnetic cycles, including four thermal cycles between 4.2 K and room temperature. declined almost linearly over the 10 000 rated electromagnetic cycles. was nearly constant for 70% of the rated electromagnetic cycles, which implies the existence of a fatigue limit in the conductors. For 85% of the rated cycles, a very sharp degradation of approximately 0.2 K occurred. Some type of large deformation of strands, such as buckling, may have caused this sharp degradation. The effective strain degraded linearly with the electromagnetic force on the cable. The gradient after 10 000 cycles was 1.5 times greater than that before cycling. After 10 000 cycles, the ac losses of both conductors considerably decreased to less than half of those before cycling. These ac losses before cycling were less than a fourth of those of toroidal field conductors. After the test campaign, destructive inspection of the conductor clarified that on average, the distribution of residual strain along the cable was almost uniform at 32 ppm. It was also clarified that some strands were visibly deformed under a high magnetic field, whereas strands under a low magnetic field did not appear to be deformed. The deformations of the central solenoid cable were larger and wavier in subcables than those observed in the toroidal field cable. This plastic deformation of the strands could be one of the major reasons for the degradation during cyclic operation.

Conductors for ITER Central Solenoids

Critical current of cable-in-conduit conductors for ITER TF coils was measured using a pair of short cable-in-conduit conductors, which are electrically connected from each other at the bottom joint. It was found from these test results that the measured critical current was lower than that evaluated from the critical current performance of a single strand. One of the explanations for this phenomenon is a nonuniform current distribution due to local degradation caused by strand buckling. To study the influence on the conductor performance, the author developed a new analysis model for the calculation of bending strain due to buckling and then, combined this with the electrical circuit model, which consists of lumped and distributed circuits for the conductor and upper/bottom joints, respectively. Simulation results show that when local degradation exists, nonuniform current distribution is established. This indicated that conductor performance can be degraded by local degradation such as strand buckling.

Neutron diffraction measurement of internal strain in the first Japanese ITER CS conductor sample

The Japan Atomic Energy Agency (JAEA) has a responsibility to procure nine International Thermonuclear Experimental Reactor (ITER) toroidal field (TF) coils and 19 TF coil cases. The JAEA started sub- and full-scale trials to qualify and optimize the manufacturing procedures of TF coils and TF coil cases from August 2012. The major outcomes of these trials are development of an automatic winding system with high accuracy in conductor length measurement ±0.01%, evaluation of the conductor elongation of about 0.06% due to heat treatment, demonstration of the manufacturability of a full-scale radial plate with high accuracy, and establishment of the TF coil case manufacturing procedure satisfying tight tolerances. The JAEA thus started the series production of TF coils from the end of 2013 and TF coil cases from the beginning of 2014. Until now, 11 double-pancake (DP) windings were completed, and seven DPs were heat treated with elongation of the conductor from 0.063% to 0.077%, which are well consistent with the expected values. In addition, three main bodies of the basic segments of TF coil cases are being assembled. These successful achievements indicate good progress in the procurement of ITER TF coils.