Tomone Suwa

Impact of Cable Twist Pitch on

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.

T_{cs}

The ITER central solenoid (CS) is a highly stressed magnet that must provide 30 000 plasma cycles under the ITER prescribed maximum operating conditions. To verify the performance of the ITER CS conductor in conditions close to those for the ITER CS, the CS insert was built under a USA-Japan collaboration. The insert was tested in the aperture of the CSMC facility in Naka, Japan, during the first half of 2015. A magnetic field of up to 13 T and a transport current of up to 60 kA provided a wide range of parameters to characterize the conductor. The CS insert has been tested under direct and reverse charges, which allowed a wide range of strain variation and provided valuable data for characterization of the CS conductor performance at different strain levels. The CS insert test program had several important goals as follows. 1) Measure the temperature margin of the CS conductor at the relevant ITER CS operational conditions. 2) Study the effects of electromagnetic forces and strain in the cable on the CS conductor performance. 3) Study the effects of the warmup and cooldown cycles on the CS conductor performance. 4) Compare the conductor performance in the CS insert with the performance of the CS conductor in a straight hairpin configuration (hoop strain free) tested in the SULTAN facility. 5) Measure the maximum temperature rise of the cable as a result of quench. The main results of the CS insert testing are presented and discussed.

-Degradation and AC Loss in

Japan Atomic Energy Agency (JAEA) is procuring 100% of the ITER Central Solenoid (CS) conductors. The CS conductor is required to maintain the performance under 60000 pulsed electromagnetic cycles. JAEA tested two internal-tin Nb3Sn conductors for the CS at the SULTAN test facility. As a result of destructive examination, the twist pitches of both of the cables satisfied requirements of the ITER Organization (IO). The current sharing temperatures Tcs of each sample were 6.6 and 6.8 K before cyclic operation, and the Tcs values were 6.8 and 6.9 K after 9700 electromagnetic cycles, including three warm-up/cooldowns, respectively. The Tcs performance of both samples satisfied the IO requirement. The ac losses of CSKO1-C and CSKO1-D were approximately half of typical bronze-route CS conductors at 2 and 9 T. The ac loss at 45.1 kA after the cycling was 1.5 times higher than that without the transport current. An almost constant strain of the jacket was observed after the test as a result of the residual strain measurement. Therefore, the deformation of the cable might have been homogeneous along the conductor axis. Because of the higher Tcs of CSKO1-D than CSKO1-C, JAEA started the manufacturing of the CS conductor with the same specification as CSKO1-D.

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

The optimization of the heat treatment of Nb3Sn conductors for toroidal field coils in ITER was attempted to improve the current sharing temperatures (Tcs). Using the strand, we chose the pattern at 570°C for 250 h and 650°C for 100 h as the best, which increased the critical current and maintained the residual resistivity ratio higher than 100. The behavior of the critical current of the strand vs. the magnetic field, temperature, and strain was also improved. This pattern was used on two conductors, and their performances were tested. Tcs was evaluated over 1000 electromagnetic cycles and one thermal cycle. A sharp Tcs degradation occurred at 50 cycles. Then Tcs decreased linearly. Although this tendency was similar to the conductors that were heat treated with the original pattern, the degradation rates were improved. The ac losses (Q) before cycling were approximately 10% lower than those of the original pattern. Q after cycling became almost equivalent between two patterns. The conductor was inspected after the test, which showed that the conductor under the high-magnetic-field zone had contracted by approximately 600 ppm during the test. Some clearly deformed strands were observed under the high-magnetic-field zone, which could degrade Tcs.

Conductors for ITER Central Solenoids

We describe herein the characteristics of a Nb3Sn cable inserted into a conduit (cable-in-conduit conductor) for the International Thermonuclear Experimental Reactor toroidal field (TF) coil and central solenoid (CS). During insertion, the pulling force almost linearly increases as a function of the length Ii of cable is inserted. The slope of these curves for the CS cables are approximately 74% that for the TF cable, although the mass per unit length of the CS cable is approximately 63% that of the TF cable. Thus, friction between the CS cable and the conduit is slightly greater than that between the TF cable and the conduit. The number Np of rotations at the cable point for the TF cable increases to 50 almost linearly versus Ii. For Ii <; 150 m, Np for the CS cables also increases almost linearly with a slightly greater slope than for the TF cable. However, the slope decreases, and Np becomes constant at 30 for Ii ) 600 m. During compaction, the number Nt of rotations at the tail of the TF cable, the 613-m-long CS cable, and the 918-m-long CS cable increases almost linearly versus compacted cable length to 23, 36, and 69, respectively. The X-ray transmission imaging of the CS conductor clarifies the distributions of the fifth-stage twist pitch of the cable (Ip) over the entire length of the conductor. These results are consistent with a geometric analysis based on Np and Nt. The results for Ip peak at the cable point; thus, a sample of the conductor should be taken from the point to investigate how Ip elongation affects conductor performance.

ITER Central Solenoid Insert Test Results

The Japan Atomic Energy Agency is responsible for procuring all amounts of central solenoid (CS) conductors for International Thermonuclear Experimental Reactor, including CS jacket sections. The conductor is cable-in-conduit conductor with a central spiral. A total of 576 Nb3Sn strands and 288 copper strands are cabled around the central spiral and then wrapped with stainless steel tape whose thickness is 0.08 mm. The maximum operating current is 40 kA at magnetic field of 13 T. CS jacket section is circular in square type tube made of JK2LB, which is high manganese stainless steel with boron added. Unit length of jacket sections is 7 m, and 6400 sections will be manufactured and inspected. Outer/inner dimension and weight are 51.3/35.3 mm and around 100 kg, respectively. Since the CS conductor suffers 60000 cycles of high electromagnetic force in the lifetime, severe requirements were specified for jacket sections in terms of not only high mechanical performance at 4 K but also of the size of initial defects in the jacket section. The minimum allowable defect size is estimated to be 2 mm2 × 0.2 mm by linear elastic fracture mechanics. Eddy current test (ECT) and phased array ultrasonic test (PAUT) were developed for non-destructive examination. The defects on inner and outer surfaces can be detected by ECT. The defects inside the jacket section can be detected by PAUT. These technologies and the inspected results of more than 700 jacket sections are reported in this paper.

Analysis of Internal-Tin Conductors for ITER Central Solenoid

The International Thermonuclear Experimental Reactor (ITER) magnet system is composed of four kinds of large superconducting magnets. Those magnets are manufactured by using cable-in-conduit conductors (CICCs). In the production process of the CICCs, the superconducting cable is inserted into a stainless steel jacket. In a previous study, the rotation of the cable was observed during the insertion. Moreover, the twist pitch of the cable was elongated by the rotation. The influence of the fifth stage twist pitch on the conductor performance is smaller than the lower stage twist pitch. However, longer twist pitches could restrict good current transfer in the joint region in the production of ITER coils. It was difficult to observe the behavior of the twist pitch elongation in the whole length of the cable, because the cable was covered by the jacket. In this study, the dummy cable was inserted into the jacket with windows to investigate the fifth stage twist pitch, rotation, and torsion of the cable in whole length. After insertion, it was found that the cable was untwisted and twisted at the point and tail of the cable, respectively. Moreover, the distribution of the twist pitch can be estimated from the rotation of the cable at the point and the entrance of the jacket during the insertion of the dummy cable. The fifth stage twist pitch distribution can be evaluated from both rotations in the production of the central solenoid conductors.

Optimization of Heat Treatment of Japanese

The superconducting property of Nb3Sn strands is very sensitive to strain. The transverse electromagnetic loading has been considered as a major origin of the degradation of Nb3Sn cable-in-conduit conductors (CICCs) due to the local bending. Since the bending pitch is around 5 mm due to contacting of strands compacted by the electromagnetic transverse loading, there is a possibility of a large bending strain with small deflection of strands. The bending strain of the strands cannot be evaluated from only the small deflection obtained visually. Measuring bending strain of Nb3Sn strand in CICCs is important for evaluating the conductor performance. Neutrons, which have a large penetration depth, are a powerful tool to evaluate the internal strain of Nb3Sn in the CICC. This paper shows that the bending strain in Nb3Sn strands of CICCs can be determined by the neutron diffraction profile nondestructively and quantitatively.

\hbox{N}{ \rm b}_{3}\hbox{S}{\rm n}

The ITER Central Solenoid (CS) is composed Nb3Sn cable-in-conduit conductor. Short twist pitch cable design is appropriate for the CS cables to maintain the current sharing temperature under electromagnetic loading cycles. In the production process of the short twist pitch CS cable, Nb3Sn strands, and Cu strands are cabled tightly and compressed to get circular shape. However, these processes induced strong interstrand contact force and the strands deformation. The strand performances are degraded by the severe indentation on the strands at the contact point between the strands, although the strands are deformed before the heat-treatment. Therefore, influence of the indentation on critical current and residual resistive ratio were investigated in two types of bronze-route and an internal-tin Nb3Sn strands which are used for the CS cable, in order to determine the threshold of indentation depth in the strands. Also, transverse cross-sectional observation by Electron Probe Micro Analyzer was carried out on the indented strands to investigate the damage in the cross-section. Before August 2016, 37 CS cables were manufactured. On 37 CS cables manufactured before August 2016, numbers of indentations on the Nb3Sn strands were investigated to confirm whether there was indentation on the Nb3Sn strands which degrades the strand performance.