Hidemasa Ozeki

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

Under the International Thermonuclear Experimental Reactor (ITER) project, the Japan Atomic Energy Agency (JAEA) is procuring all of the Nb3Sn conductors for the Central Solenoid (CS). The CS consists of six vertically stacked modules. The height and outer diameter of the CS are approximately 13 m and 4 m, respectively. The CS has a circular five stage cable. All of approximately 43 km of Nb3Sn CS cables will be manufactured in Japan. Before mass-production start, the jacketed cable conductors should be tested in the SULTAN facility in Switzerland to confirm their superconducting performance. The original cabling design had relatively long twist pitches and is referred to as the normal twist pitch (NTP) conductor. The NTP conductor test results revealed decreasing the current sharing temperature (Tcs) with increasing number of electro-magnetic (EM) load cycles. Therefore, a short twist pitch (STP) design was proposed and the STP conductors were also tested. The STP conductor results showed that the Tcs is stable during EM cyclic load tests. Because the conductors with STP have a smaller void fraction in the cable area than those with NTP, a higher compaction ratio during cabling is required and the possibility of damage on strands increases. The STP cable technology was developed in collaboration among Japanese cabling suppliers and JAEA. Several key technologies will be described in this paper.

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

The Japan Atomic Energy Agency is responsible for the procurement of the central solenoid (CS) conductor for ITER. The CS conductor uses a circular-in-square type sheath tube (jacket, outer dimension: 49 mm × 49 mm square, inner diameter: 32.6 mm) made of high manganese stainless steel JK2LB. The CS jacket suffers high electromagnetic force with 60,000 cycles during its lifetime. Prior to mass-production of CS jacket sections for the ITER machine, production of 170 units of jacket sections has been carried out. During production, it was confirmed that dimensional and mechanical performances satisfy the ITER requirements. The nondestructive examination procedure for the CS jacket sections has been developed. As a result, mass-production processes of CS jacket have been established.

Conductors for ITER Central Solenoids

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.

ITER Central Solenoid Insert Test Results

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.

Cabling Technology of

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.

Conductor for ITER Central Solenoid

The performance of two mass-produced Nb3Sn conductors for the ITER central solenoids was tested for the first timmass-produced Nb3Sn conductorse. One was cut from the forward end of an 80-m-long conductor, and the other was cut from the forward end of a 918-m-long conductor. The fifth-stage twist pitches of these conductors were lengthened by approximately 16% during cable insertion and compaction with conduit. The current sharing temperatures Tcs were measured over 20 000 electromagnetic cycles, including four thermal cycles between 4.2 K and room temperature. The Tcs of the former conductor increased and became almost constant through the cycling. In contrast, Tcs of the latter conductor not only increased but also decreased slightly against cycling. The Tcs decline rate after 10002 cycles was -4.50 × 10-6 K/cycle. If this rate is assumed to continue after 20000 cycles, Tcs would decrease by -0.27 K over 60000 cycles. Even so, Tcs is higher than an acceptance criterion of 6.5 K at 60 000 cycles. The ac losses Q of both conductors at a current of 0 kA were almost the same as or slightly lower than Q of a short sample conductor whose fifth-stage twist pitch was not lengthen. On each conductor, Q at 40 kA was approximately 10% higher than that at 0 kA; thus, the effect of the transport current on Q was not large.

Establishment of Production Process of JK2LB Jacket Section for ITER CS

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.