Fumiaki Tsutsumi

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

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.

Impact of Cable Twist Pitch on

The Japan Atomic Energy Agency (JAEA) has developed jacketing technologies for ITER Toroidal Field (TF) and Central Solenoid (CS) conductor. Full scale TF and CS conduits were fabricated using carbon‐reduced SUS316LN and boron‐added (∼40 ppm ) high manganese stainless steel (0.025C ‐22Mn ‐13Cr ‐9Ni ‐0.12N: JK2LB), respectively. Welding condition was optimized so that back bead does not interfere a cable insertion. The weld joint samples were compacted by a compaction machine that was newly constructed and tested at 4.2 K. Mechanical characteristics at 4K of CS, TF conduits and CS welded joint satisfied ITER mechanical requirements. TF welded joint shows slightly lower value of 0.2% yield strength (885 MPa) than that of ITER requirement (900 MPa). The TF conduit contains nitrogen content of 0.14%, which is minimum value in ITER specification. The lower nitrogen content may be caused by the release of nitrogen from molten metal during non‐filler welding resulting in a 4 K strength decrease. To satisfy the ITER requirements, minimum nitrogen contents of conduit should be increased from 0.14% to 0.15% at least. Therefore, JAEA successfully developed TF and CS conduits with welding technologies and finalized the procurement specification for ITER conductor jacketing.

T_{cs}

In March 2010, the Japan Atomic Energy Agency (JAEA) was the first to start the mass production of toroidal field (TF) conductors among the six parties who were procuring TF conductors in the International Thermonuclear Experimental Reactor project. The height and width of the TF coils are 14 m and 9 m, respectively. The conductor is a cable-in-conduit conductor with an operating current of 68 kA. A circular multistage superconducting cable is inserted into a circular stainless steel jacket with a thickness of 2 mm. A total of 900 Nb3Sn strands and 522 copper strands are cabled around the central spiral and then wrapped with stainless steel tape whose thickness is 0.1 mm. The superconducting cables are inserted into the jacket assembled using the automatic butt Tungsten Inert Gas welding technique. Cable insertion is one of the key technologies in the jacketing process because the gap between the inner surface of the jacket and the outer diameter of the superconducting cable is only 2 mm in diameter. It was observed that the cabling pitch of the destructive sample is longer than the original pitch at cabling. JAEA carried out the tensile tests of the cable and the measurement of the cable rotation during the insertion to investigate the cause of the elongation. The cause of elongation was clarified, and the results are described in this paper.

-Degradation and AC Loss in

The Japan Atomic Energy Research Institute developed a jacket material called JK2LB (0.03C-22Mn-13Cr-9Ni-1Mo-0.2N-B) for the Central Solenoid (CS) conductor in the International Thermonuclear Experimental Reactor (ITER). To demonstrate production feasibility of CS conductor jacket, trial fabrication of a full size jacket using hot extrusion followed by cold drawing of the JK2LB billets was performed. As a result of dimensional measurement, the ITER dimensional requirement for circle-in-square jacket has been achievable. We achieved the requirement of 0.2% yield strength >1000 MPa and KIC(J)ges130 MParadicm for solution treated + aged jacket. It has been observed that applied cold work strongly affects the toughness of jacket before and after aging. We estimate that C and N reduction will be required to achieve the required strength and fracture toughness for ITER CS jacket. The fabrication R&D has prepared us for mass production of jacket for the ITER CS conductor procurement

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

The design and manufacture of Nb3Sn conductors for ITER toroidal field (TF) coils have many technical challenges. Although it was demonstrated in the ITER model coil project that the conductors have a sufficiently high performance and the engineering design is valid, unexpected issues arose. Through both theoretical and experimental efforts improved conductors were developed. The Japan Atomic Energy Agency started to procure improved conductors for TF coils as part of the ITER project. Because the required tonnage of Nb3Sn strands is quite large compared with past experience and the required superconducting performance is higher than that of the model coils, quality control techniques are very important for the successful manufacture of the strands. Approximately 60?ton of Nb3Sn strands have been successfully completed under a severe quality control regimen and all strands meet ITER specifications. This paper summarizes the technical developments leading to the first successful mass production of ITER TF conductors.

Conductors for ITER Central Solenoids

This paper provides qualification test results of large forgings and thick hot rolled plates of austenitic stainless steels, newly developed JJ1 (0.03C-12Cr-12Ni-10Mn-5Mo-0.24N), and 316LN containing high nitrogen of 0.2%, to be used in coil cases of the ITER toroidal field coils. The distributions of tensile properties at liquid helium temperature (4K) in products, average strengths with standard deviations at 4K, temperature dependence of strengths are evaluated to qualify the materials and to establish a reasonable quality control method to be applied to mass production materials for the ITER toroidal field coils. It is also demonstrated that temperature dependence of strengths are expressed by quadratic curves, which are expressed as a function of carbon and nitrogen contents and strengths at room temperature.Copyright

Cabling Technology of

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.

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

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.