Y. Nabara

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.

Characterization of ITER

Japan Atomic Energy Agency has developed four types of strand which can be used in the ITER TF coils. One is a strand made by an internal tin process strand and the others are bronze process strands. The achieved critical current density is more than 790 in the bronze process strands and more than 980 in the internal tin process strand under 4.2 K temperature and 12 T magnetic field and there is hysteresis loss of less than 770 mJ/cc under 3 T cycle. Since these strands are utilized with an external strain, it is necessary to evaluate strain dependency to confirm the ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring to produce uniform axial compressive or tensile strain along the strand length, a strand being soldered on the outer surface of the ring. The detailed strand characteristics were investigated subjecting the developed strands to a magnetic field from 10 T to 13 T, a strain from about 0.8% to 0.5%, and a temperature from 4.2 K to the critical temperature. When the critical current is normalized to that under the conditions where strain is intrinsically zero, the bronze process strands exhibit better performance than the internal tin process strand. However, the three bronze process strands do not exhibit the same -strain characteristics. Two types of scaling relations are applied to the data, and good expressions of strand performance were obtained by the least square method within 3 A as RMS.

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

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.

Strands Under Strain-Applied Conditions

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.

Examination of Japanese Mass-Produced

The influence of the expected Lorentz loading and time dependent operating conditions of a magnet on the conductor AC loss is experimentally simulated by a cryogenic cable press that applies cyclic mechanical loading. A series of ITER conductor tests with the press have commenced and we report on the results from the first set of two TF conductors, which have the option-II cabling scheme but consist of NB3Sn strands from different manufacturers. With the press, we apply a transverse load of 578 kN/m and the load cycle is repeated up to 30,000 times. As a function of load cycles, we measure the cable mechanical stiffness, interstrand contact resistances, and the coupling loss. When compared with a previously measured option-II type conductor, the present conductors have higher initial losses. However, they showed greater cable displacement and larger increase in contact resistance with load cycles. This is due to the lower cable stiffness thought to be related to the lower axial strand stiffness, resulting in greater cable displacement than the previous cable. Consequently, the two conductors tested here have lower losses already within the first few cycles.

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

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.

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

Japan Atomic Energy Agency has successfully developed Nb3Sn strand which fulfills ITER requirements. Because Nb3Sn is very susceptible to external strain which reduces critical current, critical temperature, and critical field, it is necessary to evaluate strain dependency of these Nb3Sn strands to confirm an ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring and a strand is soldered on its outer surface. This shape produces uniform axial compressive or tensile strain along strand length by expanding or closing the opening of the ring. Critical current can be measured by the apparatus under a magnetic field up to 15 T, a temperature range of 4.2 K to 15 K, and strains. The maximum allowable current is about 300 A. The details of the apparatus and results of strand characterization are presented.

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

Stability and quench analyses of the ITER TF coils are performed with a combination of three computer codes to obtain an accurate prediction. A one-dimensional code ldquoGandalfrdquo with an adaptive mesh is used mainly for these analyses. The overall thermohydraulic analysis is performed with a quasi three-dimensional code ldquoVINCENTArdquo and the boundary conditions are provided to Gandalf. In quench analyses, the two-dimensional ANSYS model was used to estimate the thermal diffusion from the heated conductor to the cold radial plate. The analyses show that the TF coils have sufficiently high minimum quench energy (around 300 mJ/cc-strand) against the mechanical disturbance and the disturbance due to a plasma disruption. The maximum temperature obtained in the quench analysis is allowable, for the condition of the detection voltage of 0.2 V, delay time of 2 s and the discharge time constant of 11 s. It is confirmed that the coils will be operated with reasonable margin and discharged without any damage if an unexpected quench occurs.