Development of cable-in-conduit conductor for ITER CS in Japan
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The National Institutes for Quantum and Radiological Science and Technology has the responsibility to develop a cable-in-conduit conductor (CICC) for the ITER central solenoid (CS). Qualification tests of CICCs fabricated in the initial development stage were carried out at the SULTAN test facility; the superconducting performance (Tcs = current-sharing temperature) was found to be degraded by the repeated cyclic loading that simulates realistic ITER operating conditions. From destructive examination and neutron diffraction tests, this degradation appears to result from bending strain on the strands generated by electromagnetic forces. In response, the cabling of the CICC was optimized by shortening the twist pitch to make it stiffer against electromagnetic forces. No Tcs degradation of the optimized CICC was seen in the subsequent SULTAN test; further, a CS insert (CSI) test was performed at the CS model coil test facility, which included hoop strain for a more realistic simulation of ITER conditions. Good performance was also achieved in the CSI test.
KeywordsITER Cable-in-conduit conductor Superconducting magnet system Central solenoid
The National Institutes for Quantum and Radiological Science and Technology (QST), acting as Japan’s domestic agency in ITER, has responsibility to develop the CS conductor [5, 6]. There are 49 conductors, each having the length of several hundred meters, which are to be produced for the CS. The operating scenario for the CS is pulsed operation that induces 15 MA of plasma, with 30,000 repetitions and a burn duration of 400 s ; the CS conductor must therefore survive severe repeated cycles of electromagnetic force (EM). A requirement of the CS conductor is no degradation of Tcs from the EM cycles.
In 2010 and 2011, qualification tests of the CS conductor initially developed were performed, showing unexpected degradation of Tcs in the EM cycles. Investigations were undertaken to understand this degradation [8, 9]; one of the most effective techniques was neutron diffraction analysis. This analysis clearly showed strand deformation  as the cause of the Tcs degradation; consequently, the authors decided to optimize the conductor design to prevent this deformation. The status of this work is described in this paper.
To re-design the conductor [11, 12, 13], the twist pitch of the cable was shortened. It was expected that a shorter twist pitch configuration would reduce the strand deformation under EM cycles; however, this optimization produced another problem: dented strands during the cabling process. To overcome this issue, QST improved the cabling process by applying empirical criteria to the dented strand ; an overview is presented in this paper.
In 2012, a qualification test of the optimized conductor with a shorter twist pitch was performed and showed no degradation of Tcs with EM cycles . With this success, series production of the CS conductors could begin, and in parallel, to confirm the performance of the optimized short twist pitch conductor under conditions which more precisely duplicate the ITER CS operation, a central solenoid insert (CSI) was tested in 2015. The results satisfied all the CS requirements with a sufficient margin for Tcs . The status is also described in this paper.
2 ITER CS conductor
3 Initial design of the CS conductor
Major parameters of the CS conductor initial design
(2SC + 1Cu) × 3×4 × 4×6
Inner cable diameter
49 mm × 49 mm
4 Consideration of the degradation
Researchers have tried to understand this unexpected strand deformation in the LLZ [9, 22]; one mechanism was assumed: strand buckling. In this mechanism, thermal compressive forces originally acted on strands inside the conduit as a result of differences between the thermal contraction properties of materials in the conductor, operating during the heat treatment and cool-down process. On the other hand, a large void was generated in the LLZ because EM loading pushed strands forward toward the HLZ. Since the large void allows strands to move easily, strands were compressed and then buckled by the thermal compressive forces, and as a result, strand buckling occurs in the LLZ .
5 Optimized conductor design
6 CS insert test
As the CSI is a single-layer solenoid, strain induced by the circular geometry (hoop strain) can be included in the evaluation of the CS conductor performance, which is not possible in the straight-line conductor samples in the SULTAN test. In addition, CSMC can generate an external field up to 13 T. Thus, the purpose of the CSI test is to evaluate Tcs under real CS operation (i.e., SOD conditions: 13 T of external field and 40 kA of current) with hoop strain. An additional objective was to compare the results to the SULTAN tests. Therefore, the CSI tests also included the SULTAN conditions of 11.5 T of external field and 45 kA of current. The requirement criteria for Tcs are 5.2 K and 6.5 K in the SOD and SULTAN conditions, respectively, as discussed above.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Human and animals rights
No human participant and animal were involved in this study.
- 6.Nabara Y et al (2016) Performance analysis of mass-produced Nb3Sn conductor for central solenoid in ITER. IEEE Trans Appl Supercond 26(3):4200705Google Scholar
- 8.Bruzzone P et al (2012) Test results of ITER conductors in the SULTAN facility. In: Proceedings of 24th IAEA Fusion Energy Conference IAEA CN-197, p 536Google Scholar
- 13.Nabara Y et al (2014) Impact of cable twist pitch on Tcs degradation and AC loss in Nb3Sn conductors for ITER central solenoids. IEEE Trans Appl Supercond 24:4200705Google Scholar
- 14.Takahashi Y et al (2014) Cabling technology of Nb3Sn conductor for ITER central solenoid. IEEE Trans Appl Supercond 4:4802404Google Scholar
- 16.Martovetsky N et al (2016) ITER central solenoid insert test results. IEEE Trans Appl Supercond 26(4):4200605Google Scholar