Cable-in-Conduit Conductors

Cable-in-Conduit Conductors (CICC) are a common form of superconducting cable used in large-scale applications. There are several varieties of CICCs, but they all consist of many small, stabilized superconducting wires contained within a conduit through which a coolant (typically supercritical He II) flows. Figure 1 shows the cross section of a number of different CICCs.

CICCs have many advantages. The large surface area on the fine wires, for example, provides good heat transfer with the coolant, resulting in good stability. This advantage allows the conductor to respond to large and varying heat loads while still superconducting. The conduits themselves can be made very strong, allowing the conductor to tolerate large electromagnetic forces, and the conduits may also easily be electrically insulated from each other, permitting high voltage differences between the adjacent conductors. Such advantages make CICCs particularly attractive for the large superconducting magnets used in fusion energy experiments.

CICC devices are characterized by varying electromagnetic fields and thus experience large forces and heat loads. In fact, CICCs have become the workhorse conductor for superconducting magnets in fusion experiments. Figure 2 provides a view of the ITER machine, currently under construction in France, showing the various magnet systems. All the coils use CICC. Other examples of fusion energy experiments that use CICCs include JT60SA in Japan and the SST-1 in India.

There are disadvantages to these conductors as well. Due to the fraction of space occupied by the coolant and conduit walls, the engineering current density (defined as the current carried divided by the conductor cross section) of CICCs is relatively low and the ability to create highly precise uniform magnet fields with CICCs is limited. This disadvantage means CICCs are unsuitable for applications such as accelerator magnets and MRI magnets, devices that tend to use other conductor options such as Rutherford Cable.

Researchers have created CICCs with both NbTi and Nb3Sn superconductors and work has started to develop high temperature superconductor (HTS)-based CICCs. A recent review of CICC work, including a discussion of HTS based CICCs, is given in L. Muzzi et al., “Cable-in-Conduit Conductors: Lessons from the Recent Past for Future Developments with Low and High Temperature Superconductors,” Superconductor Science and Technology, Vol. 28, 2015.

N. Koizumi presents a good overview of the ITER magnet system in “Progress of ITER Magnet Procurement,” Physics Procedia, Vol. 45, 2013. Manufacturing of CICCs is discussed in P. Lee (Ed.), Engineering Superconductivity, Wiley, 2001. R. Zanino et al. presents a study of the complex thermal-hydraulics associated with CICCs in “A Review of Thermal-hydraulic Issues in ITER Cable-in-Conduit Conductors,” Cryogenics, Vol. 46, 2006. For an example of the use of CICCs in applications other than fusion energy, see I.R. Dixon et al., “The 36-T Series-Connected Hybrid Magnet System Design and Integration,” IEEE Transactions on Applied Superconductivity, Vol. 27, No. 4, 2017. ■