The use of Nb₃Sn in fusion: lessons learned from the ITER production including options for management of performance degradation

Abstract

The overall 30 year Nb 3 Sn conductor program for ITER provides an unusual opportunity to look at the issues created by the application of novel Nb 3 Sn technology on a large scale. ITER design criteria have evolved to make use of the features of the industrialized material, exploiting advantages as well as managing the disadvantages. There are lessons from the successes and failures to be learned for the future application of very high current Nb 3 Sn strands in fusion and high energy physics. The behaviour of Nb 3 Sn strands in the large compound ITER conductors is still producing surprises after 25 years of development, industrial production and testing, with the Nb 3 Sn strain sensitivity and brittleness producing many more subtle design impacts than originally foreseen. The results now obtained at the end of the ITER production suggest that, as a novelty for Nb 3 Sn, there are options for conditioning the strands in the ITER conductors that can be applied during commissioning and which could reduce the impact of brittleness by as much as one third.

Superconductors for fusion: a roadmap

“…Similar methods to those already used in the early days of ITER [16] could stimulate DEMO engineering in the magnet area and link these two activities. There are interesting new proposals regarding compact high-field tokamaks as pilot fusion plants, which would not rely on ITER superconducting technology, but the lessons learned from ITER show how improvements can be made in the engineering layout of a tokamak fusion reactor.…”

“…However, R&D progress or breakthroughs in several fields, e.g. target loads and physics or materials, would have a big impact and open up the available design space 16 .…”

“…In [118], we can see that this coil was operated in a steady condition at a few tens of millikelvins below T cs , and then quenched going above this value. For Nb 3 Sn CICC with a low n-index (below 10 [16,119]), the situation is very different as the coil can be operated even above T cs in a kind of semi-resistive mode [16]. In this case, only the cooling power available is limiting the maximum operating temperature.…”

“…This is likely to also be valid for an HTS conductor with low index value. Due to the issue of Nb 3 Sn CICC degradation management [16], the ITER project revised the initial conductor criteria [120] in this way but a posteriori. It is important to note that even large Nb 3 Sn that does not experience degradation also has a low transition index.…”

Electromechanical behaviour of REBCO coated conductor toroidal field coils for ultra-high-field magnetic-confinement plasma devices

“…Ultra-high magnetic fields can decrease particle drifting and disruption, and significantly increase the possibility of fusion reactions. The international thermonuclear experimental reactor (ITER) has a fusion energy gain factor of 10 and aims to achieve a peak magnetic flux density of 11.8 Tesla by using low temperature superconducting (LTS) Nb 3 Sn conductors [2]. However, things may change with the emergence of the second-generation high temperature superconducting (HTS) rare-earth barium copper oxide (REBCO) coated conductor, a technique that was not introduced when the ITER project was launched but which has reached a certain degree of maturity so that there are now tens of manufacturers worldwide [3].…”