On the Evaluation of Current Sharing Temperature Within Conductors of ITER Toroidal Field Coils in Operation

Abstract

The nineteen ITER Toroidal Field (TF) coils (including a spare one) consists of seven double pancakes (DP) of cable-in-conduit conductors (CICC), each containing 900 superconducting Nb₃Sn strands. In total eight strand manufacturing companies were involved, either relying on the bronze route or internal tin process. As a result, the strand, and thus conductor, performance can vary significantly within the same TF coil. This peculiarity inherent to the ITER project makes the evaluation process of coils stability, linked to the current sharing temperature (Tcs), more delicate. Following an extensive conductor testing campaign carried out over several years within the SULTAN facility, degradation of the different TF conductors was observed along electromagnetic and thermal cycling. Relying on the ITER critical current scaling law and associated parameters for TF strands, effective strain properties and n-values were extracted from the SULTAN Tcs measurements. In parallel, recent simulations conducted with MULTIFIL (mechanical code) on TF CICC have shed some light on the strands mechanical state after cool-down and along electro-magnetic cycling, highlighting the importance of the bending strain. Capitalizing on these results, on the SULTAN database, and on the literature (already mentioning the bending strain effect on conductor degradation), we aim at refining the existing methodology of performance scale-up from strands to conductors by introducing the bending strain. Ultimately, we apply the model in an attempt to conciliate SULTAN Tcs measurements and MULTIFIL mechanical results.