Exploring the limits of a very large Nb₃Sn conductor: the 80 kA conductor of the International Thermonuclear Experimental Reactor toroidal field model coil

Abstract

In a phase II experiment on the International Thermonuclear Experimental Reactor (ITER) toroidal field model coil (TFMC) the operation limits of its 80 kA Nb 3 Sn conductor were explored.To increase the magnetic field on the conductor, the TFMC was tested in the presence of another large coil: the Euratom LCT coil. Under these conditions the maximum field reached on the conductor was around 10 T.This exploration has been performed at constant current, by progressively increasing the coil temperature and monitoring the coil voltage drop in the current sharing regime.Such an operation was made possible thanks to the very high stability of the conductor.The aim of these tests was to compare the critical properties of the conductor with expectations and to assess the ITER TF conductor design. These expectations are based on the documented critical field and temperature dependent properties of the 720 superconducting strands which compose the conductor.In addition the conductor properties are highly dependent on the strain, due to the compression appearing on Nb 3 Sn during the heat treatment of the pancakes and related to the difference in thermal compression between Nb 3 Sn and the stainless steel jacket. No precise model exists to predict this strain, which is therefore the main information which is expected from these tests.The method to deduce this strain from the different tests is presented, including a thermohydraulic analysis to identify the temperature of the critical point and a careful estimation of the field map across the conductor.The measured strain has been estimated in the range −0.75% to −0.79%. This information will be taken into account for ITER design and some adjustment of the ITER conductor design is under examination.

Cable-in-conduit conductors: lessons from the recent past for future developments with low and high temperature superconductors

“…Historically, the evolution of CICC technology and its main development steps have been extensively discussed within a list of papers [19][20][21][22][23][24] but, to our opinion, time is mature enough for a further update on the state-of-the-art of this technology, that has undertaken a series of important steps in the last years. We limit our considerations to those aspects that in our view were subject to major changes and clarifications in the recent past, thus not addressed in a comprehensive way by previous reviews.…”

“…We limit our considerations to those aspects that in our view were subject to major changes and clarifications in the recent past, thus not addressed in a comprehensive way by previous reviews. In fact, our aim is not to go back over the whole history of CICC development, but we refer to the papers listed above [19][20][21][22][23][24] for all design features that are only marginally recalled or not even discussed here.…”

Coupled mechanical–electromagnetic–thermal–hydraulic effects in Nb3Sn cable-in-conduit conductors for ITER

“…The nature of the problem at hand is intrinsically multiphysics, as the critical current carried by the cable will depend on the magnetic field, thermal field (temperature) and mechanical field (strain) distributions. A lot of work has been devoted in the past to the analysis of the DC performance of the TFMC (Savoldi et al 2002, Heller et al 2003, Zanino et al 2003b, Zanino and Savoldi Richard 2003, Duchateau et al 2004, Zanino et al 2004, Ulbricht et al 2005, although the assessment of the latter was originally beyond the scope of this experiment. Probably as the most important outcome of this previous work, the reduction in performance (albeit quantitatively dependent on the strand scaling used) of a Nb 3 Sn conductor at increasing electromechanical load (I × B) was reported for the first time and confirmed also in the case of the ITER Central Solenoid Model Coil (Zanino et al 2003a).…”

Review of Nb3Sn conductors for ITER

“…Indeed, a lower 'n' leads to a lower effective magnetic field B eff in the conductor (due to the field gradient across the cable) [8], also a low 'n' means that it is possible to operate slightly above the current sharing temperature T cs without much excess of Joule heating, as observed during the model coil tests. On the other hand, a low 'n' means a non-negligible electric field does exist even at the operating temperature T op , 1 K below T cs .…”