Exploring the limits of a very large Nb_3Sn conductor the 80 kA conductor of the ITER Toroidal Field Model Coil

摘要

In Phase II experiment of the International Thermonuclear Experimental Reactor (ITER) Toroidal Field Model Coil (TFMC) the operation limits of its 80 kA Nb_3Sn conductor were explored. To increase the magnetic field on the conductor, the TFMC was tested in presence of another large coil: the Euratom-LCT coil. Under these conditions the maximum field reached on the conductor, was around 10 teslas. 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 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_3Sn during the heat treatment of the pancakes and related to the differential thermal compression between Nb_3Sn 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 thermohydraulical 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.