Characterization of transport properties variations with magnetic field and temperature of ITER-candidate NbTi strands

Abstract

While the International Thermonuclear Experimental Reactor (ITER) conceptual design retained the Nb 3Sn for toroidal field (TF) and central solenoid (CS) coils, the low working field (around 6 T) promoted the choice of NbTi for the poloidal field (PF) coils. EU has carried out the experimental study of industrial NbTi strands and cables with different internal structures in order to choose the one which generate the lowest losses when used in the PF operating conditions (i.e. pulsed field). CEA has contributed to this project through the experimental study of the transport properties variations with respect to magnetic field and temperature of two candidate strands. One of them contains an internal CuNi barrier and the other is Ni-plated. A homemade cryostat is used to control the temperature of the sample which is wound on a VAMAS-like mandrel. J c measurements are presented here and subsequently the parameters deduced from scaling laws and their variation with temperature between 4.2 and 7 K and with field up to 11 T. A comparison between the two strands characteristics and ITER PF coils criteria is also discussed. The results are in good agreement with literature and lie inside an acceptable range in spite of some discrepancy with the ITER PF criteria: a recent thermo-hydraulic simulation confirmed it. In the future, this study, completed by AC losses measurements on cabled strands, should help to optimise the strands performances below the ITER PF security margins.