Florida electro-mechanical cable model ofNb3Sn CICCs for high-field magnet design

Abstract

Performance degradation of Nb3Sn cable-in-conduit-conductors (CICCs) is a critical issue in large-scale magnet designsuch as in the International Thermonuclear Experimental Reactor (ITER) andthe series-connected hybrid (SCH) magnets currently under development at theNational High Magnetic Field Laboratory (NHMFL). The critical current Ic ofNb3Sn conductors is strongly affected by thermal pre-strain in strand filaments in a CICC fromdifferential thermal contraction between strands and conduit during cooling down afterheat treatment. Mitchell and Nijhuis recently introduced strand bending underlocally accumulated Lorentz force for the interpretation of observed transverseload degradation, defined as the Ic reduction due to strand bending and contactstress at strand crossing with respect to the expected Ic from strand data atthe thermal compressive strain. In this paper, a new numerical model of CICCperformance has been developed based upon earlier work by Mitchell and Nijhuis.The new model, called the Florida electro-mechanical cable model (FEMCAM),combines the thermal bending effects during cooling down and the electromagneticbending effects during magnet operation, as well as effects due to strand filamentfracture. We present the FEMCAM formulation and benchmark the results againstabout 40 conductor tests of first-cycle performance and 20 tests that include cyclicloading. We also consider the effects of different jacketing materials on CICCperformance. We conclude that FEMCAM can be a helpful tool for the design ofNb3Sn-basedCICCs and that both thermal bending and transverse bending play important roles in the performanceof Nb3Sn CICCs.