Change of interstrand contact resistance and coupling loss in various prototype ITER NbTi conductors with transverse loading in the Twente Cryogenic Cable Press up to 40,000 cycles

Abstract

The multistrand NbTi conductors for the Poloidal Field (PF) Coils of the International Thermonuclear Experimental Reactor (ITER) are subjected to heavy transverse loading due to the Lorentz forces in the coils. The current in the multistage Cable-In-Conduit Conductors (CICC) exceeds 50 kA and the magnetic field reaches up to more than 6 T for a few tens of thousands of pulses. The large transverse forces, accumulating from strand to strand over the cable cross-section, cause a severe deformation of the cable bundle inside the conduit and this goes along with electromagnetic, mechanical, and thermohydraulic effects. In order to study the electromagnetic and mechanical behaviour in more detail, a Cryogenic Cable Press is build to simulate the effect of the Lorentz forces on a conductor comparable to the present design for ITER in magnet operating conditions. The magnetisation of the conductor (and from this the coupling loss expressed in nτ) and the interstrand resistance ( R c) between various strands and strand bundles inside the cable can be measured along the loading history, starting at virgin condition and accordingly subjected to various loads. The results, all obtained on eight full-size ITER type NbTi conductor samples with a variety of cable strand layout and coatings, are reported here. A consistent correlation is found between the experimental AC loss and interstrand contact resistance ( R c) results. It is also observed that there is a strong impact of cyclic loading on the AC loss and R c which may change up to orders of magnitude. The variation of the AC loss due to transverse cyclic loading of CICC conductors in ITER coils can be accomplished by reducing the void fraction. The results point out that cyclic loading with a significant number of cycles, sufficient to reach a saturation after having passed the peak transverse resistance, should be included in next tests on large NbTi CICC's and PF Model Coils as the AC loss and ability of current sharing among strands will vary along the loading history.