Compressive pre-strain in Nb3Sn strand by steel tube and effect on the critical current measured on standard ITER barrel

The large Cable-In-Conduit Conductors (CICC) designed for the magnet coils in the International Thermonuclear Experimental Reactor (ITER), are composed of Nb₃Sn strand bundles in a Stainless Steel (SS) or Incoloy conduit. Both, the thermal contraction of the strand composite and the conduit material, define the final pre-strain after cooling down and thus affect the strand critical current (I_c). The transverse forces, introduced when charging a coil, in addition affect the overall strain state due to bending and pinching of strands. Recently, periodic bending tests were applied on strand samples without additional axial compressive pre-strain. Here we explore the method of swaging a SS-tube around a strand to imitate the cool-down strain effect of the conduit. The experimental results of the I_c measurements at 12 T and 4.2 K are presented for a Nb₃Sn PIT strand with and without swaged SS tube on both Ti-6Al-4V and SS standard ITER sample holder (barrels). The effect of gluing the sample to the barrel is also investigated. The intrinsic strain state of the samples is verified by measurement of the I_c versus applied strain with the Pacman spring.