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ENEA - Fusion division

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Nb3Sn Material

Contact: Luigi Muzzi
e-mail: muzzi@frascati.enea.it
tel.: +39 06 9400 5391
fax.: +39 06 9400 5393

In the frame of R&D activities related to ITER, we have participated, in the last years, to an intense activity for the qualification of industrially produced Nb3Sn strands with advanced performances (Advanced Strands). Such activity has been launched by European Fusion Development Agreement (EFDA) in order to stimulate Europe regarding the production expertise for Nb3Sn wires to be used for the ITER toroidal field and central solenoid coils.
During the experimental campaigns of the strand qualification , we have performed electric (critical current, resistance), magnetic (magnetization) and structural features (diameter, Cu:non-Cu ratio) measurements . The results have shown that the developed strands satisfy ITER’s last requirements, with transport current not lower than 200 A (at 12T, 4.2K), a Cu:non-Cu ratio of about 1, with histeresys losses lower than 1000 kJ/m3 of superconductor during a standard cycle of magnetic field: ±3T a 4.2K.

SEM image of multifilament Nb3Sn strands produced with “advanced” technique>SEM image of  multifilament Nb<sub>3</sub>Sn strands

Another activity, carried out in collaboration with the French CEA organization concerned the study of new advanced Nb3Sn strands mechanical properties , with particular attention to the influence of bending strain on the strands transport properties. The bending strain, caused by the geometry of the strands inside the conductor, plays an important role in lowering the conductor performances of in the operative regimes. At a strand level, the bending causes processes of current transfer among the filaments inside of it, from the region of the strand which undergoes the highest strain to that at lower strain, and the general effect on the transport properties depends on strand geometric features and on the capability to redistribute current inside it. Moreover, in the conductors, the bending strain acts on strands overlapping to a uniform longitudinal strain, caused by a higher thermal contraction of the steel jacket.

In order to test the strands in regimes as similar as possible to the operative conditions of ITER toroidal field coils, a technique has been developed whose principal footsteps are described as following:

  1. the strand is jacketed in a thin steel compacted pipe;
  2. the strand is wound on a cylindrical sample holder, on which the thermal treatment of Nb3Sn formation is carried out;
  3. the strand is transferred, at room-temperature, on a different steel sample holder, with a different diameter respect to the first one.

As result of this procedure the filaments of Nb3Sn are submitted to a strain bending uniformly distributed on relevant lengths of conductor.

The transport properties have been measured either on the bare strain, before being inserted in the steel pipe and before the application of the bending strain, and on the jacketed strand.
For this last one, the characterisation has been done either before the application of the bending, and after the transfer process (step c), i.e. after the application of the bending.
From the comparison of the results it has been deduced that the percentage degradation in the critical current due to the application of the longitudinal strain (due only to the action of the steel jacket ) is between 55% at 14T and 37% at 10T. As far as the additional effect of the bending is concerned, a degradation of 25% results at 14T which decreases lowering the applied magnetic field.

The system has been later modelled with finite elements codes and the comparison of the experimental results with the available theoretical models allowed to obtain important information about the process of current transfer inside the strands.