Deformation Behaviour and Cabling Degradation of Nb3Sn Wires

• Simon Hopkins (CERN)

For the production of accelerator magnets, Nb3Sn wires are assembled in the form of Rutherford cables, a transposed strand configuration in which the wire is subjected to significant mechanical deformation. Cabling is time-consuming and costly, so for the qualification and acceptance testing of wire, uniaxial rolling is often used as a proxy for cabling deformation. The effect of this deformation on the internal structure of a wire before heat treatment, and its impact on the superconducting properties after heat treatment, strongly depends on the wire design. Cabling and rolling degradation has been widely studied for established wire types, especially the Restacked Rod Process (RRP®) wire produced by Bruker OST. In the context of CERN’s High Field Magnet (HFM) programme, the need has arisen to explore a broader range of wire designs: both commercially available wires of different designs or larger diameters procured for magnet development, and novel wires developed in collaboration with wire manufacturers ultimately working towards the target non-copper critical current density (Jc) of 1500 A mm-2 (16 T 4.2 K) defined for the proposed FCC-hh hadron collider. In this seminar, a comparative study of the deformation behaviour of several designs of RRP®, powder-in-tube (PIT) and distributed tin wire by electron microscopy and image analysis will be presented. The degradation of critical current (Ic) and residual resistance ratio (RRR) will be analysed, both statistically over large-scale procurement, and as a function of rolling reduction for individual spools, and correlations with wire designs identified. Based on these results, the test procedures and acceptance criteria for wire and cable qualification will be critically assessed, and the effects of wire design will be considered in the context of the prospects for future wire development.