Shape memory effect of cable-in-conduit conductors

Abstract

In fusion magnets, cable-in-conduit conductors (CICCs) are foreseen, consisting of an outer conduit and an inner niobium-tin cable produced by a simultaneous heat treatment. The heat treatment and the different thermal contractions of the various conductor materials lead to prestress onto the niobium-tin filaments during cool down, degrading the superconducting parameters considerably. These parameters show, during external mechanical loading, a maximum dependence on strain (E). For example, the critical current (I/sub c/) of steel jacketed CICCs increases by about a factor of 2 at /spl epsi//spl ap/0.7% and decreases again at further straining (B=13 T, T=4 K). This characteristic is reversible up to /spl epsi//spl ap/1.2% in stressed and unstressed condition of the conductor. If the conductor is plastically deformed at 4 K up to a remaining strain (/spl epsi//sub r/) and then warmed up to 295 K, /spl epsi//sub r/, decreases by about a factor of 5, which is coupled to a phase transition resulting in a change of volume in the microstructure of the steel conduit. This reduced /spl epsi//sub r/ remains constant after cooling to 4 K again (shape memory) and the critical current corresponds to this decreased /spl epsi//sub r/. In case of an accidental over-straining of a CICC, a further temperature cycle of 4 K/spl rarr/295 K/spl rarr/4 K would recover I/sub c/. Recovery strain results with different jacket materials (aged) of Type 316L, 316LN, Incoloy 908, and titanium are investigated.