Modelling analysis of pre-stress effect on upper critical magnetic field and critical current of Nb3Sn composite wire

An analytical model to describe the variations of the upper critical magnetic field and critical current at 4.2K of the superconducting multifilamentary Nb3Sn composite wire caused by the application of pre-stress at room tempearture was presented and applied to experimental results. Main results are summarized as follows. (1)The variations of the upper critical magnetic field and critical current could be described well as a function of pre-stress by combining the Ekin's scaling law with the elastic/plastic mechanical calculation. (2)It was demonstrated experimentally and theoretically that the upper magnetic filed and critical current can be improved by the pre-stressing treatment through the control of residual strain of Nb3Sn. (3)The strength distribution of the Nb3Sn filaments was estimated for the first time by extracting the influence of breakage of Nb3Sn filaments from the change in critical current of pre-stressed composite. (4)The procedure to predict the relation between critical current at 4.2K and pre-stress at room temperature for engineering-scale long samples was presented based on the present model. An example of the application indicated that the permissible overall pre-stress on composite, below which all Nb3Sn filaments transport current without breakage, is reduced by approximate to 20 % for long specimens of 300m in comparison with that for the short specimens of 25mm, while the average strength of Nb3Sn for 300 m is reduced by approximate to 60 % from that for 25mm.