The study of Cu fraction influence on Nb/sub 3/Sn strand for ITER performance

Abstract

Ahsfmet--In framc of Se.C.R.E.T.S. international task Nb3Sn strands with 50 nnd 60 % of Cu fraction have been produced and romprrativc research of their performance (Q, I,, Jt, RRR, etc.) have bccn carricd out. Strand design and processing wcre identical. However, thc dccrcasc of Cu fraction vrluc has influenced on RRR value as well us Cr-phted and barc strands. By method of subsequent approximntion the estimntion of diffusion coefficients of Cr into Cu by R275IR20 values of both strands have been carricd out. These calculations have shown that Nb,Sn strand after renction at the standard regime (575'C 150 11 f 65OoC 200 h at lo-' Torr) characterized the value of diffusion cocfficicnt -lo-" cm%, that has led to Cr penetration into oxygen free Cu in not more than 40 pm of depth. Thc cnlcuhtions of RHR intcgral vsliic vcrsus reaction timc and Cu thickness have also been done. The relation of diffusion coefficient and microstracture of Cu-stabilization has been discussed.

Influence of heat treatment excursion on critical current and residual resistivity ratio of ITER Nb3Sn strands

“…It is known that RRR of Cr plated strands decreases with increasing heat treatment time [19][20][21][22][23]. In order to verify that Cr layer is the main contributor to RRR degradation, the Cr layer is removed from a set of x from Cr/Cu interface and at time t can be calculated by a complementary error function [19],…”

“…This is a very challenging measurement, due to extremely high detection sensitivity needed. Previous measurements by laser mass spectrometer [20], and high sensitivity EDS [23] only had limited success.…”

“…Moreover, it has been reported that Residual Resistance Ratio (RRR) of Cr plated ITER strands decrease with increasing heat treatment time [19][20][21][22][23]. This has been attributed to Cr diffusion into copper stabilizer.…”

“…This is a very challenging measurement, due to the extremely high detection sensitivity needed. Previous measurements by laser mass spectrometer[20], and high sensitivity EDS[23] had only limited success.In order to demonstrate the validity of our RRR calculation code, a few additional sets of RRR versus heat treatment time data are plotted in figure…”

Effect of Interfaces and Cr Diffusion on Stabilizing Cu Conductivity in Nb₃Sn-Strands

“…However, it is well known that after the prolonged annealing aimed at the solid-state diffusion formation of the superconducting Nb 3 Sn phase due to interaction between Nb filaments and bronze matrix the RRR values decrease considerably and demonstrate high spread [2]. In [3][4][5] the RRR drop at the diffusion annealing of the Cr-plated Nb 3 Sn-starnds is attributed to Cr diffusion. However, other factors affecting the residual resistance of stabilizing Cu must be taken into account.…”