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

TL;DR

This study investigates how heat treatment excursions affect the critical current and residual resistivity ratio of ITER Nb3Sn strands, finding that while critical current remains stable, residual resistance ratio decreases with higher temperature and longer duration.

Contribution

It introduces a diffusion-based model and numerical code to predict residual resistance ratio changes due to heat treatment variations in Nb3Sn strands.

Findings

Critical current remains stable despite heat treatment variations.

Residual resistance ratio decreases with higher temperature and longer heat treatment.

A diffusion model successfully predicts residual resistance ratio changes.

Abstract

Heat treatment is critically important to the performance of Nb3Sn superconducting strands. For very large Nb3Sn magnet coils, such as the International Thermonuclear Experimental Reactor (ITER) Central Solenoid (CS) coils, heat treatment carries risk of temperature and time excursion, which may result in performance degradation. Therefore, it is prudent to study the effect of possible excursion on Nb3Sn performance. In this study, Nb3Sn strands used for ITER CS coils are heat treated at different temperatures for different times. Their critical current, residual resistance ratio and hysteresis losses are measured. It is found that in the range we studied, critical current and hysteresis losses do not change significantly. Residual resistance ratio, however, decreases with increasing heat treatment temperature and time. This is attributed to the diffusion of metallic elements from the plated Cr layer to the copper stabilizer. Based on a model of metallic elements diffusion, a numerical code is developed to predict residual resistance ratio as a function of heat treatment temperature and time.