Effect of Thermal Loading on Nb3Sn CICC Performance

The ITER CICC will undergo a number of cool-down and warm-up cycles over the lifetime of the plant. The standard SULTAN based ITER conductor qualification test normally includes one thermal cycle in the test sequence. In many samples a performance degradation was observed following this thermal loading. In order to investigate the effect of multiple thermal cycles on the TF conductor short sample, additional repeated thermal cycles to liquid nitrogen temperature were carried out on the left leg of the CNTF3 sample and the JATF5 sample. Thermal cycles using SULTAN are very time consuming, about four days, with a corresponding cost of around 32 kEuro. Ten thermal cycles will give an estimation of the degradation upon repeated thermal loading, but would require a prohibitive amount of time in SULTAN, and therefore cause a significant delay in the testing of other time critical samples. As a large fraction of the change in thermal contraction occurs between room temperature and liquid nitrogen temperature, a purpose made facility and program was developed. This ad-hoc facility allowed faster, more cost effective thermal cycles that crucially did not interfere with SULTAN's ongoing test program. Cooling and heating was provided by means of forced flow nitrogen. The sample was contained within a vacuum to prevent the formation of moisture or ice. During warm-up, a heater distributed around the CNTF3A was also used. These cycles were performed both before and after electromagnetic loading. The results of these tests indicated that thermal loading before the first electromagnetic load cycle did not result in a worsening of the conductor performance. The tests following repeated thermal cycling after electromagnetic loading show a thermal cycle causes a performance degradation but a large number of consecutive thermal cycles do not appear to have a significant effect.