Current-sharing Temperature Characteristics of ITER Central Solenoid Insert Coil

The performance of the ITER central solenoid insert (CSI) conductor was tested in 2015. The current-sharing temperatures (Tcs) were measured over 16,000 electromagnetic cycles, including three thermal cycles between 4.2 K and room temperature. Tcs under the initial magnetization (IM) condition (13 T, 40 kA) of the CSI conductor not only increased, but also decreased between 6.71 and 6.84 K against cycling; then Tcs became almost constant at 6.74 K. Thus Tcs under the IM condition, was approximately 1.5 K higher than the specification of 5.2 K throughout the test. The slope of the hoop strain (εhoop) on the CSI conductor against the electromagnetic force was 1.55×10-4 % m/kN (in εhoop > 0) and 1.39×10-4 % m/kN (in εhoop < 0). Taking the effect of εhoop into account, the Tcs of the CSI under the SULTAN simulated condition (11.5 T, 45.1 kA) was equivalent to that of the SULTAN test after around 10,000 cycles. Before around 10,000 cycles, especially at the initial charge, the Tcs of the SULTAN test was lower than that of the CSI test. It is assumed that the hoop strain in the CSI test accelerated a strain relaxation, which increased the Tcs from the initial charge. When the strain fully relaxed and Tcs stopped increasing after around 10,000 cycles, the Tcs of the SULTAN test became equivalent to that of the CSI test. Given this perspective, the CSI test and SULTAN test were consistent. In εhoop > 0, the absolute value of the effective strain (εeff) of the CSI test decreased (i.e., Tcs increased) against the electromagnetic force (Fr) because the effect of the positive εhoop on the increase in Tcs exceeded the effect of the Fr on the decrease in Tcs. The line of εeff −εhoop of the CSI test against Fr was nearly symmetric about the y-axis (Fr=0). Comparing the εeff −εhoop of the CSI test and the εeff of the SULTAN test, the slopes of the strain against Fr were almost the same between the CSI test and SULTAN test before cycling. The εeff of the SULTAN test became close to the εeff−εhoop of the CSI test after cycling. This CSI test demonstrated that mass-produced CS conductors are highly capable of being used in the ITER.