New Inspection Method of Termination Resistance at Room Temperature for ITER TF Coil

It is important to achieve sufficiently low joint resistance (RJ), as 3 nQ with a background field of 2 T and nominal current of 68 kA to reduce the loss by Joule heating during operation, for interdouble-pancake joints of ITER TF coil termination. However, a large cryostat is required to cool down TF coil during R
_J
measurement at 4 K because of huge mass and size of TF coil. Authors, therefore, developed an inspection method, which enables to detect some differences in R
_J
at room temperature, while it is difficult to do in nano-ohm range. R
_J
of TF coil termination is mainly determined by contact resistance between cable and copper sole of the joint box (R
_c
), and contact resistance between copper sole soldering. This paper first focuses on development of inspection method for R
_c
. R
_c
affects current penetration length (CPL) between the cable and copper sole. We thus assumed that when a joint is fabricated with proper process, i.e., R
_c
is sufficiently low, the behavior of CPL is repeatable. On the other hand, when there is some abnormality in the manufactured joint, we can find irregular behavior on CPL compared to the normal repeatable behavior. Thus, we propose an inspection method in which a current of 20 A is injected in the joint and the voltage profile along the joint is measured at room temperature. As a result of the measurement, good repeatability of CPL could be shown in actual all ITER TF joints tested, while a significant difference of CPL was observed in a joint sample which includes some disturbances.