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Description
Joint resistances of eight short samples have been measured until 2018 as a qualification test for ITER Toroidal Field (TF) coils. The joint sample consists of two short TF conductors with the length of 1.535 m, and each conductor has two joint boxes at both ends. The lower joint is a testing part that is full size joint of the TF coil. The upper joint is connected to 100 kA current leads of a conductor test facility with 9 T split coils. Voltage taps are attached at three positions in the original conductor part of the length of 0.3 m between the two joint boxes. Hall probes are attached on the lower joint box at around the center of the external field coils to measure the shielding current. The voltage drops between L-leg and R-leg conductors were measured with six voltage taps per position per conductor while holding sample currents for three minutes at 1, 15, 30, 45, 60, and 68 kA. In a few cases, the voltage drops at 3, 6, 9, and 12 kA were additionally measured. The joint resistance of the lower joint is estimated from the increase of the average voltage drop among the six taps against the currents. In all the samples, the difference among the voltages of the six taps is enlarged to the range of 0.01 mV and saturated at less than 15 kA. In addition, the obtained data show that the joint resistance is increased with current at low current and converged to a certain value. Furthermore, decay time constant of shielding currents induced in the lower joint by shut-off of the external field coils is gradually elongated with decrease of the shielding currents, which suggests that the joint resistance is decreased at low current. With a simplified electric circuit model, these phenomena can be simulated by the assumption of existence of a strand contacted to the copper sleeve with extremely low joint resistance.
