The turn-to-turn contact resistance (characteristic resistance) is a very important factor to a no-insulation (NI) (RE)Ba2Cu3Oy (REBCO) pancake coil. It characterizes the stability and charging delay of NI REBCO pancake coils. However, the ordinary sudden-discharging method cannot measure the turn-to-turn contact resistance under different conditions, such as an operating current, a temperature, and an external magnetic field. Although the turn-to-turn contact condition is strongly affected by the pressure inside of the NI REBCO coils, the feature of the contact resistance must be clarified to estimate the stability of NI REBCO coils.In this paper, we have proposed a new method to measure the turn-to-turn contact resistance applying AC current. The theory and the measurement results are also shown. The measured turn-to-turn contact resistance is reasonable, compared with that measured by the ordinary sudden-discharging method. In near future, we will measure the turn-to-turn contact resistance under different conditions by means of the proposed method.Index Terms-No-insulation winding technique, REBCO pancake coil, turn-to-turn contact resistance measurement.

No-insulation (NI) (RE)Ba2Cu3Oy (REBCO) magnets are very promising for practical applications to generate high magnetic field. The NI winding technique provides high thermal stability so that quench protection is unnecessary. However, the turn-to-turn contact resistance (sometimes called characteristic resistance) is an important factor to characterize the thermal stability and charging delay of NI REBCO pancake magnets. Although the conventional sudden-discharging method is widely used to measure the turn-to-turn contact resistance, it is not applicable to various conditions; e.g., the change of DC operating current and temperature.Therefore, we have previously proposed a turn-to-turn contact resistance measurement method using a low-frequency AC (LFAC) current. In this paper, we measured the contact resistance for different DC operating currents or change of DC current to confirm the validity of LFAC method. In addition, by heating a test coil, the contact resistances were also measured during the change of DC operating current. Since the electromagnetic stress is small enough not to change the contact condition, the change in contact resistance is negligible. The measured contact resistance was unchanged, so we could conclude that the LFAC method is effective in measurement of the contact resistance when DC operating current was applied.

The characteristics of no-insulation hightemperature superconducting (HTS) manet is dominated by the magnitude of the turn-to-turn contact resistance. A few techniques have been proposed to increase the turn-to-turn contact resistance. The relation between the turn-to-turn contact resistance and the magnet stability has also been investigated in simulations. Although the turn-to-turn contact resistance measurement was only the sudden-discharging method before, we proposed a lowfrequency-AC current (LFAC) method. Using our proposed method, it is possible to measure the turn-to-turn contact resistance under various conditions. In a previous paper, we measured it when charging DC current to a single pancake coil. In this paper, the turn-to-turn contact resistance was measured by the LFAC method when an external field of 1-3 T was applied to a single pancake coil. The coil strain was also measured, and the relation of the turn-to-turn contact resistance and the coil strain was also discussed.

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