Introduction
Rare-earth-based and Bi-based high-temperature superconducting wires have been intensively studied for use in power transmission cables and electric airplanes and so on in the high-temperature range of 65-77 K. Recently, there has been much interest in applications such as NMR and MRI in low-temperature super-high magnetic fields of 4.2 K and 20 T or higher, and in high-performance operation in the medium-temperature range of 20 K. In recent years, they have been attracting attention for applications such as NMR and MRI in ultra-high magnetic fields at low temperatures of 4.2 K and 24 T or higher, and for high-performance operation in the medium temperature range of 20 K or higher. In addition, the development of wire materials by various manufacturers has greatly improved the critical current Ic of wire materials in these operating regions, and a method for testing high current of more than several hundred A in the wire is necessary. To measure the Ic of wires with high characteristics, the following methods have been proposed: estimation from current-carrying characteristics using a small bridge[1], U-shaped sample holder[2], and pulse current[3]. So far, our group has developed an Ic measurement method using a DC constant current source and a precise voltage measurement, and now we have focused on the pulse current method.
In order to measure high-current Ic using pulsed current, the current sweep speed of the pulsed current is inversely proportional to the inductance of the measurement system, and the maximum current value is inversely proportional to the resistance, so we measured the inductance and resistance of various cables. Finally, we measured the Ic of commercial REBCO tapes in liquid nitrogen using the pulsed current.

Experimental method
The inductance per unit length of various cables was measured at a frequency of 1-100 kHz at room temperature using an LCR meter. The resistance was measured at room temperature using the DC 4-terminal method. The cables used were KIV38 mm2 twisted pair, Fujikura coaxial cable, Litz wire, and self-made coaxial cables. Using a pulsed current source, a pre-amplifier, and an oscilloscope, we measured the current-voltage characteristics of SuperPower's REBCO tape (SF4050-AP, 4 mm wide, Ic = 110 A) at 77 K self-field. The voltage of the current source was fixed as 10 V, the pulse width of the pulse current was 1-5 ms, the maximum current was 10-120 A, and the current sweep rate was of the order of A/μs. The gain of the amplifier was 1000, and a 3 kHz low-pass filter was used before amplification.

Experimental results and discussion
The twisted-pair cable had the highest inductance, and the self-made coaxial cable had the lowest. The resistance was highest for the Litz wire and lowest for the coaxial cable from Fujikura. Fig. 1 shows the current-voltage characteristics of a commercial REBCO tape measured using pulsed current. The voltage noise was about 0.5 μV. The black line is the result of power-low fitting, and the Ic and n values were 106 A and 25, respectively. The interpolation diagram shows the current-voltage waveforms at 96 A and 110 A. As shown in the voltage waveform, during the current sweep, an inductive voltage was generated due to the inductance of the sample, and it took 2 ms for the voltage to stabilize due to the low-pass filter. The voltage was averaged over 3-4 ms from the start of the current sweep.

Acknowledgement
This work was partly supported by the collaboration between Nagoya University and Suzuki Shokan, Grant-in-Aid for Scientific Research (20K15217), and NEDO.We thank Prof. Jun Imaoka, Dr. Takato Machi, Prof. Yasuo Narumi, and Prof. Arnaud Badel for providing samples and useful discussions.

References
[1] S. Fujita et al., IEEE Trans. Appl. Supercond. 29, 18466670 (2019).
[2] K. Tsuchiya et al., Cryogenics 85, 1-7 (2017).
[3] F. Sirois et al., Supercond. Sci. Technol. 23, 034018 (2010).

Keywords: Critical current measurement, REBCO coated conductors, Pulsed current