REBa2Cu3Oy (REBCO) tapes exhibit high critical current (Ic) even under high magnetic fields, making them ideal for ultra-high field magnets. Since 2022, our group has been developing a 33 T cryogen-free superconducting magnet (CSM) [1], where the REBCO tapes will be used for the innermost coil. For optimal magnet design, it is crucial to characterize these tapes across various temperatures, magnetic field amplitudes, and angles. In applications such as high-field CSMs, and more recently, compact fusion reactors and rotating machines, operation at temperatures between 4.2 K and 20 K is anticipated. However, in temperature-variable environments, due to spatial and thermal constraints, Ic is often measured using micro-bridges of REBCO tapes. Since micro-bridges are sensitive to inhomogeneities, measurements using full-width REBCO tapes are more reliable. With advancements in REBCO tape technology globally, single tapes now achieve Ic values in the kA range at low temperatures. The challenge lies in achieving high-current energization within limited space and cooling capacity. By using pulsed currents, heat generation in the probe and sample can be neglected [2]. In this study, we developed a method for Ic measurement using pulsed currents in high magnetic fields at low temperatures.
A 5 kA pulsed current source was developed using supercapacitors (DLCAP, Nippon Chemi-Con) [3], charged up to 60 V with an external DC voltage source, and regulated to generate the desired pulse current. To reduce voltage noise and improve measurement accuracy, the current waveform was controlled to follow a smooth, half-sinusoidal shape with plateaus. To suppress heat generation, the duration of the plateaus was set to less than 5 ms. Using this setup, we measured the Ic of REBCO tapes (FESC-SCH04(40), Fujikura) at temperatures ranging from 4.2 K to 77 K, and in magnetic fields up to 25 T applied perpendicular to the tape using a 25T-CSM. Additionally, these results will be compared with measurements obtained using DC large current and a precise variable temperature insert [4] to validate the accuracy of the Ic.
As a result, the current was swept at 0.1-1 kA/ms, and even in measurements at 25 T, where the electromagnetic force is the largest, the voltage noise was kept below a few μV/cm. It was verified that this system is capable of measuring transport Ic in REBCO tapes under variable temperatures from 4.2 K to 77 K and magnetic fields. Regarding the prospects of this system, in the near future, this pulsed Ic system will become a user facility for collaborative research in our site. There is also potential for extending the current probe to other high-temperature superconducting wires, such as iron-based superconductors. Furthermore, the system is expected to be adaptable to probes with a rotation stage to investigate the field angular dependence, and to evaluate electromechanical properties under tensile stress. As an advanced application, pulsed Ic measurements could be performed in flat-top long pulsed magnetic fields [5]. Preliminary results of these derivative technologies will be reported briefly.
References [1] S. Awaji et al., MT-28, 4OrA2-5 (2023). [2] F. Sirois et al., IEEE Trans. Appl. Supercond. 19, 3585 (2009). [3] Y. Tsuchiya et al., IEEE Trans. Appl. Supercond. 34, 9500207 (2024). [4] C. Barth et al., IEEE Trans. Appl. Supercond. 28, 9500206 (2018). [5] Y. Kohama and K. Kindo, Rev. Sci. Instrum. 86, 104701 (2015).
This work was partly supported by JSPS KAKENHI Grant Number JP22H01522, JP22KK0244, a project of NEDO JPNP20004, and collaborative research with Nippon Chemi-Con. We had valuable discussions with M. Leroux, and F. Sirois.