A Pulsed Current Inductive Method and Its Applications for Continuous Measurement of the Critical Current of Long Superconducting Tapes

Abstract

Recent progress in high-temperature superconducting (HTS) applications, especially the success of compact Tokamak magnets, has brought great opportunity for HTS coated conductors (CCs). The present methods for measuring the critical current of CCs, including both dc transport measurements and inductive measurements, cannot fully satisfy the requirements of material manufacturers. We have formerly proposed a pulsed current inductive (PCI) method to solve this issue, which is different from the traditional pulsed current measurement since no voltage contact is required. The signal voltage is picked up by a coil placed near the sample. The other difference is that the PCI method utilizes one ramping pulse instead of a series of flat-top pulses. The PCI method does not suffer from inductive voltage interference. It is also fast due to the one pulse feature. The heat generation during the test is even smaller than that in the normal pulsed method. The PCI method is a fast, data-robust, and sample-safe transport measurement. In this article, the influences of the test conditions, including the coil displacement, coil shape, fluctuation of the tape <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> value, and tape width, were analyzed, where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> refers to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula> power law exponent of the superconducting tape. An apparatus for reel-to-reel critical current measurement of HTS tapes was constructed based on the PCI method. An 11-m tape was tested with a relatively high resolution of 18.5 mm and a speed of 0.3 m/min, which can be upgraded with multiple sensors. The combined uncertainty of the method was estimated to be 4% + 5 A. Short sample experiments showed that the relative deviation of the PCI method from the dc transport method was mostly less than 5%, without the introduction of any fitting parameters. A good step toward real application of the PCI method has been achieved in this work. The need to systematically study the influence of sample inhomogeneity in the future is also shown.