Mechanism and mitigation of anomalous V-I characteristics in Ic measurements of fully filamentised REBCO tapes

Abstract

Abstract

Abstract Critical-current measurements of fully filamentised REBCO coated conductors using the standard four-probe method frequently produce anomalous, poorly reproducible V-I characteristics. These include premature resistive signals and step-like voltage transitions that preclude reliable determination of Ic and the n-value. By combining transport experiments with a nonlinear equivalent circuit model, we show that these anomalies originate from finite inter-filament resistance. The resistance disrupts the quasi-equipotential condition across the tape width, driving non-uniform current sharing among filaments in a pattern governed by the arrangement of current and voltage leads. On this basis, we develop two measurement strategies compatible with industrial reel-to-reel processing, neither of which requires the unfilamentised end margins used in laboratory practice. The Controlled Sequential Transition (CST) method induces a deterministic filament transition sequence, generating step-rich V-I curves from which filament-resolved Ic values are extracted through model-driven regression with approximately 5% consistency under reversal of the current injection direction. The Pre-Distributed Current (PDC) method suppresses redistribution by apportioning the transport current according to the expected Ic of each filament, yielding smooth, conventional V-I curves. Across all ten nominally identical samples at 77 K in self-field, the PDC method recovers Ic within approximately ±1.5% of the reference with voltage taps placeable on any filament. Direct readout from the CST method systematically overestimates the n-value, by +70% to +246% across the ten samples (mean +120%) — a bias that the analytical model captures semi-quantitatively. We recommend the PDC method for critical-current calibration at the tape ends and the CST method for filament-level defect diagnostics during conductor development. The results establish a measurement framework for multifilamentary REBCO tapes and illustrate a general metrological principle: when a superconducting layer is subdivided into elements connected by finite inter-element resistance, strategies that explicitly account for transverse current paths must replace the quasi-equipotential assumption of standard four-probe protocols.