The introduction of no-insulation (NI) technology is a milestone in superconducting magnet technology, raising the possibility of ultra-compact high field superconducting coils.There are significant problems yet to be resolved however, including resistive ramping losses and magnetic field drift. One area claimed to be solved by NI technology is quench protection of REBCO pancake wound coils. Data from a recent study by MagLab engineers appears to support this claim, showing that rapid quench propagation is a general characteristic of coils with low resistance (LR) between turns, which are a generalized model of NI coils.
Critical current density J c in polycrystalline or granular superconducting material is known to be hysteretic with applied field H due to the focusing of field within the boundary between adjacent grains. This is of concern in the so-called coated conductors wherein superconducting film is grown on a granular, but textured surface of a metal substrate. While previous work has mainly been on J c determined using induced or magnetization currents, the present work utilizes transport current via an applied potential in strip geometry. It is observed that the effect is not as pronounced using transport current, probably due to a large difference in criterion voltage between the two types of measurements. However, when the films are narrowed by patterning into 200-, 100-, or 80-µm, the hysteresis is clearly seen, because of the forcing of percolation across higher-angle grain boundaries. This effect is compared for films grown on ion-beamassisted-deposited (IBAD) YSZ substrate and those grown on rolling-assisted-biaxiallytextures substrates (RABiTS) which have grains that are about ten times larger. The hysteresis is more pronounced for the latter, which is more likely to have a weak grain boundary spanning the width of the microbridge. This is also of concern to applications in which coated conductors will be striated in order to reduce of AC losses.
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