Abstract
Twisting of multi-filamentary superconductors is an important step in the development of wires with AC losses at an acceptable level for AC applications. The necessary twist pitch depends on wire architecture, critical current density, matrix material, and external factors such as temperature, frequency and applied magnetic field. The development of an AC optimized MgB2 superconductor would be facilitated by a fast method to set the requirements for the twist pitch. A problem often encountered when comparing wires with different twist pitches is the degradation in critical current occurring at small twist pitches due to mechanical deformation. In this work we propose to use a non-twisted conductor to estimate the influence of twisting on the AC losses. A long superconductor is cut into smaller lengths, each simulating one third of the twist pitch, and the AC losses due to applied magnetic fields are compared between samples of different lengths. With this method, the effect of reducing the size of the loop of the coupling currents is studied without changing the superconducting parameters. AC loss measurement results are presented for a round titanium matrix MgB2 wire with simulated twist pitches between 9 mm and 87 mm.
Highlights
Medium- and high-temperature superconductors on or close to the market have the technical performances necessary for consideration in several large-scale applications, like DC [1] or low-field AC [2]-[3] cables, generator rotors [4], and induction heaters [5]
Simulating the twist pitch by cutting non-twisted wires When a multi-filamentary wire is exposed to a time-varying magnetic field, currents are induced in a loop consisting of two filaments and bridges in the metal matrix between them
These currents contribute to the total losses by an increase in the superconductor hysteresis losses and by the ohmic losses caused by the coupling currents crossing the metal matrix
Summary
Medium- and high-temperature superconductors on or close to the market have the technical performances necessary for consideration in several large-scale applications, like DC [1] or low-field AC [2]-[3] cables, generator rotors [4], and induction heaters [5]. 2. Simulating the twist pitch by cutting non-twisted wires When a multi-filamentary wire is exposed to a time-varying magnetic field, currents are induced in a loop consisting of two filaments and bridges in the metal matrix between them. Simulating the twist pitch by cutting non-twisted wires When a multi-filamentary wire is exposed to a time-varying magnetic field, currents are induced in a loop consisting of two filaments and bridges in the metal matrix between them These currents contribute to the total losses by an increase in the superconductor hysteresis losses and by the ohmic losses caused by the coupling currents crossing the metal matrix. In a non-twisted multi-filamentary superconducting wire, the loop becomes infinitely long, and the filaments are fully coupled, i.e. the filaments act as one bundle leading to larger hysteresis losses than if they were acting separately. The lengths of the samples correspond to one third of the twist pitch of twisted wires
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