Henning Taxt

AC Loss Measurements on Multi-Filamentary

The MgB2 superconductor is an attractive choice for many high-current dc applications, particularly for its low cost compared with, e.g., YBCO, and the higher operating temperature than the low-temperature superconductors. To extend the operating range to include also ac applications, the ac losses need to be determined and a path towards a low-loss conductor needs to be established and verified. To accomplish this development, systematic and reliable measurements of the losses are essential. In this paper, we present ac loss measurement results on nontwisted MgB2 wires with titanium and copper-nickel matrices. 50 hertz ac losses were measured calorimetrically in applied magnetic fields, with transport currents, and with combinations of both. The temperature range was between 25 K and Tc, the magnetic field was varied up to 300 mT, and the transport current up to 113 A. The measurement results are used as a basis to discuss the necessary ac loss reduction steps in MgB2 superconductors.

\hbox{MgB}_{2}

The MgB2 superconductor is an attractive choice for many high-current dc applications, particularly for its low cost compared with, e.g., YBCO, and the higher operating temperature than the low-temperature superconductors. To extend the operating range to include also ac applications, the ac losses need to be determined and a path towards a low-loss conductor needs to be established and verified. To accomplish this development, systematic and reliable measurements of the losses are essential. In this paper, we present ac loss measurement results on nontwisted MgB2 wires with titanium and copper-nickel matrices. 50 hertz ac losses were measured calorimetrically in applied magnetic fields, with transport currents, and with combinations of both. The temperature range was between 25 K and Tc, the magnetic field was varied up to 300 mT, and the transport current up to 113 A. The measurement results are used as a basis to discuss the necessary ac loss reduction steps in MgB2 superconductors.

Wires With Non-Magnetic Sheath Materials

MgB2 multi-filamentary superconductors are widely considered for use in dc applications. To expand the possible application range to ac apparatuses, the development of a low ac loss wire is needed. This development involves several steps, and a solid understanding of the loss mechanisms is important to optimize that process as well as for evaluating dc wires exposed to current or magnetic field ripple. In this study we discuss the coupling currents and their influence on hysteresis loss as well as on coupling current loss. We give a phenomenological explanation of the origin and behaviour of the coupling currents and describe the loss patterns for hysteresis loss and coupling current loss separately. Finally, we interpret measured ac losses in an MgB2 wire cut into different lengths representing different equivalent twist pitches. Under certain circumstances short sample lengths are shown to give inaccurate measurement results. On the other hand, short sample lengths of non-twisted wires can be used to estimate the twist pitch necessary to electromagnetically decouple the superconducting filaments. For the studied MgB2 wire, with a pure titanium matrix, the losses were mainly hysteretic, although the level of these losses was lowered when reducing the equivalent twist pitch, and thereby the coupling currents. At 35 K the reduction in loss started at an equivalent twist pitch of approximately 36 mm, where the filaments were assumed practically fully coupled. The decoupling then continued all down to the shortest equivalent twist pitch of 9 mm.

AC loss measurements multi-filamentary MgB2 wires with non-magnetic sheath material

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.