AC Loss in MgB2 Superconductors: Calorimetric Loss Measurement

Abstract

Ever since the phenomenon of superconduction was discovered, and especially after the discovery of high-temperature superconductors (HTS) in the mid-80s, there has been high expectations to what could be achieved by this technology. The possibility of reducing losses and dramatically increasing current densities pushes the limits of what is possible in most electric power applications, such as cables, transformers and motors. These uses have been proven difficult with today's superconductors, because alternating magnetic fields and currents inevitably introduce losses. The losses can be minimized by optimizing the design of the superconducting wire. Effort has been put into developing an AC wire from the HTS, but losses are still too high for most applications.It is therefore interesting to look at the recently discovered superconductor MgB2 for use in AC applications. Compared to the HTS, it is inexpensive, flexible and easy to produce in kilometre lengths. Some research and development is being done on MgB2 wires for AC application. This study can supplement the ongoing work with high quality measurements in applications-like conditions. An apparatus for loss measurement in MgB2 superconductors has been constructed and measurements on two different wires have been performed.Calorimetric measurement is used to determine the losses. That means that the temperature rise in the sample due to losses is measured. The temperature increase is then compared to temperature increase caused by a reference heater to determine the power dissipated. The AC loss measurement apparatus can supply AC transport currents of 200 Apeak, and magnetic fields up to 1 Tpeak to the superconductor sample, as well as combinations of both. Measurements can be performed at temperatures down to 20 K.Systematic AC loss measurements have been performed on two MgB2 superconducting wires.Sample 1 is a multi-filament round wire with titanium sheath. Sample 2 is a multi-filament tape with cupronickel sheath and a niobium barrier around the filaments. Losses due to AC magnetic field was determined for both wires. In addition, critical current and losses due to transport current was determined for Sample 2. The obtained results are coherent and in line with comparable loss measurements presented in literature. No further interpretation of the results is presented in this thesis.