Characterization of MgB2 Superconductors in Conduction-Cooled Systems

Abstract

Superconductivity as a phenomenon was discovered over 100 years ago and the first commercially viable materials have been in use for over 50 years. Today, the truly commercial applications in medical imaging and scientific research are limited to operating temperatures near 4.2 K. However, the discovery of high temperature superconductivity in 1986, and the discovery of superconductivity in magnesium diboride, MgB2, in 2001, have the potential for changing this. The high temperature superconductors have proven to be useful in many applications including electric machinery and power transfer. However, their wide spread application is in part limited by their steep price. This is not the case with MgB2. The cheap and easy-to-manufacture material has been shown to be competitive at applications with low to medium strength magnetic fields at an elevated operation temperature of 20 K. The most important commercial application, the magnetic resonance imaging falls within this range. These elevated operation temperatures are cost-effectively reachable with conduction-cooling and mechanical cryorefrigerators, i.e. cryocoolers. Designing and engineering of such dry system requires different approaches than their liquid cooled predecessors. To support these activities, accurate and relevant measurement data on the superconductor properties under the proper operating conditions, is required.

In this thesis, I present the approaches, challenges and solutions for obtaining valid measurement data in a conduction-cooled measurement system, focusing on the measurements of the properties used in characterizing the superconductor performance. I show how measurements in conduction-cooled system differs from ones cooled with liquid or gaseous cryogens. I discuss the importance of a proper sample holder and present considerations on how to make one. While the physical components of the measurement system are important, acquiring valid results requires proper measurement procedures. I analyze the voltage-current characteristic measurement and show how the results are easily effected by improper procedures. I consider, how the quench initiation and propagation experiments are performed, and how the quench dynamics MgB2 and high temperature superconductors differ from low temperature ones. To demonstrate the applicability of conduction-cooling, I present the design, construction and benchmarking a measurement system for determining the magnetization AC-losses in a superconductor.