A "permanent" high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen

Abstract

This thesis explores a new design for a portable "permanent" superconducting magnet system. The design is an alternative to permanent low-temperature superconducting (LTS) magnet systems where the magnet is cooled by a bath of liquid helium. The new design involves a high-temperature superconducting (HTS) magnet that is cooled by a solid heat capacitor. An apparatus was constructed to demonstrate stable operation of a permanent magnet wound with Bi2223/Ag conductor while in thermal communication with a mass of solid nitrogen. The system includes a room-temperature bore and can func tion while it stands alone, detached from its cooling source, power supply, and vacuum pump. The magnet is operated in the 20-40 K temperature range. This apparatus is the first to demonstrate the operation of a superconducting magnet with a permis sible temperature variation exceeding a few degrees kelvin while a magnetic field is maintained for a useful duration. Models are developed to predict the experimental system's warming trend and magnetic field decay. The models are validated with a good agreement between simulations based on these models and experimental results. Potential performance advantages of a solid nitrogen cooled permanent HTS (SN2/HTS) magnet system over a liquid helium cooled LTS (LHe/LTS) system are explored for various applications. The SN2/HTS system design includes a second solid heat capacitor that cools a radiation shield. Recooling of the heat capacitors is performed with a detachable cryocooler. The SN2/HTS system offers both improved stability and improved portabilit over an LHe/LTS system design.

(cont.) Design codes are constructed to comiiie the SN2/HTS system design with a LHe/LTS design for two different applicatiohs. The first application is a general permanent superconducting magnet employing a room temperature bore. The second application is a superconducting mine countermeasures system (SCMCM) that is used to sweep passive magnetic influence mines. The codes predict the important system attributes, namely minimum volume and minimum weight, that should be expected fora given set of design requirements (i.e. field magnitude and bore size, or magnetic dipole moment) and a given set of conductor properties. Their results indicate that present HTS conductor critical current and index are not yet sufficient for producing SN2/HTS systems of a size that is comparable to that expected for a LHe/LTS system. However, the conductor properties of Bi2223/Ag have been consistently improving over the last decade, and new HTS conductors are expected to be developed in the near future. Therefore, the codes are used to determine the minimum HTS properties that are necessary for constructing a cryocooled SN2/HTS system with a size comparable to that expected for a LHe/LTS system.