Stability of High-Temperature Superconducting Conductors Using the Finite Element Method

209 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1998.One of the key steps that needs to be addressed for the construction of a fusion reactor is the ability to contain the plasma. The use of superconducting magnets to produce the necessary magnetic field is one of the most promising approaches. Because high-temperature (high-$T\sb{c}$) superconductors can maintain superconducting properties in much higher magnetic fields than low-temperature superconductors, their use in fusion magnet designs may be preferred. An important issue for high-$T\sb{c}$ superconductors is stability: i.e., the ability to maintain or recover superconductivity in the event of a thermal disturbance or flux jump. Previous research in this area has generally only considered temperature-independent properties for both low- and high-$T\sb{c}$ superconductors. This is a valid assumption for low-$T\sb{c}$ superconductors that have a very limited operating range ($T\le10$ K); however, for high-$T\sb{c}$ superconductors, the temperature can vary greatly, and at cryogenic temperatures, the material properties are highly temperature-dependent. This leads to nonlinearities in the heat conduction equation. As a result, this research uses the finite element method (FEM) to determine the stability of high-$T\sb{c}$ superconductors in various operating modes. This is done by determining the critical energy deposition which is required to cause the temperature of the tape to rise uncontrollably. Also, the temperature and critical current vs. time profiles of several points in a superconducting tape are analyzed. Both two- and three-dimensional models are analyzed. For the two-dimensional model, it is shown that the types of sources and boundary conditions are limited because the third direction is not taken into account. Hence, a full three-dimensional model is necessary to apply all of the constraints (e.g., boundary conditions, location of materials). For the three-dimensional model both the critical energy vs. magnetic field and the temperature and critical current vs. time profiles are analyzed. A comparison with tests on pancake coils from the National Research Institute for Metals (NRIM) in Tsukuba, Japan is performed. These results are then used to predict the capabilities of a YBCO thin film on a metallic tape substrate that has a much higher critical current which leads to less stability when the operating current is near the critical current.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD