Thermal Modeling and Verification of a Quasi-Poloidal Stellarator Modular Coil

Controlled nuclear fusion has been the subject of experimental and analytical studies for more than forty years. A focus of research in this area has been plasma confinement using a toroidal magnetic field. The two important fusion devices used for this purpose are Tokamaks and Stellarators. The Quasi Poloidal Stellerator (QPS) is a low-aspect ratio toroidal magnetic confinement device used to contain the plasma so that Controlled Thermonuclear Reactions (CTR) can take place. An integral part of an Oak Ridge National Laboratory QPS design is the modular coil, which provides the primary magnetic field in the configuration. Since the coils are not actively cooled, the stellarator must be operated in short steps or pulses with sufficient time given to the copper conductors within the modular coils to cool down. This short pulse causes thermal stresses and deformations, which need to be carefully studied and understood in the design process. A prototype modular coil named UT Racetrack coil, was developed in the Mechanical, Aerospace, and Biomedical Engineering Department, at the University of Tennessee, to test and use in verification of thermal computer simulation models. The simulation models were developed to use in studying thermal cooling requirements, need and location for auxiliary cooling methods such as liquid nitrogen lines, use of copper chill plates as heat sinks, and temperature response of the conductor cable. Various other issues related to the physical properties of epoxy and insulations used in the QPS design, and thermal analysis of the welding and fabrication of the modular coils were also addressed and resolved in this study. It was found through the computer simulation of the welding process that the modular coils will not be damaged in the welding of the steel can during the fabrication of the QPS design. In addition, through direct experiments important thermal properties of metals and the epoxy used in the Vacuum Impregnation Process were measured and compared with the available data. Also, a specific simulation method was developed to ascertain the thermal conductivity of a composite material similar to the epoxy-packed copper cables used in the modular coils. The experimentally measured thermal properties were used in the computer simulation of the proposed QPS conductor coils as well as in the simulation of the fabricated UT Racetrack coil. All computer simulations in this thesis were done in FEMLAB®. The developed and verified computer model can now be used in prediction of the thermal stresses and deformations in the modular coil, and in the improvement of the thermal features of the proposed design, including optimization of the location of cooling cryogenic lines