Moisture Degradation of CTD-403: Testing the Application of Cyanate Ester Insulations to Fusion Devices

The fusion devices currently being developed present several challenges for magnet designers. One challenge lies within the electrical insulation, which must be able to withstand extreme temperatures (both cryogenic and elevated temperatures), large shear and compressive stresses, high operating voltages, and high levels of incident radiation. To address the need for better performing insulation systems, Composite Technology Development, Inc. (CTD) has developed CTD-403, a cyanate ester resin with increased radiation resistance, ease of processing and fabrication, low moisture absorption characteristics, and high mechanical and electrical strength at cryogenic and elevated temperatures.

In this thesis, CTD-403 resin based insulation systems were tested under the operation guidelines of the Quasi-Poloidal Stellarator (QPS). Because the coils of QPS are water cooled, it is important to understand the effects of long term humidity exposure on the insulation. The effects of humidity on moisture absorption characteristics, dimensional stability, mechanical and electrical properties were characterized. Increasing the humidity level caused a corresponding increase in the saturation level, while increasing the temperature of exposure magnified these effects causing more pronounced non-Fickian behavior. The mechanical properties degraded with an increase in humidity level. Elevated temperature effects are more pronounced on the compressive properties than the tensile properties. The glass transition temperature was more greatly affected by elevated temperature exposure as opposed to increased humidity exposure, decreasing in both instances. The dielectric strength decreases with increased humidity level and exposure time.

Based on these findings, the performance of cyanate ester resin based insulation systems is shown superior to that of traditional epoxy based resin systems in all performance measures. The degradation of the mechanical and electrical properties of the insulation from long term effects of humidity and elevated temperature exposure are shown to be within the performance criteria bounds, and the insulation is therefore recommended for use in current and future fusion programs.