Void-free epoxy castings for cryogenic insulators and seals

The design of main lead bushing which transmits heat leak loads by conduction to the supercritical helium stream is discussed. The bushing, which consists of epoxy resin cast about a copper conductor, must be electrically insulated, vacuum tight and be capable of withstanding the stresses encountered in cryogenic service. The seal design of the bushing is especially important; leakage from either the helium system or the external environment into the vacuum will cause the magnet to quench. The epoxy resin casting must resist mechanical loads caused by the weight of leads attached to the bushing and thermal stresses transmitted to the epoxy via the conductor. The epoxy resin is cast about the conductor in such a way as to provide the required vacuum tight seal. The technique by which this is accomplished is reviewed. The elimination of voids in the epoxy acts as stress concentrating discontinuities during cooling to or warming from 4K. The types of voids that could be expected and their causes are described. Techniques employed to eliminate voids within the cast resin portion of the bushing are reviewed.