The application of high temperature superconductors to power apparatus has progressed greatly in the last few years. Several groups have designed and built prototype cables utilizing liquid nitrogen as both coolant and in some designs also as the electrical insulation. For the successful employment of these designs in the field, utility companies have to be convinced of their operational reliability. Cables and their associated power apparatus have the inherent problem of an ambient to cryogenic temperature difference, exceeding 200K. This has to be overcome with the constraints of good thermal efficiency, high current flow and controlled electric stress. In the specific case of the termination, for example, the conflicting requirements of high electrical conduction and low thermal conductivity mean that at maximum continuous rating there will be significant thermal input into the low temperature region. The construction of a liquid nitrogen cryostat capable of optical measurement of bubble behaviour with applied fields designed and built by the authors is described. This has been used to study the influence of applied electric field on thermally induced bubble nueleation as related to the use of liquid nitrogen as a dielectric in superconducting power apparatus. This paper describes experimental results for thermally induced nucleate boiling from an artificial cavity in a uniform d.c. electric field. Bubble departure frequency, pre-departure volume and aspect ratio with increasing uniform d.c. electric field strength arc reported.
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