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Several groups working in the field of power transmission using superconducting cables have proposed the use of flexible cables [1,2]. In these cables, the insulation between the inner and outer conductors is a dielectric tape; at the operating temperature, the interstices between the tapes become impregnated by helium coolant.

The Brookhaven National Laboratory is engaged in the development of a superconducting ac power-transmission system. A cryoelectric test facility for evaluating the performance of flexible superconducting cables is now under construction at this laboratory. This outdoor facility will be able to test cables of several hundred meters length and represents a sizable investment in refrigeration and power test equipment. Before this construction was undertaken a laboratory test facility was operated to ascertain the performance of the cryogenic components. The facility for evaluating short sections of cable is described.

A program to measure the thermal conductivity of braided superconductors in several epoxy-supported coil forms has been in progress for the last two years. This presentation covers only this portion of the program, which also included determination of baffling on heat transfer and the surface coefficients for both liquid and supercritical helium.

The end product of the process involving the engine under discussion is liquid nitrogen. This is obtained by cooling and liquefying air and distilling it. There are several methods of obtaining liquid air: by cooling and using Joule-Thomson expansion; by cooling and production of work using mechanical expansion; or by using a combination of the two. A method employing mechanical expansion is preferred because of the higher process efficiency and the resulting higher yield. As an example, the system tested was run using cooling and Joule-Thomson expansion with a resulting yield of 90 liters per hour of 99.7 percent pure liquid nitrogen. The same plant, using cooling, Joule-Thomson expansion, and a reciprocating expansion engine produces 200 to 245 liters per hour of pure liquid nitrogen. This paper will describe the engine, point out operating problems and give the results of performance tests with some conclusions.