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The experimentally measured ac losses in superconducting niobium of a size and shape suitable for practical power cables has been shown to be of such small magnitude that ac superconducting cables warrant serious consideration.

Development of superconducting power transmission cables operated at liquid helium temperature requires a cryogenic system with extremely low heat-leak characteristics. A thermal envelope, housing the superconducting cables and having the highest performance superinsidation systems, still exhibits high heat-leak losses in relation to the electrical losses generated by the superconductor. This paper describes a conceptual superconducting transmission line configuration using multiple-layer, vacuum-insulated pipe with intermediate thermal shielding to reduce the helium-level refrigeration load. The intermediate metallic shields, anchored at discrete temperature levels on a vent vapor tube, and sensible refrigeration of the vent vapor stream intercept heat leak to the lower temperature helium compartment, thus upgrading thermal performance of the insulated pipe and bringing heat-leak losses in line with superconducting losses.

The thermal ratings of forced cooled and self-cooled cable runs are reviewed in order to assess the merit of forced cooling. These ratings, along with those of forced cooled joints, are employed to show that the ampacity of a 138 kV, 2000 kc mil high pressure oil filled pipe type system can be increased by at least 100o through forced cooling by circulating the pipe oil. The refrigeration requirements to permit this increased ampacity is determined, and the flow schemes which will meet these requirements are outlined along with the generic design of a refrigeration package to implement the preferred flow scheme. This generic design is used as the basis for an economic analysis which shows that single forced cooled cable ampacities greater than the ratings of a pair of self-cooled cables can be achieved at a 20% cost saving over the use of two self-cooled cables to meet the same load requirement. The emergency ratings of these forced cooled cables which correspond to the usual 100 and 300 hour emergency ratings are found to be 8 and 6% of the 100% duty cycle rating.

TWENTY-FIVE years ago, one of us1 suggested an ‘external work’ machine to be used for the liquefaction of the gases of low boiling point, in particular helium and hydrogen, in a process similar to that used by Claude for the liquefaction of air2. The essential feature of the suggestion was to replace the conventional cylinder and piston of the expansion engine by one constructed from an elastic metal bellows. Such a bellows engine would form a gas-tight unit which could be suspended in a vacuum for thermal insulation, and one would expect the engine to operate with a high degree of reversibility. Though it was experimentally verified at the time that metal bellows could be used at hydrogen temperatures, the design of an engine was not pursued because the problem of fatigue fracture seemed to require extensive development work.

In a previous paper,1 a program underway at Oak Ridge National Laboratory to investigate the high voltage electrical properties of fluids and solids for cryogenic systems was described. As the first result of these investigations, we report here on measurements of the breakdown strength of liquid helium boiling at atmospheric pressure, under voltages up to ± 130 kV de, and in gaps of up to 2. 5 mm. Preliminary results with ac voltage have also been obtained.

The major instability problem in Nb3Sn ribbon superconductors is known to be due to flux jumps caused by magnetic field components perpendicular to the face of the ribbons (H$$_\perp$$). Likewise, techniques which are intended to prevent this instability are also well known and include (1) reducing the width of the ribbon, (2) using low current rated material in regions of large H$$_\perp$$), (3) paralleling the ribbon with good electrical conducting material, and (4) providing as much cooling as possible. The decision, in 1968, to employ Nb3Sn ribbon in IMP, a large plasma physics research facility, was made without full knowledge of how bad the instability really was. While difficult force problems and machining and assembly problems were being solved, experiments were performed to determine the best way to reach the IMP quadrupole design point. The stability question was answered by performing experiments with relatively large test coils called ''cusp coils'' which showed that stabilization could be achieved by cowinding 0.006 in. aluminum ribbon along with the Nb3Sn ribbon

In recent years a great deal of attention has been directed to the application of superconductivity to electrical equipment, in particular, superconducting power transmission lines [1]. Liquid helium is being investigated in a dual role as the dielectric and coolant in some cable projects presently under way [2–4], in view of its extremely low dielectric loss (< μ rad). Most of the information available on the breakdown strength of liquid helium is limited to either uniform or quasiuniform field conditions. Nevertheless, in any practical system one often encounters bends, corners, and projections. Frozen contaminants on the electrode surface, for instance, water vapor, could provide serious nonuniform field configurations. It is therefore necessary to evaluate the performance of the dielectric under strongly inhomogeneous field conditions. In order to do this the ASTM standard electrode and the point-plane electrode systems were employed. Although the main emphasis was directed toward measurements on liquid helium, in certain cases liquid nitrogen and cold helium gas were also studied. It was found that the deterioration in dielectric strength of liquid helium in strongly inhomogeneous field conditions is much more severe than that of well-degassed transformer oil. Also, in the case of the point-plane electrode configuration, liquid helium exhibits a strong polarity effect markedly different from that of transformer oil. The breakdown data were also subjected to statistical analysis in order to determine the form of the distribution function.

Although the mechanical behavior of many commercial metals has been well tabulated for temperatures down as low as 20°K and in some cases 4°K, relatively few data are available on experimental alloys in this range. Just what properties may be obtained is somewhat uncertain. This paper deals with an investigation into the tensile and Impact properties and austenite stability of several austenitic manganese steels.

Technical and economic considerations indicate that it is advantageous to operate cryogenic underground cables at high voltages in an optimum range between approximately 100 and 500 kV. Several programs under way in the United states for the development of high voltage superconducting or cryoresistive cables were recently summarized by Belanger.1 An appendix to his paper summarizes the most important work in progress in other countries. In particular, the U. S. Atomic Energy Commission has funded a program to investigate several different superconducting cable concepts. The design of these high voltage cryogenic cables will require engineering data on the dielectric behavior of cryogenic liquids and solid insulators similar to that which already exists for room temperature liquid and solid insulation.

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