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Several types of thin polymer film insulation are being evaluated for possible use in superconducting underground transmission cables. Measurements are reported on the partial discharge inception voltages of threelayer disc samples of 102-?m polyethylene and 61-?m polycarbonate for temperatures and pressures of the helium impregnant ranging from 5.0 K and 0.3 MPa to 11.5 K and 1.5 MPa. Butt gaps are simulated by punching 1 mm diameter holes in some of the layers. The effect of aluminized shielding around the holes is discussed. The observed inception voltages vary strongly with the helium conditions, increasing rapidly as the density increases. The results suggest that the voltage is capacitively divided between the polymer layers and helium-filled gaps, with partial discharge occurring when the breakdown strength of the helium is exceeded.

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.

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.

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.