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Displacement results are usually used as the criterion for distortion of castings during casting and heat treatment processes. However, the displacement results consist of both contraction and distortion. Contraction is a kind of uniform volume change, which can be solved by enlarging the castings by a contraction coefficient. However, the actual distortion has to be deleted by giving inverse distortion or by mechanical correction method. In this paper, algorithms are presented to evaluate the distortion of castings. One method is to subtract the uniform contraction from the displacement results. The other is to calculate the deflection angle between the normal direction of the discretised surface triangle of the original casting shape in stereolithography format and that of the simulated casting shape. The third one is the application of actual machining allowance in the evaluation of distortion. These methods are validated by three case studies, including a heavy hydroturbine blade casting.

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.

Several types of thin-film plastic insulation are being evaluated for possible use in ac superconducting underground transmission cables. The materials include polyolefins, polycarbonates, polysulphones, and nylons, in thicknesses ranging from 25µ to 125µ. Multiple-layer samples of the films are placed between 9.5 mm diameter, spherical stainless steel electrodes, and the film and electrodes encapsulated in epoxy. The epoxy casting is arranged to allow injection of liquid or gaseous helium between the plastic layers, at pressures up to 5 atm and a temperature of 4.2 K. Each casting contains eight pairs of electrodes.

The dielectric for a flexible superconducting power transmission cable will necessarily be in the form of multi-layered tapes, impregnated by supercritical helium, wound with butt gaps to provide flexibility.1 Major consideration for the choice of an appropriate dielectric are that the dielectric should posses suitable mechanical properties at low temperatures (5–10°K), the material should have very low dielectric loss (< 20 μ radian) and it should possess high enough dielectric strength. Economic considerations dictate that the cable should operate at very high electric stress levels. For example, the Brookhaven National Laboratory superconducting cable design2 specifies an operating stress 10 MV/m, free of any significant partial discharge up to a stress of 20 MV/m. So far no dielectric material simultaneously satisfying all the mechanical and electrical requirements cited above has been found and the search for a suitable dielectric tape is still in progress. In order to arrive at an appropriate material it is necessary to evaluate various materials in terms of their mechanical and electrical characteristics. It is in this connection that we undertook a program of measurement of dielectric strength and partial discharge inception levels of polymeric insulating tapes at cryogenic temperatures.

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.

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.