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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 measurement of the stored energy of graphite taken from the lattice of the ORNL Graphite Reactor is described. The release of 26 cal/g of stored energy was observed upon heating to 218°C. The measurements were made using a new technique that depends upon the transfer of heat by radiation from the walls of the calorimeter to the sample.

Measurements of magnetic properties of superconducting In5Bi3 are reported. The single-crystal samples exhibit almost reversible magnetization curves and have resistivity ratios between 33 and 75. The results of the measurements in the superconducting state together with those of additional galvanomagnetic measurements allow estimates of some electronic constants of In5Bi3. The electron mean-free paths calculated in this way are about 20 times longer than the coherence length, i.e., the pure limit is approached very closely. This leads to the conclusion that In5Bi3 is an intrinsic type-II superconductor with a Ginzburg-Landau parameter κ=2.25±0.05. The transition temperature is 4.16±0.01 K and the thermodynamic critical field atT=0 is 340±5 Oe. A comparison of the temperature-dependent constants κ1 and κ2 with the predictions of the theory for the pure limit shows deviations up to 8% for κ1. For κ2 the agreement is good.

Hysteresis loop measurements were conducted on several commercial iron‐nickel alloys during neutron irradiations at 90°K and at temperatures up to 175°C, in both zero and saturating magnetic fields. Isochronal annealing tests were also performed on the alloys after irradiation. The low‐temperature irradiation results indicated that magnetic properties are affected not by the point defects themselves but by their motion and eventual disposition. The radiation‐induced changes appeared similar in many ways to those observed in these alloys upon magnetic annealing.Experiments on the alloys at temperatures from 70° to 175°C in a saturating magnetic field showed that the activation energy of the radiation‐induced process was 0.3 eV. In all cases the radiation‐induced changes were annealed out at temperatures below the usual ordering temperature for Ni3Fe. This recovery process appeared to be dependent upon the state of the magnetization of the sample.

The superconducting properties of Tc‐V alloys with compositions ranging from 0 to 100 at.% V have been investigated in fields up to 15 kOe by measurements of the bulk magnetization determined ballistically. A phase diagram of the Tc‐V system has been determined by several metallurgical techniques. The sequence of crystal structures at 1500°C with increasing vanadium content is hcp→bcc→CsCl→bcc. The technetiumrich bcc phase when quickly cooled to room temperature exhibits superlattice x‐ray diffraction lines of the CsCl structure. The superconducting transition temperature of the technetium‐rich hcp alloys increases with vanadium concentration from 7.73°K to a maximum for the system of 11.3°K at the limit of vanadium solubility (∼6.5 at.% V). This transition temperature remains constant for the two‐phase alloys. The transition temperature decreases progressively with increasing vanadium content in the CsCl structure, and at compositions in the 50 to 80 at.% V interval no evidence of superconductivity is ob...

The superconducting magnetization behavior and transition temperatures of single crystals of Nb were investigated prior to and after a series of fast neutron irradiations (E>1 MeV) in the Oak Ridge Research Reactor at a temperature of 40°C.1 In addition to increases in the upper critical field Hc2 and small changes in the transition temperature after irradiation, it has been found that defects have been created with the ability to pin magnetic flux as evidenced by an increase in the nonequilibrium behavior of the magnetization.A greater degree of flux pinning in single‐crystal Nb can be attained by fast‐neutron irradiation than has heretofore been reported for pinning introduced by mechanical deformation, and for fast‐neutron doses of approximately 2×1019 neutrons/cm2 the magnetization approaches that of a completely irreversible type II superconductor. The initial magnetization cycle and subsequent hysteresis loop for an irradiated sample are shown in Fig. 1 as well as the magnetization cycle prior to ir...

Using a conventional ballistic technique, magnetization curves of a superconducting Pb‐27% Tl alloy have been obtained from the transition temperature Tc=6.43°K down to 1.70°K. The thermodynamic critical field curve indicates that this alloy is of the strong coupling type. The Ginzburg‐Landau parameters deduced from several features of the magnetization curves are given and their temperature dependence is compared to theoretical models.

Neutron irradiation effects in the precipitation hardening alloy, nickel‐beryllium, were measured using the ferromagnetic Curie temperature as an index of the amount of precipitate formed. Effects found previously in a similar precipitation hardening alloy, copper‐beryllium, were not found upon irradiation at approximately room temperature. However, an irradiation for an integrated fast flux of 4×1017 n/cm2 at a temperature of 300°C showed that 1.3 atomic percent more beryllium in the form of the compound Ni–Be precipitated from a supersaturated solid solution than from a similar specimen held at 300°C but not irradiated.

A technique is described for the electrical discharge machining of prolate spheroid specimens from metallic rods using conventionally milled electrodes. Specimens of Nb, Ta, Re, and Nb–Ta alloys have been satisfactorily prepared. For single crystals of Nb, x‐ray studies indicate the absence of any recrystallization after heat treatment of the prolate spheriods to remove strains.

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.

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.

A single crystal of vanadium (resistivity ratio Γ = 450) was irradiated in the Oak Ridge Research Reactor at a temperature of 40°C for a series of fast neutron fluences. The sample was a prolate spheroid approximately 25 mm in length, with a minor diameter of about 3 mm Prior to and after each irradiation isothermal bulk magneti­zation measurements were carried out at various temperatures in the superconducting regime. The sample was also investigated in a Hartshorn mutual inductance bridge before and after each irradiation. The bridge apparatus1 was arranged so that the sample could be excited by a sinusoidal magnetic field of adjustable frequency ω and amplitude h0 in a superposed dc biasing field H. Both fields were parallel to the long axis of the sample. The bridge was balanced with the sample in the Meissner state, so that the output was proportional to the ac magnetic permeability and could be viewed on an oscilloscope. A lock-in amplifier at the bridge output tuned to the fundamental frequency detected signals in quadrature proportional to μ’ and μ”, the real and imaginary components of the fundamental portion of the ac permeability, which is generally nonlinear.

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.

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.