摘要:

An experimental study has been conducted to measure the flow of magnetic flux through the wall of a specimen of Nb‐25% Zr, and thereby to infer the magnetic induction and currents in the material. It was found that if at some magnetic field the specimen were heated to aboveTcand recooled, then if the applied field were subsequently changed at a constant rate, the rate of flux flow into or out of the sample would increase linearly with time. If measured at a series of magnetic fields, the rates of these increases, or the flow rate slopes were found to be a smoothly increasing function of average applied field. Calculations based on eight published critical‐current density models did not satisfactorily explain these observations. It was found, however, that the empirical current‐density modelJc= &agr;c[(&mgr;0Hc2−B) / (B0+B)] did, in fact, give excellent agreement with the measured flow‐rate behavior. In this paper the experimental and analytical results are described and the new critical‐current density model is compared with earlier ones.

ISSN:0021-8979    

DOI:10.1063/1.1659540

出版商:AIP    

年代:1971

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1659543

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A method is developed to adapt the conventional definition of the noise temperature to amplifiers with superconducting inputs. In particular this method is applied to a crossed‐film cryotron amplifier with a superconducting input to calculate its noise temperature. We then compare it with that of other amplifiers and masers. In addition to the noise temperature, a new criterion is evolved here: to estimate the figure of merit of amplifiers and detectors. This involves a new parameter called herein the action factor, which is the product of the smallest energy measurable by a device, with its smallest possible response time. The action factor and the noise temperature are worked out for the cryotron and a few other amplifiers and detectors for comparison. The advantage of the cryotron amplifier is brought out.

ISSN:0021-8979    

DOI:10.1063/1.1659544

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Thermal noise causes a random frequency modulation of the self‐oscillation of a Josephson junction, and the temperature of the noise source can be determined by analysis of the generated signal. We show that a thermometer based on this principle would be theoretically capable of measuring temperatures in the microkelvin range, and describe a prototype thermometer which has recorded noise temperatures down to 0.075 K.

ISSN:0021-8979    

DOI:10.1063/1.1659545

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Thin‐film tunnel junctions using a semiconductor rather than an oxide barrier have been fabricated. Niobium, used for the bottom layer, and the semiconducting center layer were deposited using rf sputtering techniques, while the top layer was evaporated lead. Sputtering was done from high‐purity single‐crystal slices of Ge, Si, and GaAs and from polycrystalline InSb. Superconductive tunneling was observed in most samples. The semiconductor tunnel junctions were somewhat light/heat sensitive and also exhibited a nonlinearI‐Vcharacteristic at voltages greater than the superconducting energy gap to a greater extent than comparable oxide devices. Excess current was noted in the devices at voltages below the superconducting gap and the measured gap of the niobium film was found to be reduced from that of pure material. Although initial devices were inferior to good oxide junctions, present results show that techniques of rf‐bias sputtering and sputter etching have strong possibilities for the production of deposited insulator junctions.

ISSN:0021-8979    

DOI:10.1063/1.1659547

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

This paper reports what is believed to be the first direct observation of the drift of single fluxons through a superconductor. This has been achieved by using an ultrasensitive cryotron amplifier to sense the changes in persistent current which occur in a superconducting film loop when a fluxon is drifting into or out of the loop. The loop is deposited on an insulated superconducting ground plane; this decreases the inductance sufficiently so that the current changes induced by single fluxons entering or leaving the loop can be sensed. It has been found possible to nucleate single fluxons by current pulsing narrow films which are superimposed over but insulated from the loop. The fluxons are found to be nucleated at the intersections of the edges of the film loop with the superimposed films, and then swept into or out of the loop by the Lorentz force exerted by the loop current. The fluxon velocity component normal to this current has been measured for tin and is found to be less than a factor of two different from the values predicted by certain of the present theories of fluxon motion.

ISSN:0021-8979    

DOI:10.1063/1.1659548

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Loss measurements were made at 60 Hz on 6‐m lengths of 1‐ and 3‐cm‐diam superconducting niobium tubes used as the inner conductor of a coaxial line. These tubes carried transport currents up to 1700 and 5300 A, respectively. The losses were found to fit the relationP=fAEc1(H/Hc1)n,wherefis the frequency,Ais the surface area,Ec1is the energy loss per unit surface area per cycle atHc1, the lower critical magnetic field, andHis the peak magnetic field at the surface of the superconductor. The exponentnequals 3 forH≤Hc1andn≥4 forH≥Hc1. The ac losses showed a 25% linear increase with temperature from 4.2 to 5 K. For the case of an eccentric coaxial line, where the current and magnetic field vary about the circumference of the superconductor, the ac losses were found to agree with those predicted from calculated values of the current and magnetic field distributions. In particular, when the peak surface field is less thanHc1andRis the ratio of the maximum to minimum surface magnetic fields about the circumference of the superconductor, the losses were a factor (3R2+2R+3)/4Rgreater than those found at an equivalent current for a concentric configuration.

ISSN:0021-8979    

DOI:10.1063/1.1659549

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The hysteresis losses in commercial type‐II superconductors preclude the use of these materials for ac superconducting power cables. While recent work has shown that ac cables employing high‐purity Nb may have acceptably low losses, it is clear that dc cables would better exploit existing superconductivity technology and make possible the use of commercial NbTi and Nb3Sn. Coincidentally, there has been a lively interest in dc transmission in the electric power industry due partly to advances in SCR converters and partly to increased emphasis on system stability. In this paper the relevance of these developments is discussed. Several alternative designs for superconducting dc cables utilizing commercial superconductors are discussed and design and cost calculations are presented. Areas where additional experimental work is needed are identified.

ISSN:0021-8979    

DOI:10.1063/1.1659551

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The opportunities opened by the use of cryoresistive and superconducting materials in underground transmission systems have led C.G.E. and L'Air Liquide to undertake, in close cooperation, a cryocable program which started in 1966. A first set of problems associated with the development of cryogenic cables deals with the cable system: design, safety, terminal equipment including leads, cryogenic equipment, refrigerators, and problems related to overload capability and reliability. A second set concerns the cable itself, i.e., scientific and technological problems associated with the conductor, the electrical insulation, and the thermal exchange between conductor and helium. We gained useful experience on the design problems and on the technological problems involved in the construction of a cryoconducting cable. A 20‐m aluminum cable cooled down to 25°K with pressurized helium flow was built and tested with 3500‐A dc under 20 kV; results are presented. On this model we solved the following types of problems. First, mechanical problems concerning cooling down of the cable, thermal contraction of the pipes, electrical insulation and conductors, construction of an invariable cable constituted by elementary helicaly wound conductors were solved. Second, thermal problems of reduction of heat leaks, conception of thermal insulation, and segmentation of vacuum jackets were solved. Third, electrical problems of design of 300°–‐25°K leads were solved; this problem of losses at both ends is, in proportion, more important for short model than for long cable. Finally, refrigeration problems of helium and nitrogen flows, thermal shields and design of refrigerators (optimal capacity and spacing) were solved. In order to solve problems concerning the cable itself, research has been done on superconducting materials, electrical insulation and heat exchange. Surface losses and critical currents of superconductors were compared. The losses mainly depend on the environment of conductors (electrical and thermal interface impedance between superconductor and stabilizing material, thermal impedance of the conductor versus helium, geometric irregularity of layers leading to transverse magnetic fields···). Results were presented on electrical insulation. A satisfactory dielectric is a key factor in a high‐capacity cable system. Measurements of the dielectric strengths of plastic tapes, immersed in liquid and hypercritical helium, have been made; the values obtained for 100‐&mgr; tapes are about 100 kV/mm and are similar with ambient temperature values. Breakdown strength has been measured for different electrode spacings and at various pressures. Dielectric losses are responsible for important thermal losses in cables; loss tangents of several materials have been measured at various temperatures and frequencies; the values decrease down to 5×10−6. Vacuum is not only a thermal insulation medium but also an electrical insulation environment. Dielectric strength and field emission currents have been measured at cryogenic temperatures for vacuum spacings and solid insulation in vacuum. The laws of thermal transfer between conductor and helium were studied for conventional configurations. Exchange coefficients were calculated and correlations between these coefficients and pressure drop were established. This experience is now used for the design of a superconducting cable. Several dc and ac cable configurations are under study. One of these systems consists of coaxial cables, in a pressurized helium pipe, with multilayer impregnated helium insulation. Another consists of pipes and multilayer insulation under vacuum. Preliminary calculations were made in order to optimize geometrical and electrical features of a superconducting ac cable. The values of electric field, current density, voltage, and ratio between coaxial cable diameters can be chosen to obtain a minimum value of the sum of costs relative to dielectric losses, reactive power compensation, and cable cost. Design studies of a superconducting high‐power cable are now going on; research and tests on superconducting materials, insulating materials, heat exchange, refrigeration and safety will be continued. Our aim is to design a model a few tens of meters long which will be built in 1972 to show that many of the problems involved in the construction of superconducting cables in the range of several thousand MVA have been solved.

ISSN:0021-8979    

DOI:10.1063/1.1659552

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Alternating current superconducting cables which can be readily integrated into utility systems require both a superconductor with low characteristic ac losses and a high‐performance cryogenic system to minimize the cost of heat pumping the thermal losses to the atmosphere. Moreover, the resulting cable systems must operate with the long‐term reliability expected of utility systems. Recent publication have treated the measurement of the superconducting material properties and the conceptual electrical and cryogenic design for a superconducting cable. This paper will discuss a cable configuration and examine its electrical and cryogenic design features with principal emphasis placed upon the relationship of these features with present technological capabilities. Fabrication and installation feasibility and the expected levels of performance and reliability can be inferred from the practical operating cryogenic systems which are used as a guide for the selection and design of the cryogenic enclosure for the cable.

ISSN:0021-8979    

DOI:10.1063/1.1659553

出版商:AIP    

年代:1971

数据来源: AIP