Current distribution calculation for a superconducting multi-layered power cable

The calculation method for the current distributions within a coaxial multi-layered power cable consisting of helically wound long superconducting tape conductors have been studied. The analytical expressions for the self- and mutual inductances of the coaxial helical long thin arch-shaped tapes are analytically and numerically compared with straight arch-shaped and flat tapes. It is confirmed that these expressions are consistent with each other. The current distributions within a coaxial 3-layered cable are calculated, using the self- and mutual inductances obtained due to the thin approximation. The effect of the non-uniform gaps between tapes in a coaxial multi-layered power cable is studied.     

Circuit analysis model for AC losses of superconducting YBCO cable

Information about AC losses and electromagnetic behaviour is essential when designing superconducting cables. In this work, AC losses of coaxial YBCO cables are determined using circuit analysis based computational model tailored for the needs of the YBCO cable design work. In the equivalent circuit superconducting layers are connected in parallel, the layers have an inductive coupling between each other and AC loss within a layer generates an effective resistance. The layer currents can be solved from a set of circuit equations. The computational model takes into account that the current in the cable creates magnetic field, which generates magnetisation loss and affects strongly the critical current of the YBCO tapes. The model was applied on several coaxial superconducting YBCO cable designs, which had nominal currents of 1-10 kA (rms). Low AC loss values were predicted for these compact YBCO cable designs. For example, AC losses less than 4 W/m were predicted for 10 kA cables.     

50 Hz loss measurements on superconducting niobium coaxial conductors

A method is described for the measurement of ac losses in a cylindrical conductor forming part of a coaxial superconducting pair. Results given for 25 μm niobium are substantially lower than previous values obtained for this material when tested in the form of narrow strips, and show that, if used as the conductor in a superconducting power cable, the material could carry a peripheral current density of 360 A cm^?1 with tolerable losses (< 10 μW cm^?2) at temperatures of up to 7 K.     

Development of Superconducting Coaxial Cables for Cryogenic Detectors

Fast readout with low noise is essential to apply superconducting detectors to such experiments as time-of-flight mass spectrometry or X-ray spectroscopy, where high counting efficiency is required. We have developed a thin seamless superconducting coaxial cable employing NbTi alloy in both of the outer and inner electrical conductors. The outer diameter is 1.60 mm, and both conductors are separated by dielectric material of PTFE. The coaxial cable revealed to become superconductive below about 10 K. The thermal conductance was measured between 0.4 and 6 K and consistent with literature. Performance at high frequency was also measured at 4.2 K. The attenuation was very small and less than 1 dB up to about 7 GHz. The effect of mechanical treatment to thermal and electrical properties of NbTi alloy seems to be small in the present forming process of coaxial cable.      

Fabrication of sections of a Nb3Sn-based rigid superconducting cable

Using Nb₃Sn layers deposited onto the outer and inner surfaces of copper tubes 30 and 50 mm in diameter, we have fabricated sections of a rigid superconducting coaxial cable up to 1 m in length. The highest current-carrying capacity of the cable at 4.2 K was 800–850 A/mm, which corresponded to a critical current density of (5.0–5.5) × 10¹⁰ A/m² in the Nb₃Sn layer.     

Ultrafast response of superconducting transmission lines

The authors report investigations of picosecond transient propagation on normal and superconducting transmission lines and results of a variety of lines that include YBa₂Cu₃O_7-x (YBCO) coplanar lines, a superconducting coaxial cable, and a dielectric-matched gold-line structure. A previously developed algorithm for analyzing transient propagation was used to identify the dominant mechanisms for signal distortion in most of these cases, and the essential properties of all tested to date are summarized for a direct comparison     

Controlling for outer-diameter of superconducting cable for PF Cable-In-Conduit Conductor of ITER

Poloidal Field (PF) coils, made of CICC (Cable-In-Conduit Conductors), are the most important part of ITER (International Thermonuclear Experimental Reactor) superconducting magnet system. The structure of superconducting magnet system in ITER and the CICC are introduced. The main factors influenced the cabling of the superconducting cable for CICC is discussed. Analyzed the outer diameter controlling technique and the relationship between ac loss and structure parameters of the superconducting cable. The technique route of cabling has been established. Especially, the technique of dies setting for shaping + multi-rolling compacting dies + half-dies for diameter keeping is explained in detail, which can efficiently control the diameter and the cabling quality of the superconducting cable, without injury to strands line. The technical parameters for cabling is fixed and one 765 m long dummy cable and one 115 m long superconducting cable of CICC for PF coil is manufactured successfully for ITER the last.     

Low Temperature Properties of a Superconducting Niobium Coaxial Cable

Semirigid coaxial cables with seamless metal shields are promising for readout from sensitive devices operating below liquid helium temperature. Low thermal conduction of such cables are also essential to reduce heat penetration into cryogenic temperature. We have developed thin semirigid coaxial cables employing niobium-titanium and niobium in both center and outer conductors, taking advantage of low thermal conductivity and extreme small electrical resistivity of superconductors. We assembled an adiabatic demagnetization refrigerator and measured thermal and electrical characteristics of those superconducting coaxial cables below T _c. Thin niobium coaxial cable with an outer diameter of 0.86 mm showed two-orders lower thermal conduction than expected, which is considered as the effect of impurity of niobium and forming process. Small attenuation was observed up to high frequency above 10 GHz at 3 K.     

Temperature rise in a superconducting cable during overload

Approximate calculations are presented for the temperature rise and temperature fluctuation in an ac superconducting cable during current overload. The calculations are for a cable, which it is intended will remain superconducting throughout the overload. These show that because of the low value of T_c and low J_c niobium-titanium is not a suitable material to use. Niobium-zirconium with a 1 mm copper backing will carry continuous overloads of up to × 9 and a first cycle overload of up to × 11 provided it is fully stabilized against flux jumps. (Normal surface current rating = 4 × 10⁴ A m^?1). Niobium-tin can in principle carry very large overloads, but the limiting factors are its poor mechanical properties, the difficulty in producing thick layers of the material, and stabilization.     

Long flexible transfer lines for gaseous and liquid helium

Screened flexible four-fold coaxial transfer lines for gaseous and liquid helium with lengths of 5 to 50 m have been successfully built and tested. The lines for gaseous helium have to supply and return a mass flow of 5 to 10 g s⁻¹ at temperatures ranging between 350 and 20 K for cooldown or warmup of superconducting magnets. The lines for liquid helium have to supply up to 100 ℓ h⁻¹ for final cooldown or up to 25 ℓ h⁻¹ for normal operation of superconducting magnets. The hydrodynamic and thermal performance characteristics of the lines have been measured. The results are encouraging and it can be envisaged to use this kind of lines for superconducting magnets in high energy accelerators.     

Laying method for superconducting feeder cable along railway line

The superconducting feeder cable shrinks in cooling process from the room temperature to the temperature of liquid nitrogen. When a long superconducting feeder cable is constructed in a railway line, it is needed to consider countermeasures against the cable shrink due to cooling force. We report on a suitable method of laying the cable in 300-m class on the test track. We also report that the method is suitable for this cable because no buckling and rupture points were observed in the X-ray radiograph taken over the whole length of the cable after its installation.     

Superconductive dc transmission lines: design study study and cost estimates

A conservative design study of a dc superconductive power line based on present day technology is presented. Power is transmitted by opposing currents in coaxial conductors, formed from helically wound tapes of composite, with a potential difference between them. Conductor temperature stability and fault current conditions are shown to be the factors which dominate the conductor cost; both are analysed. The capital cost of a cable is expressed as a function of a cable dimension, the basic cable parameters, and the material properties. This cost may be minimized with respect to cable radius. The resultant optimum magnetic fields vary little with power rating and are quite low at about 1–2 × 10⁵ A m^?1. The capital cost of such cables, exclusive of terminal costs, is less than the cost of conventional dc cables at power levels above about 1 GVA. Although extensive development would be necessary, the basic principles needed to build a dc superconductive cable are known.     

Miniature superconducting high frequency coaxial lines

The use of superconductors in coaxial cables designed for high frequency transmissions leads to an appreciable reduction of both the losses and physical dimensions. In this paper, after recalling the definition of the electrical parameters of the coaxial lines, we review the theoretical and experimental studies of the miniature superconducting coaxial lines. An attenuation constant of 0.5 dBkm^?1 has been achieved at 1 GHz and 4.2 K with a 2 mm external diameter line having lead - plated copper conductors and a fluoroethylene - propylene dielectric. Such performances can only be achieved with lines of high geometrical and structural qualities, having few physical irregularities and without periodic non-uniformities. The results are promising for a new teletransmission medium and may be compared with the two other challengers, guided waves and optic fibres. However, the need for a cryogenic environment has for the present hindered extensive industrial development     

Cryopower transmission studies in Europe

European studies on cryopower transmission are focused on semiflexible and totally flexible superconducting cables. Intense investigations of cable components are being performed. Nb/Cu and Nb/Al composite conductors have been developed, which exhibit 50 Hz surfaces losses well below the design limit of ac cables at 4–6 K and surface peak flux densities up to 0.1 T. Measurements of breakdown voltages and dissipation factors at 4 K indicate that wrapped tape electrical insulation, impregnated with helium, may be suitable for superconducting cables, the achievable dielectrical strength being about 10 kV mm^?1 (rms). But further investigations are necessary within this topic. Different types of dc and ac cable joint and terminal have been designed or already constructed. A number of larger laboratory model tests with cable sections up to 50 m length have been performed or are planned for the near future with the aim of studying size effects, the interaction of different components and to demonstrate the feasibility and performance of superconducting cables.     

Application of phase-stabilized optical fiber in transmission ofreference and IF signals in VLBI observation

A phase-stabilized optical fiber (PSOF) cable, which is 880 m long, was successfully used to transmit a 100-520 MHz IF signal and 10 MHz reference frequency signal in Mark III very long baseline interferometry (VLBI) observations. Phase stability of the 1760-m round-trip PSOF cable was measured by a dual mixer time difference system. Results of the measurements show that the stability of the PSOF cable system is about 50 times better than that of an ordinary coaxial cable system. The measured phase fluctuations are smaller than the tolerance limit required for a coherence loss of less than 20% in VLBI observations with an integration time of several hundred seconds. Internal accuracies of the international geodetic VLBI observations, in which the PSOF cable system is used at a station, are proven to be in the level of the present-day standard. The results show that the performance of the cable is at least comparable with, and probably better than, that of coaxial cables of much shorter length widely used in VLBI stations     

Current test on a flexible superconducting core for a 2 GVA ac cable

A 5 m long prototype co-axial flexible superconducting cable core has been made and tested at currents up to 30kA. The inner and outer tubular conductors were both formed from helically-laid strips, and the dielectric between was lapped polyethylene tape. The dielectric was tested in separate experiments. The conductor strips contained layers of nobium, niobium-zirconium and high conductivity copper. The axial contraction of the cable core was restrained by titanium tie-rods and the lay angles of the conductor strips were chosen so that the core tightened radially on cooldown. Lead-filled termination cylinders between the ends of the cable and the current leads inhibited the formation and propagation of normal regions at high currents. Local and average ac loss measurements were made from 4.7 to 10.2 K and at current densities between 10 and 200 A mm^? with very satisfactory results.     

ac losses in cables of twisted multifilment superconductors

The losses due to coupling currents are calculated for a superconducting cable. The cable consists of multifilamentary superconducting wires which are twisted and surrounded by a material of different conductivity. It was found that if the wire twist pitch is different from the cable twist pitch, the losses diverge as 1/(1-f), where f is the volume fraction of wire in the cable. The time constant of the cable is increased by a similar factor. These large losses are caused by currents flowing between the wires of the cable and they can be minimized by choosing the correct ratio of cable pitch to wire pitch.     

Dielectric losses in a polyethylene insulated superconducting cable between 4 and 22 K

The dielectric losses in a 5 m length of a superconducting cable, insulated with lapped polyethylene tape, have been determined between 4.3 K and 22 K with an inductively coupled ratio arms bridge which gave a resolution in tan δ of about 10^?6. The cable sample was built to a specification appropriate for power transmission at 132 kV and 6 kA; it was impregnated with helium at 0.4 MPa and cooled by a flow of cold helium circulated from a refrigerator.In the temperature range 4 to 5 K, applicable to a cable with niobium conductors, the dielectric loss increased slightly with electric field, and the observed tan δ (relative to tan δ of the reference capacitor) was 20 × 10^?6 at the maximum test stress of 7 MV m^?1. It is thought that a significant part of this loss can be attributed to the bedding layers and electrostatic screens between the dielectric and conductors.Tan δ showed a minimum at 18 K, which is encouraging for future developments of superconducting cables which might use hydrogen as a coolant and dielectric impregnant. The observed variation of tan δ with temperature is consistent with the existence of a low temperature relaxation mechanism with low activation energy.     

Electrical characteristics of a dc superconducting cable

A dc superconducting cable is ideal for transmitting large blocks of electrical power over a long distance. However, it must be designed to operate reliably within the constraints of the electrical system. Therefore, system analysis must be performed for each application. The conductor losses caused by the harmonics on the dc must be within the design goals; a system fault should not drive the cable normal with eventual damage to the cable and interruption of power flow; and, the dielectric system of the cable must be designed to be compatible with the expected transient voltages by proper insulation coordination. Transient overvoltages are of concern to electrical power systems; these are especially critical to cryogenic cables because of the susceptibility of the cryogenic enclosures to these transients. This paper discusses the electrical system constraints which are particularly applicable to a dc superconducting cable and shows how such a cable can be designed to be compatible with the electrical system.This paper also summarizes the work on low temperature dielectrics performed at Los Alamos. It shows the variation of break down voltage of dielectric materials, in sheet form and cable configuration, with temperature and pressure under dc and impulse voltage. The surface flashover characteristics with large creepage distance as well as electrical conductivity of dielectric materials at cryogenic temperatures are discussed. These studies are essential for the design of high voltage apparatus operating in cryogenic environments.     

Thermal conduction measurement of miniature coaxial cables between 0.3 and 4.5 K for the wiring of superconducting detectors

Thermal conduction of thin coaxial cables made of 70-30 CuNi and stainless steel 304 with a PTFE insulator was measured in a temperature range between 0.3 and 4.5 K. Thermal conductivity of the 70-30 CuNi and PTFE was measured independently and their contribution to the thermal conduction of the coaxial cables was investigated. The thermal conductivity of the 70-30 CuNi differed from the literature by 25% at 0.36 K and 40% at 4.2 K, and the alloy exhibited a weak temperature dependence, which indicated the effects of mechanical treatment. It has been confirmed that the thermal conduction of the coaxial cables are low enough to keep a cold stage of ³He cryostats at a temperature below 0.3 K, even when one hundred cables are installed between 0.3 and 3 K for the read-out of superconducting tunnel junction arrays. The cables were installed in a cryogen-free ³He cryostat, and the operation below 0.3 K was successful.