Current-carrying Element Research Articles

Is a Propagating Infinite Plane Wave a "Radiation Field?"

The electric and magnetic fields of the Hertzian dipole, an infinitesimal time-varying current-carrying element with opposite charges residing at either end (via the continuity equation), are fundamental to the understanding of the fields of time-varying current and charge sources and, in particular, in solving for field patterns from current sources such as the half-wavelength antenna and various antenna arrays. That is, given the fields of the Hertzian dipole, one can sum (integrate) the fields of groupings of these infinitesimal current elements to determine the fields of any current source.

Generalised electromagnetic forces in an absolute nodal coordinate formulation

This paper focuses on the mechanical, dynamic numerical modelling of an electrical-current-carrying cable using a non-linear absolute nodal coordinate formulation. The derived equations are an expansion of the mechanical equation of motion. They allow interdisciplinary mechanical and electrical research of the cable dynamics where large electrical currents are present in the cables. A vector of the generalised electromagnetic forces in the absolute nodal coordinate formulation for a current-carrying finite element of arbitrary length is derived. For the cable elements a filament approach is used to determine the magnetic flux density. To round off the method’s applicability, the contact forces that can occur between free-moving cables are also considered. The generalised electromagnetic forces are derived and expressed with the pre-computed matrices. As the numerical testing shows, the presented method is computationally very effective. The equations are general and can be applied to any similar dynamic systems.

Fine structural characterization of the elements of a Nb-Ti superconducting cable

A niobium-titanium alloy (Nb+47%Ti) superconducting cable that acts as a current-carrying element in the magnetic system of the international thermonuclear experimental reactor (ITER) has been subjected to cold deformation by drawing. Its microstructure, phase composition and properties have been monitored via atomic force microscopy, electron microscopy and optical microscopy methods at the intermediate stage of cold drawing upon the transition of Ø1.3→Ø1.2mm. Plastic deformation centers are established to be localized at the breakage sites of the cable, and the shape and chemical composition of Nb-Ti fibers are determined at the defect-free region and breakage sites, as well. A partial lack of diffusion Nb barriers is also highlighted around Nb-Ti fibers at the breakage sites.

Current-carrying element based on second-generation high-temperature superconductor for the magnet system of a fusion neutron source

Application of current-carrying elements (CCEs) made of second-generation high-temperature superconductor (2G HTS) in magnet systems of a fusion neutron source (FNS) and other fusion devices will allow their magnetic field and thermodynamic stability to be increased substantially in comparison with those of low-temperature superconductor (LTS) magnets. For a toroidal magnet of the FNS, a design of a helical (partially transposed) CCE made of 2G HTS is under development with forced-flow cooling by helium gas, a current of 20–30 kA, an operating temperature of 10–20 K, and a magnetic field on the winding of 12–15 T (prospectively ~20 T). Short-sized samples of the helical flexible heavy-current CCE are being fabricated and investigated; a pilot-line unit for production of long-sized CCE pieces is under construction. The applied fabrication technique allows the CCE to be produced which combines a high operating current, thermal and mechanical stability, manufacturability, and low losses in the alternating modes. The possibility of fabricating the CCE with the outer dimensions and values of the operating parameter required for the FNS (and with a significant margin) using already available serial 2G HTS tapes is substantiated. The maximum field of toroidal magnets with CCEs made of 2G HTS will be limited only by mechanical properties of the magnet’s casing and structure, while the thermal stability will be approximately two orders of magnitude higher than that of toroidal magnets with LTS-based CCEs. The helical CCE made of 2G HTS is very promising for fusion and hybrid electric power plants, and its design and technologies of production, as well as the prototype coils made of it for the FNS and other tokamaks, are worth developing now.

Special features of reconstruction of defect structure of epitaxial films of CeO2 and La2Zr2O7 in alternating magnetic field

Results obtained by methods of X-ray spectroscopy and atomic force microscopy (AFM) and special features of reconstruction of the defect structure of epitaxial films of cerium oxide CeO2 and lanthanum zirconate La2Zr2O7 during their processing in an alternating magnetic field or during their magnetic structural processing (MSP) are reported. The epitaxial films of CeO2 and La2Zr2O7 are seed and barrier layers of a buffer current-carrying element of a high-temperature superconducting second-generation wire (HTSC-2 wires). Changes in the crystalline structure of epitaxial films of CeO2 and La2Zr2O7 are reflected in changes in their X-ray diffraction patterns and AFM data. The X-ray spectroscopy indicated a change in the crystal structure of a biaxially textured substrate of Ni–5% W alloy as a result of MSP of sample buffer CeO2/La2Zr2O7. The obtained data indicate the existence of complex and fast processes in the crystal structure of epitaxial films of CeO2 and La2Zr2O7 and of Ni–5% W alloy at MSP, which result in irreversible changes in the crystal structure of these materials; in other words, it is found that, in the epitaxial films of CeO2 and La2Zr2O7 and in the Ni–5% W alloy, a magnetic structural effect is observed in an alternating magnetic field [1–3].

Stable overload conditions of high-temperature superconductors at alternating current injection

The stability of alternating current injected into a high-temperature superconductor or into a current-carrying element on its basis is studied under weak cooling. The stability conditions of the current varying with time by a sinusoidal law are studied versus its frequency. It is shown that before unstable states set in, the peak values of the electric field intensity, current, and temperature in the superconductor are higher than the values determining a thermal electrodynamic stability boundary of the current permanently flowing through the superconductor—the so-called thermal quench current. It is found that ultimate stable alternating currents cause high stable thermal losses in superconductors; these losses being not considered in the modern theory of losses. Such stable conditions can be referred to as overload conditions. Analysis shows that there are characteristic times determining the time intervals within which alternating current is stable under overload conditions. Main thermoelectrodynamic mechanisms behind the existence of these intervals are formulated. They explain why the superconductor stable overheating and induced electric field reach high values before the injected alternating current becomes unstable. The existence of overload conditions considerably extends the application area of high-temperature superconductors.

Temperature factor for magnetic instability conditions of type – II superconductors

The macroscopic development of interrelated electrodynamics and thermal states taking place both before and after instability onset in type-II superconductors are studied using the critical state and the flux creep concepts. The physical mechanisms of the non-isothermal formation of the critical state are discussed solving the set of unsteady thermo-electrodynamics equations taking into consideration the unknown moving penetration boundary of the magnetic flux. To make it, the numerical method, which allows to study diffusion phenomena with unknown moving phase-two boundary, is developed. The corresponding non-isothermal flux jump criteria are written. It is proved for the first time that, first, the diffusion phenomena in superconductors have the fission-chain-reaction nature, second, the stability conditions, losses in superconductor and its stable overheating before instability onset are mutually dependent. The results are compared with those following from the existing magnetic instability theory, which does not take into consideration the stable temperature increase of superconductor before the instability onset. It is shown that errors of isothermal approximation are significant for modes closed to adiabatic ones. Therefore, the well-known adiabatic flux jump criterion limits the range of possible stable superconducting states since a correct determination of their stability states must take into account the thermal prehistory of the stable magnetic flux penetration. As a result, the calculation errors in the isothermal approximation will rise when the sweep rate of an external magnetic field or the size of the superconductor’s cross-sectional area increase. The basic conclusions formulated in the framework of the critical state model are verified comparing the experimental results and the numerical analysis of the stability conditions and the temperature dynamics of the helicoid-type superconducting current-carrying element having real voltage–current characteristic.On the whole, the non-isothermal stability conditions expand the existence of allowable stable superconducting states. The non-isothermal approximation permits also to link the theories of the losses, the magnetic instability and the thermal stabilization of superconductors, which are independently developed.

Maximum allowable currents in YBa2Cu3O7 superconducting tapes as a function of the coating thickness, external magnetic field induction, and cooling conditions

Maximum allowable (ultimate) currents stably passing through an YBa2Cu3O7 superconducting current-carrying element are determined as a function of a silver (or copper) coating thickness, external magnetic field induction, and cooling conditions. It is found that if a magnetic system based on yttrium ceramics is cooled by a cryogenic coolant, currents causing instabilities (instability onset currents) are almost independent of the coating thickness. If, however, liquid helium is used as a cooling agent, the ultimate current monotonically grows with the thickness of the stabilizing copper coating. It is shown that depending on cooling conditions, the stable values of the current and electric field strength preceding the occurrence of instability may be both higher and lower than the a priori chosen critical parameters of the superconductor. These features should be taken into account in selecting the stable value of the operating current of YBa2Cu3O7 superconducting windings.

Development of Corrector Magnet Designs for the SIS 300

As part of participation in project FAIR, IHEP has been developing designs of fast-cycling superconducting corrector magnets for the SIS 300 ring. There will be four types of corrector magnets: a steering dipole with vertical and horizontal dipole coils, a chromaticity sextupole, a resonance sextupole and a multipole magnet, consisting of quadrupole, sextupole and octupole coils. The paper presents requirements for the magnets, optimizations of the cross-section and 3D geometries, main magnetic characteristics, selection of the mechanical structure of the magnet to confine all forces affecting the coil. Since the magnets are fast-cycling, special attention is given to the choice of the current-carrying element by taking into account quench processes. Numerical analysis of AC losses and magnet temperature margin will also be presented.

Current Stability Boundary of Liquid Helium Cooled YBCO Tape

To make an YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> superconducting current-carrying element, it is important to determine the boundary of the limiting currents flowing stably in a superconductor. Therefore, the current stability analysis of Cu/Ag/YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> /Hastelloy coated tape cooled by liquid helium for coil quench design was performed. It was assumed that an external magnetic field is parallel to its surface and varies in the range from 10 T to 30 T. Under these conditions, the stable current modes of a superconducting tape are limited by the transition from the nucleate to the film boiling regimes. This current instability problem was studied using static zero-dimensional model when the thickness of a copper layer was varied in the wide range. It is shown that the instability currents increase practically linear with increasing the copper thickness. Current-sharing analysis depicts that this current-carrying capacity variation is due to the corresponding increase of the current stably flowing in the copper layer. Namely, the calculations indicate the existence of the stable sharing current, which is about 0.7 A per 1 ¿m copper thick at 30 T. As a result, the stable current sharing may occur at currents that exceed noticeably the critical currents of the tape in the wide variation range of the external magnetic field.

Bacteriorhodopsin as an electronic conduction medium for biomolecular electronics

Interfacing functional proteins with solid supports for device applications is a promising route to possible applications in bio-electronics, -sensors, and -optics. Various possible applications of bacteriorhodopsin (bR) have been explored and reviewed since the discovery of bR. This tutorial review discusses bR as a medium for biomolecular optoelectronics, emphasizing ways in which it can be interfaced, especially as a thin film, solid-state current-carrying electronic element.

Results of Contact Resistance Distribution in NbTi and Nb$_{3}$Sn ITER Conductor Termination

The contact resistance distribution among the current-carrying elements of three ITER conductor terminations is measured in the JORDI (joint resistance distribution) test facility at CRPP. One sample is based on Nb3Sn strands, the other two are NbTi cable-in-conduit with different cable pattern and termination layout. The current from a 10 kA power supply is split by precision, low temperature shunt resistors into up to 96 parallel channels, each connected to the first or second cable stage of the conductor sample. The voltage drop is sensed between each current carrying element and the equi-potential surface of the joint connecting the conductor termination to the facility current leads. The current distribution in normal operation of a superconducting coil (imposed voltage) is straightforward deduced from the resistance distribution measured in JORDI under imposed current balance. For the same termination layout, the resistance distribution of the Nb3Sn sample is much flatter than in the NbTi samples.

Results of Contact Resistance Distribution in ITER Size Conductor Termination

JORDI (JOint Resistance DIstribution), a dedicated test facility for the measurement of contact resistance distribution in full-size, ITER type conductor terminations, has been successfully commissioned at CRPP. In contrast to earlier attempts, where the contact resistance in terminations was measured in current unbalanced condition, in JORDI the contact resistance is measured under imposed balanced current distribution among all current-carrying elements of the termination. The current from a 10 kA power supply is split by precision, low temperature shunt resistors into 98 parallel channels, each connected to the first cable stage of a full size ITER conductor. The voltage drop, in the range of few tens of nanovolt, is simultaneously sensed between each current carrying element (first cable stage) and the equipotential surface of the joint between the conductor termination and the facility current lead. We report first results of the contact resistance distribution in a prototype of typical ITER terminations, made of NbTi based cable-in-conduit conductor. The results of the measurements are analyzed based on a simple statistical model

A Fast Wave Antenna for ICRF Plasma Heating

The excitation of slow waves by the fast wave antenna could lead to the undesirable heating of plasma periphery and must be suppressed. The effects of slow wave excitation are analyzed using a one dimensional analytical model. The effect of conversion of fast wave to slow wave at the plasma edge is discussed. The efficiency of the slow wave generation with the different components of the alternating current and the surface charge which is induced on the antenna current-carrying elements is studied. Qualitatively, the minimum slow wave excitation could be achieved if: the electrostatic fields induced by the antenna are shielded effectively; the currents in the antenna elements and those ones induced in the shield by the antenna magnetic and electrostatic fields are directed mainly perpendicular to the steady magnetic field. In practice, both the requirements above mentioned could not met rigorously. Thus, all the existing antennas excite slow waves. Prom this point of view, properties of unshielded and shielded strap antennas are discussed. A new antenna which provides the minimum slow wave excitation is proposed. It consists of the strap current-carrying element and a number of grounded shield elements of the similar design. The antenna impedance properties are analyzed in the framework of the continuos one-dimensional model. The analysis showed that the antenna impedance could be lower than that one for simple strap antenna. Like a TEM-mode transmission line it has resonances which could be used for decreasing the reactive part of the impedance which makes easier the matching of the antenna with feeding electric circuits. The newly proposed antenna could be used both in large scale and small plasma devices. The experimental testing of it in a mirror device in comparison with a standard strap antenna is of primary interest.

Stability of the superconducting state of a multiconductor current-carrying element during the induction of current in it

A model which permits estimation of the temporal variation of the temperature and the current in the components of a superconducting cable is proposed. The permissible currents which can be induced in a six-strand current-carrying element without destroying its superconducting properties are determined. It is shown that in the presence of inductive coupling between the conductors, the position of the cable component in which instability is initiated is not a regular function of the current induction rate if there is a spread in the nonlinearity parameters of the current-voltage characteristics of the components.

Kinetics of the irreversible propagation of a thermal instability in the presence of a nonuniform temperature distribution over the cross section of a superconducting composite

The conditions for the irreversible propagation of a normal zone along a composite superconductor are investigated within the model of a continuous medium with consideration of its transverse thermal conductivity under the assumption of a uniform distribution of the current over the cross section of the wire. The numerical experiments performed for a currentcarrying element of circular cross section with variation of the cooling rate and the transverse dimensions are compared with known results of the one-dimensional theory. It is shown that the one-dimensional theory, as opposed to the two-dimensional theory, leads to underestimated values of the velocity of a thermal instability. The size effect modifies the propagation conditions of a normal zone to the greatest extent as the heat-transfer coefficient increases. This law is based on an increase in temperature at the center of the wire with a simultaneous decrease on its surface, as a result of which the mean temperature over the cross section of the composite increases as its radius increases.

Characteristics of the formation of a resistive zone in spatially bounded multiple-wire superconducting current-carrying elements

An analysis is made of possible mechanisms for the propagation of a normally conducting zone in a superconducting multiple-wire current-carrying element spatially bounded in one direction. It is shown that irreversible growth of the resistive zone is characterized by the formation of a flattened front propagating in the spatially unbounded direction.

The method of dynamic separation and its application to quantitative thermal analysis of microelectronic devices by laser interferometry and reflectometry

We present an original method for processing experimental data in quantitative thermal analysis by laser interferometry and reflectometry on normally functioning microelectronic devices. This method hinges on the different temporal behaviours of thermally induced reflectometric and interferometric responses of a device under electric excitation. It permits separation of interferometric contributions related to heat diffusion from those induced `instantaneously' by the heat source's temperature variation. A useful result is that, instead of complex non-stationary modelling, it suffices to perform just stationary modelling of the separated contributions to derive thermal data of interest, namely the device's functioning temperature, the material's thermoreflectance coefficient and so on. A detailed description of the development of the proposed method is given for a current-carrying element in an integrated circuit.

A superconducting hydrogen-cooled switch on Nb-Sn tape

Presented are our results on R & D and testing of a superconducting switch (SCS). For a current-carrying element a tape was used with a layer from Nb 3Sn produced via a liquid-phase diffusion deposition on a Nb-Zr substrate match-oxidized for optimal performance. A peculiar feature of the switch is its operation at 12K and cooling with solid hydrogen, sublimated from a porous coppermade, cryopumped coolant-accumulator vessel. A short closed-cycle configuration was made with operating current 130A, - superconducting magnetic system (SCMS) - SCS.

Force-cooled current leads for the force-cooled superconducting magnets of the Nuclotron

Force-cooled current leads for force-cooled superconducting magnets have been constructed and tested. The current leads were designed for nominal currents of 6 and 9 kA. The current-carrying element consists of a copper box with welded shields and its cooling surface is 0.9 m 3. The specific heat influx at nominal current is 1 W kA −1, the hydraulic resistance 10 kPa and the electrical insulation strength 2–20 kV. The 6–9 kA current leads have now been operating in the test arrangement of the Nuclotron over a long period of time.