Superconducting magnets in the world of energy,especially in fusion power

Industrial applications of superconducting magnets are only feasible in the near future for superconducting homopolar machines and possibly MHD generators. In both cases there are no longer any major problems with the superconducting windings or cryogenic systems, but technological problems connected with other components are governing the chances of large scale applications.For superconducting synchronous machines, after the successful operation of machines in the MVA range, a new phase of basic investigations has started. Fundamental problems which could not be studied in the MVA machines, but which influence the design of large turbo-alternators, must now be investigated.Fusion power by magnetic confinement will probably be the largest field of application for superconducting magnets in the long run. The present research programmes require large superconducting magnets by the mid-1980s for the experimental reactors envisaged at that time.In addition to dc windings, pulse-operated superconducting windings are required in some systems, such as Tokamak. The high sensitivity of the overall plant efficiency and the active power demand of the pulsed windings require great efficiency from energy storage and transfer systems. Superconducting energy storage systems would be suitable for this, if transfer between inductances could be provided with sufficient efficiency. Basic experiments gave encouraging results.In power plant systems and electric machines an extremely high level of reliability and availability has been achieved. Less reliability will not be accepted for systems with superconducting magnets. This requires great efforts during the development work.     

The Mott-Hubbard insulating state and orbital degeneracy in the superconducting C60(3-) fulleride family

Electron correlation controls the properties of important materials such as superconducting and magnetoresistive transition metal oxides and heavy fermion systems. The role of correlation in driving metal-to-insulator transitions assumes further importance because many superconducting materials are located close to such transitions. The nature of the insulating ground state often reveals the dominant interactions in the superconductor, as shown by the importance of the properties of La2CuO4 in understanding the high-temperature-superconducting cuprates. The A3C60 alkali metal fullerides are superconducting systems in which the role of correlation in both the normal state and the superconducting pairing mechanism is controversial, because no magnetic insulator comparable to the superconducting materials has been identified. We describe the first example of a cubic C60(3-) system with degenerate orbitals that adopts the Mott-Hubbard insulating localized electron ground state. Electron repulsion is identified as the interaction that is suppressed on the transition to metallic and superconducting behaviour in the fullerides. This observation is combined with ab initio calculations to demonstrate that it is the orbital degeneracy that allows the superconducting cubic A3C60 fullerides to remain metallic while provoking electron localization in systems with lower symmetry.     

Superconducting quantum detectors in YBCO

The discovery of high-temperature superconductors has led to great efforts to find potential applications, including the development of photon detectors. We review the limitations of the different approaches proposed for realizing superconducting photon detectors. The structure and operation of a new quantum superconducting kinetic inductance detector (QSKID) with a SQUID readout circuit is described. The QSKID is made from a superconducting loop where the photosignals are generated in response to photoinduced changes in the condensate's kinetic inductance. The QSKID operates in the zero-resistance superconducting state, thereby circumventing Johnson noise.     

A variational approach to magneto-elastic buckling problems for systems of ferromagnetic or superconducting beams

Based upon a variational principle derived in a preceding paper, expressions for the magneto-elastic buckling values for ferromagnetic or superconducting systems are given. These relations are evaluated for systems of slender beams. Explicit buckling values are calculated for a single ferromagnetic or superconducting beam of arbitrary cross-section, and for systems of two parallel ferromagnetic or superconducting rods. In the analysis needed for the calculation of the intermediate (i.e., rigid-body) and the perturbed magnetic fields, an intensive use of methods inherent in the theory of complex functions is made. In conclusion our results for a set of two superconducting rods are compared with the results of a mathematically less complicated, but also less rigorous, theory.     

Review of accelerator magnet design in the world

It is pointed out that the construction of large particle accelerators and storage rings requires the design and fabrication of large numbers of bending, focusing, and correction magnets. Normal conducting iron yoke magnets are standard for low fields. Two designs representing efforts for manufacturing cost reduction are discussed. Emphasis is placed on superconducting magnets necessary for high fields. Many details of the well-established cos&thetas; one or two layer coil design using NbTi superconductor are presented. Peculiarities in the use of Nb₃Sn superconducting material are mentioned. Examples of alternative superconducting magnet designs are given. Finally, the performance of HERA (Hadron-Electron Ring Accelerator) magnets produced in quantity by industry is presented     

Investigation of superconducting magnetic systems using experiment planning

The advisability of applying the theory of experiment planning to a wide range of problems which study the static and dynamic characteristics of superconducting magnetic systems is demonstrated.Techniques used for the investigation of superconducting elements are presented. Also example plots of the analytic dependences of indices, which characterize the quality of the operation of systems with superconducting elements, are given with the application of experiment planning.     

Refrigerator-recirculator systems for large forced-cooled superconducting magnets

Forced-cooled superconductors are viewed as a promising alternative in the development of high field superconducting magnets for future fusion devices. The high current density cable superconductor is protected against thermal instabilities by forcing (single phase) supercritical helium through the cable.The cryogenic cooling system for a forced-cooled superconducting magnet works as a refrigerator and a reciculator at the same time. The paper discusses the conceptual design of the cooling systems for forced-cooled superconducting magnets with the overall objective of reducing the refrigeration costs. The general conclusion of this article is that economic cooling systems must employ efficient cold pump recirculators in which the large flow demanded by the forced-cooled superconducting magnet is confined to the cold end of the refrigerating column. If the liquid helium pump efficiency is less than 40%, systems employing elevated temperature compressors are more economic.     

Electron - phonon effects in heavy fermion systems

A discussion of ultrasonic propagation in normal and superconducting heavy fermion systems is presented. In the normal state the temperature and magnetic field dependence of elastic constants is discussed. The regions of adiabatic and isothermal sound propagation are shown. In the superconducting state attenuation effects and B-T phase diagrams are discussed for different substances. The Grüneisen parameters as the important electron-phonon coupling constants in the different phases are given and discussed.     

Cryogenic helium gas circulation system for advanced characterization of superconducting cables and other devices

A versatile cryogenic test bed, based on circulating cryogenic helium gas, has been designed, fabricated, and installed at the Florida State University Center for Advanced Power Systems (FSU-CAPS). The test bed is being used to understand the benefits of integrating the cryogenic systems of multiple superconducting power devices. The helium circulation system operates with four sets of cryocooler and heat exchanger combinations. The maximum operating pressure of the system is 2.1 MPa. The efficacy of helium circulation systems in cooling superconducting power devices is evaluated using a 30-m-long simulated superconducting cable in a flexible cryostat. Experiments were conducted at various mass flow rates and a variety of heat load profiles. A 1-D thermal model was developed to understand the effect of the gas flow parameters on the thermal gradients along the cable. Experimental results are in close agreement with the results from the thermal model.     

Search for New High-Tc Superconductors at Elevated Temperatures

Journal of Superconductivity and Novel Magnetism - One of the long-standing challenges in experimental superconductivity are the sustained efforts to achieve stable and cheap superconducting...     

Superconducting resonance systems in precise measuring units

This paper presents the results of studies on high stability oscillators with superconducting resonance systems incorporated, which operate at superhigh and radio frequencies. A superconducting resonance system was also used to build a model superconducting gravimeter of 10^?10 g sensitivity.     

Refrigeration for rotating superconducting windings of large ac electric machines

The rotating superconducting windings of large ac machines must be supplied with enough refrigeration at an adequate temperature. The problems associated with bringing liquid helium to the rotating winding structure are analysed. Existing designs as well as newly proposed systems are used to draw the general guidelines for the design concepts of cooling systems for rotating superconducting windings.     

Critical Radius and Magnetic Field for Vortex Nucleation on a Superconducting Spherical Surface

Spherical, two-dimensional electron systems are realized in nanoshells and multielectron bubbles in helium. In this contribution, we investigate the superconducting state of such shells using a variational Ginzburg-Landau formulation. A phase diagram as a function of the sphere radius and the magnetic field is derived, and we calculate a critical radius below which vortices cannot be nucleated on the superconducting spherical surface.     

Normal zone propagation within a superconducting magnetic system and cryostabilization of superconductivity

A theoretical study of normal zone propagation along composite conductors in contact with liquid helium which accounts for the resulting rate of heat release and non-uniformity of heat conduction coefficient is described. The Maddock-James-Norris theorem is shown to be true only for a constant heat conduction coefficient. The formulae are given for the speed of the normal zone propagation. Equations have been obtained which describe the collapse dynamics or normal zone growth within the superconducting magnetic systems; time of collapse and normal zone growth being evaluated. Non-linear heat waves of a new type are assumed to exist in a composite in addition to the solution with an interface of normal and superconducting regions.     

Application of superconductivity in the controlled thermonuclear research program

Applications of superconductivity will be needed in each of the three magnetic confinement systems being developed in the AEC controlled thermonuclear research program. Fusion plasmas must be confined by magnetic fields instead of solid walls. The tokamak and magnetic mirror confinement systems will need superconducting magnets to produce the confining magnetic field. The theta-pinch confinement system will need superconducting energy storage coils and homopolar machines to provide energy for pulsed magnetic fields. The AEC is supporting developments for these three systems but it is not yet known which magnetic confinement systems will be used in fusion power reactors. The technology problems associated with these applications of superconductivity are described in the paper.     

A “permanent” high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen

A new design for a portable “permanent” superconducting magnet system is explored. The design involves a persistent-mode high-temperature superconducting (HTS) magnet that is cooled by a solid heat capacitor. The system is an alternative to permanent low-temperature superconducting (LTS) magnet systems where the magnet is cooled by a bath of liquid helium.An apparatus was constructed to demonstrate stable operation of a permanent magnet wound with Bi2223/Ag conductor while in thermal communication with a mass of solid nitrogen. The apparatus includes a room-temperature bore and can function while it stands alone, detached from its cooling source, power supply, and vacuum pump. The magnet is operated in the 20–40 K temperature range. This apparatus is the first to demonstrate the operation of a superconducting magnet with a permissible temperature variation exceeding a few degrees kelvin. Models are developed to predict the experimental system's warming trend and magnetic field decay. The models are validated with a good agreement between simulations based on these models and experimental results. The results indicate that present HTS conductor critical current and index are not yet sufficient to provide field strengths and field decay time constants that are required for typical persistent-mode applications.     

Continuous Low‐Bias Switching of Superconductivity in a MoS2 Transistor

Engineering the properties of quantum electron systems, e.g., tuning the superconducting phase using low driving bias within an easily accessible temperature range, is of great interest for exploring exotic physical phenomena as well as achieving real applications. Here, the realization of continuous field‐effect switching between superconducting and non‐superconducting states in a few‐layer MoS₂ transistor is reported. Ionic‐liquid gating induces the superconducting state close to the quantum critical point on the top surface of the MoS₂, and continuous switching between the super/non‐superconducting states is achieved by HfO₂ back gating. The superconducting transistor works effectively in the helium‐4 temperature range and requires a gate bias as low as ≈10 V. The dual‐gate device structure and strategy presented here can be easily generalized to other systems, opening new opportunities for designing high‐performance 2D superconducting transistors.     

Conceptual Design of a Field Coil for 5 MW HTS Synchronous Machine

Compared with a conventional rotating machine, a superconducting rotating machine fabricated by High Temperature Superconducting (HTS) tape has superior performance and efficiency due to the HTS field coil for the rotor which can generate high magnetic flux intensity. The two primary factors for the design of the HTS rotational machine are how to construct the optimal magnetic field path through the air gap located between the rotor and the stator and how to enhance the linkage magnetic flux density between the armature coil in the stator and the field coil in the rotor. A 5 MW HTS motor for ship propulsion is planned for development in early 2011 by a Korean collaboration group of KERI and DOOSAN Heavy Industry. As a part of this R&D efforts, we designed and analyzed the field coil for a 5 MW HTS synchronous motor. In this paper, the computational results of the magnetic field distribution on the whole winding regions of the HTS field coil of the superconducting rotating machine will be also presented and discussed.     

Phase-Slip Phenomena in Proximitized NbN/NiCu Superconducting Nanostripes

Ultrathin superconducting/ferromagnet (S/F) nanostripes are very interesting systems to investigate both the physics involved on nanoscale size and as light sensitive elements for superconducting single photon detectors. The electrical transport properties of NbN(8 nm)/NiCu(10 nm) nanostripes are presented down to a temperature of 4.2 K. A number of voltage steps were observed on the current-voltage characteristics, and they were investigated at different temperatures. A possible explanation in terms of active phase-slip phenomena has been proposed on the basis of the time-dependent Ginzburg-Landau theory, leading to an estimation of the inelastic electron-phonon relaxation time around 0.8 ps. The latter value was found to be in good agreement with a relaxation time, independently measured by femtosecond transient optical reflectivity experiments performed on the same bilayer.