James William Bray

Susceptibility calculations for alternating antiferromagnetic chains

Earlier work of Duffy and Barr consisting of exact calculations on alternating antiferromagnetic Heisenberg spin‐1/2 chains is extended to longer chains of up to 12 spins, and subsequent extrapolations of thermodynamic properties, particularly the susceptibility, are extended to the weak alternation region close to the uniform limit. This is the region of interest in connection with the recent experimental discovery of spin‐Peierls systems. The extrapolated susceptibility curves are compared with corresponding curves calculated from the model of Bulaevskii, which has been used extensively in approximate theoretical treatments of a variety of phenomena. Qualitative agreement is observed in the uniform limit and persists for all degrees of alternation, but quantitative differences of about 10% are present over the whole range, including the isolated dimer limit. Potential application of the new susceptibility calculations to experiment is discussed.

The Spin-Peierls Transition

In a spin-Peierls transition, a spin-lattice system consisting of one-dimensional antiferromagnetic linear chains in a 3-D lattice progressively dimerizes and thereby becomes nonmagnetic at T = 0. Like the usual Peierls transition, this is a soft-mode transition associated with a “fermi-surface-driven” instability (in a pseudo-fermion representation). We discuss the character of the transition and make predictions concerning the dynamic structure factor.

Development of a High Speed HTS Generator for Airborne Applications

General Electric, under contract with the Air Force Research Labs (AFRL), has successfully developed and tested a high speed, multimegawatt superconducting generator. The generator was built to demonstrate high temperature superconducting (HTS) generator technology for application in a high power density Multimegawatt Electric Power System (MEPS) for the Air Force. The demonstration tested the generator under load conditions up to 1.3 MW at over 10,000 rpm. The new MEPS generator achieved 97% efficiency including cryocooler losses. All test results indicate that the generator has a significant margin over the test points and that its performance is consistent with program specifications. This demonstration is the first successful full-load test of a superconducting generator for the Air Force. In this paper we describe the development of the generator and present some key test results used to validate the design. Extrapolation to a higher power density generator is also discussed.

Superconductors in Applications; Some Practical Aspects

The first blush of success in the search for a new superconductor is usually a high transition temperature, Tc. However, all power applications of superconductors and most other applications requires good current carrying capacity, usually characterized by a critical current Ic, within substantial magnetic fields, usually characterized by a critical magnetic field Hc. Furthermore, a number of other characteristics must be satisfied before commercial success can be obtained, such as acceptable cost, mechanical strength, stabilizers, and appropriate insulation materials. I examine a number of superconductors, starting with the workhorse NbTi, and look at the hard road to success for the successful commercialization of a new superconductor. I also review the applications, potential and actual, in which superconductors might be used.

HIGH MAGNETIC-FIELD BEHAVIOR OF MEM-(TCNQ)2

Abstract The charge-transfer complex MEM(TCNQ)2 is a spin-Peierls system with a non-magnetic, singlet ground state at T=0. We report high-field magnetization data which provide some evidence for a new magnetic spin-Peierls phase at fields above 190 kOe. The experimental results are compared to those for TTFCuBDT.

Performance of an HTS Generator Field Coil Under System Fault Conditions

High-temperature superconducting (HTS) coils are generally stable against transient thermal disturbances. Protection against spontaneous quenches is not a main design issue for an HTS coil. However, HTS coils used in many electric devices such as motors, generators, transformers, and current limiters will operate under over-current fault conditions, which may result in a coil quench and thermal runaway. Those electric devices should be able to ride through some grid fault conditions and remain functional. This requires a certain over-current capability of the HTS coils. This paper discusses the over-current requirements from grid faults and the thermal transient responses of a BSCCO coil. It presents the analysis results of the coil subjected to over-current pulses at different operating conditions. The study also investigates the thermal runaway current and its relationship with the over-current pulse

High-Temperature Superconducting Homopolar Inductor Alternator for Marine Applications

This paper describes a high power density high-temperature superconducting (HTS) electric machine topology that is scalable for marine propulsion and power generation. The design, currently being pursued for airborne applications, is based on homopolar inductor alternator (HIA) technology, which is new within HTS applications. The basic machine design configuration of the HTS HIA is based on a stationary HTS field excitation coil, a solid rotor, and an advanced but conventional stator comprising liquid-cooled air-gap armature winding and an advanced iron core. High power density is obtained by the enhanced magneto-motive force capability of the HTS coil, the increased airgap flux density and armature current loading, and the high tip velocity of the rotor. Preliminary scaled up designs look attractive for three marine applications: propulsion drive, primary ship power generation, and power generation modules. The generators are driven directly by the turbines without the additional complexity of a clutch and gear system. A conceptual design study of a 36-MW 3600-r/min generator, a 4-MW 7000-r/min auxiliary generator, and a 36-MW 120-r/min and 4-MW 132-r/min propulsion motor are summarized.

High Power Density HTS Iron Core Machines for Marine Applications

We present a conceptual design study to develop high power density high temperature superconducting (HTS) machines for two navy applications: primary ship propulsion power generation and drive, and power generation modules (PGM). The objective of the study was to evaluate the design of a 36 MW, 3600 rpm generator, a 36 MW, 120 rpm propulsion motor, and a 4 MW, 7000 rpm power generation module. The generators are to be driven directly by the turbines without the benefit of clutch or gear. Several design concepts were evaluated, including iron core rotors and homopolar inductor alternator topologies. Engineering trade-offs make the re-introduction of ferromagnetic iron attractive for a number of reasons. High power density is obtained by the increased airgap flux density and armature loading, and, for high speed applications, the high tip velocity of the rotor.

Development of a high speed multi-megawatt HTS generator for airborne applications

General electric, under contract with the Air Force Research Labs (AFRL), has successfully developed and tested a high speed, multimegawatt superconducting generator. The generator was built to demonstrate high temperature superconducting (HTS) generator technology for application in a high power density multimegawatt electric power system (MEPS) for the air force. The demonstration tested the generator under load conditions up to 1.3 MW at over 10,000 rpm. The new MEPS generator achieved 97% efficiency including cryocooler losses. All test results indicate that the generator has a significant margin over the test points and that its performance is consistent with program specifications. This demonstration is the first successful full-load test of a superconducting generator for the air force. Here we report key features of the generator and test results.

Transient Capability of Superconducting Devices on Electric Power Systems

Superconducting devices operating within a power system are expected to go through transient overload conditions during which the superconducting coil has to carry currents above the rated values. Designing the coil to remain superconducting through any possible fault scenario can be cost prohibitive, necessitating operation beyond the critical current for short periods. In order to set operating limits and design adequate protection systems for superconducting devices connected to a power system, the region of safe operation of these devices has to be described with general capability curves. Existing standards that define limits for these over-current situations are based on copper winding experience that do not apply to devices with superconducting components because of the highly nonlinear interaction between magnetic fields, operating temperature, and current density in the superconductor, and the rapidly varying material properties at cryogenic temperatures. In this paper, the behavior of superconducting coils during over-currents is discussed and a simplified capability curve is described to help standardize device capabilities. These curves are necessary to aid power system designers in appropriately designing the system and associated protection systems.