Mechanism of the High-Tc Superconducting Dynamo: Models and Experiment

High-Tc superconducting (HTS) dynamos are experimentally proven devices that can produce large, >kA, DC currents in superconducting circuits, without the thermal leak associated with copper current leads. However, these DC currents are theoretically controversial, as it is not immediately apparent why a device that is topologically identical to an AC alternator should give a DC output at all. Here, we present a finite-element model, and its comparison with experiment, which fully explains this effect. It is shown that the DC output arises naturally from Maxwell’s laws, when time-varying overcritical eddy currents are induced to circulate in an HTS sheet. We first show that our finite-element model replicates all of the the experimental electrical behavior reported so far for these devices, including the DC output characteristics, and transient electrical waveforms. Direct experimental evidence for the presence of circulating eddy currents is also obtained through measurements of the transient magnetic field profile across the HTS tape, using a linear Hall array. These results are also found to closely agree with predictions from the finite-element model. Following this experimental validation, calculated sheet current densities and the associated local electric fields are examined for a range of frequencies and net transport currents. We find that the electrical output from an HTS dynamo is governed by the competition between transport and eddy currents induced as the magnet transits across the HTS tape. These eddy currents are significantly higher (∼1.5X) than the local critical current density J_c, and hence experience a highly non-linear local resistivity. This non-linearity breaks the symmetry observed in a normal ohmic material, which usually requires the net transport current to vary linearly with the average electric field. The interplay between local current densities and non-linear resistivities (which both vary in time and space) is shown to systematically give rise to the key observed parameters for experimental HTS dynamo devices: the open-circuit voltage V_oc, the internal resistance R_int, and the short-circuit current I_sc. Finally, we identify that the spatial boundaries formed by each edge of the HTS stator tape play a vital role in determining the total DC output. This offers the potential to develop new designs for HTS dynamo devices, for which the internal resistance is greatly reduced and the short circuit current is substantially increased.New Zealand (NZ) MBIE Endeavour Grant No. RTVU1707 NZ Royal Society Marsden Grant No. MFP-VUW1806

A new benchmark problem for electromagnetic modelling of superconductors: The high-T <inf>c</inf>superconducting dynamo

The high-Tc superconducting (HTS) dynamo is a promising device that can inject large DC supercurrents into a closed superconducting circuit. This is particularly attractive to energise HTS coils in NMR/MRI magnets and superconducting rotating machines without the need for connection to a power supply via current leads. It is only very recently that quantitatively accurate, predictive models have been developed which are capable of analysing HTS dynamos and explain their underlying physical mechanism. In this work, we propose to use the HTS dynamo as a new benchmark problem for the HTS modelling community. The benchmark geometry consists of a permanent magnet rotating past a stationary HTS coated-conductor wire in the open-circuit configuration, assuming for simplicity the 2D (infinitely long) case. Despite this geometric simplicity the solution is complex, comprising time-varying spatially-inhomogeneous currents and fields throughout the superconducting volume. In this work, this benchmark problem has been implemented using several different methods, including H-formulation-based methods, coupled H-A and T-A formulations, the Minimum Electromagnetic Entropy Production method, and integral equation and volume integral equation-based equivalent circuit methods. Each of these approaches show excellent qualitative and quantitative agreement for the open-circuit equivalent instantaneous voltage and the cumulative time-averaged equivalent voltage, as well as the current density and electric field distributions within the HTS wire at key positions during the magnet transit. A critical analysis and comparison of each of the modelling frameworks is presented, based on the following key metrics: number of mesh elements in the HTS wire, total number of mesh elements in the model, number of degrees of freedom (DOFs), tolerance settings and the approximate time taken per cycle for each model

CFD analysis of the thermal state of an overhead line conductor

At present commercial CFD packages such as Fluent, ANSYS CFX, and Star-CD are widely used for investigation of heat and mass transfer processes in various fields of engineering. These codes can also be successfully applied to estimate the thermal state of major components of electrical distribution networks, such as overhead lines, underground cables and transformers. This paper presents some results obtained from numerical modelling of the temperature field in the Lynx overhead conductor in both cross and parallel wind conditions using 2-D and 3-D CFD models. The CFD results obtained demonstrate that for an applied load of 433 A and considering the summer rating (Lynx conductors ER P27 [1]) the maximum temperature in the conductor is considerably lower (16 degrees) than the prescribed design conductor temperature. This indicates that there is headroom for increasing the ampacity of the conductor

Research data supporting "Origin of the DC output voltage from a high-Tc superconducting dynamo"

Research data supporting [Origin of the DC output voltage from a high-Tc superconducting dynamo]. Please see the README file for a description of the dataset

Modeling HTS non-insulated coils: A comparison between finite-element and distributed network models

High-temperature superconducting (HTS) non-insulated (NI) coils have the unique capability to bypass current through conductive turn-to-turn contacts, mitigating the possibility of a catastrophic failure in the event of a quench. However, this turn-to-turn conductivity leads to a significant increase in the coil decay/charging time constant. To understand this phenomenon, several modeling techniques have been proposed, including the lumped and distributed network (DN) circuit models, and more recently the finite-element (FE) models. In this paper, the decay results obtained from modeling HTS NI pancake coils using both a DN model and a 2D FE model approach are evaluated and compared. Steady-state fields, and transient charging and decay behaviors are calculated with each model and the results compared. Key differences are highlighted, including the computation speed and the capturing of various physical phenomena. Both models exhibit non-exponential decay during initial coil discharge due to current redistribution between the inner and outer turns. In addition, the FE model exhibits other effects arising from current redistribution in both the radial and axial directions, including remanent magnetization, and variation of the “apparent total inductance” during charging. Simulations of sudden discharge have also been analyzed using the common “lumped circuit” formula. This shows that extracted values for the apparent surface contact resistance between coil windings can differ by more than a factor of 5 from the initial input value. Our results confirms the optimal choice of architecture for future NI coil models and emphasize that caution should be exercised when interpreting experimental results using the lumped circuit approach

Properties of narrow gap quantum dots and wells in the InAs/InSb/GaSb systems

The properties of InSb quantum dots grown by metal organic vapour phase epitaxy are summarised as deduced from photoluminescence, magneto-photoluminescence, and far-infrared modulated photoluminescence experiments. A technique is described for shifting the emission of these dots to lower energy by coupling them with a narrow InAs quantum well, leading to the demonstration of electroluminescence at similar to 2.3 mum. (C) 2003 Elsevier B.V. All rights reserved