Y. Yang

University of Southampton, Southampton, England, United Kingdom

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Publications (73)64.79 Total impact

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    ABSTRACT: Performance of state of the art MgB2 multifilamentary conductor at a required bend radius is essential for many applications including but not limited to magnets and motors. The characterisation is generally done with benchmark transport Ic but further detail can be seen in IV characteristics which are undertaken in this paper. Two conductors with the same architecture but different diameters, 0.89 and 0.45 mm were measured from 32 K to 20 K in self-field in conditions of as received and deformed to a 50 mm bend diameter, corresponding to strains of 1.4 % and 0.7 % respectively. The qualifying 0.45mm conductor was further measured in background fields up to 3 T. The smaller diameter wire was found to have no signs of degradation of critical behaviour in Ic or IV characteristics.
    Journal of Physics Conference Series 05/2014; 507(3):032061.
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    ABSTRACT: IV data of MgB2 conductors were obtained over more than 3 voltage decades from 0.1 to 400 μV/cm and currents up to 600 A, revealing the characteristic n number to increase rapidly with decreasing temperature from n = 20 around 33 K to above 60 by 25 K. Constant temperature, current-voltage, (IV) characteristics of magnesium diboride conductors over a sufficient voltage range to determine n are not available in the literature and remain a serious experimental challenge. Conductors, instrumented with voltage and temperature sensors, were mounted on a cryofree, variable-temperature-controlled platform within a custom built cryostat. To achieve constant temperature measurement, the stability had to be improved by reducing the thermal path either longitudinally or laterally. Both dc current and transient measurements were implemented and refined until good agreement was obtained. At the highest stable voltage reached, for a given temperature, any further increase led to a rapid thermal propagation until the sample was fully resistive. In carbon-doped samples, the temperature-dependent transport critical current density is best described by two linear functions with different Tc 's. The lower Tc function has the steeper increase in transport critical current with decreasing temperature.
    IEEE Transactions on Applied Superconductivity 01/2013; 23(3):8001304-8001304. · 1.20 Impact Factor
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    ABSTRACT: A series of short circuit tests have been performed on a coreless (air-core) high-temperature superconducting (HTS) rotor, installed inside a conventional stator. The machine was driven like a generator using an 11 kW drive motor, to a maximum frequency of 20 Hz. The short circuit was activated at the stator terminals using a three-phase contactor switch and a remote trigger. Tests were conducted with a range of values for each of the following parameters: i) initial frequency, between 4 and 20 Hz; ii) operating field current, between 5 and 80 A; and iii) external resistance, between 14 and 160 mΩ. For each investigation, the other parameters were fixed. Large current surges were observed in both the stator (3 times its nominal current) and in the rotor (1/4 of the initial operating current). Some localized thermal measurements of a coil in the HTS winding were also recorded but, during the transient events, these measurements are invalidated by inductive voltages. The temperature is believed to remain steady during the short circuit event. The rotational kinetic energy of this rotor was almost half of its counterpart, but it was the power supply and the series resistor that absorbed most of the magnetomotive energy after the short-circuit. These components are essential parts of the HTS generators' overall system and their operating limits must also be considered in new designs.
    IEEE Transactions on Applied Superconductivity 01/2013; 23(3):5201505-5201505. · 1.20 Impact Factor
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    ABSTRACT: Following the development of twisted-pair cables prepared with High Temperature Superconducting (HTS) tapes and their initial tests at 4.2 K in liquid helium at CERN, the cable samples of 2 m lengths were subsequently tested in flowing helium gas at temperatures between 10 K and 77 K at University of Southampton. A cryostat with optimized hybrid HTS current leads was purposely built for the tests up to 2.5 kA. The cryostat has two separate helium flow conduits, each accommodating a twisted pair and allowing independent temperature control. With the completion of the tests on the twisted-pair cables, a 5 m long semi-flexible Nexans cryostat was also set up for the testing of prototype HTS links assembled at CERN. The link, which is optimized for application to the remote powering of LHC 600 A electrical circuits, consists of a compact multi-cable assembly with up to 25 twisted-pair 600 A HTS tapes. The cables are cooled by a forced-flow of helium gas the inlet temperature of which can be changed in order to compare the electrical performance over a range of temperatures. The paper reports on the results of powering tests performed on the individual cables and the integration process for the forthcoming tests of the prototype links.
    Physics Procedia 01/2012; 36.
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    ABSTRACT: A two-stage conversion system is described for regulating the DC current in a superconducting coil, for example, the field winding of a superconducting synchronous machine. An ambient temperature converter provides a low, controlled current to a cryogenic converter, co-located with the superconductor, which steps up the current by over a hundred times to supply the superconducting coil. An array of parallel-connected metal oxide field effect transistors (MOSFETs) minimise losses in the cryogenic converter and a low-frequency, self-oscillating control technique is used for the DC link which minimises the capacitor requirements and ensures orderly switching conditions. Closed-loop control of the superconductor current is achieved through the ambient temperature converter, with current sensing power MOSFETs used at low temperature for current measurement. Practical results including measurements made on a superconducting machine are used to illustrate the system performance.
    IET Power Electronics 01/2012; 5(6):739-746. · 1.52 Impact Factor
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    ABSTRACT: The prospect of medium/high field superconducting magnets using 2G HTS tapes is approaching to reality with continued enhancement in the performance of these conductors. Direct measurements of 1D adiabatic quench initiation and propagation of a Cu-stabilized 2G conductor have been carried out with spatial-temporal recording of temperature and voltage following the deposition of various local heat pulses to the conductor at different temperatures between 40K and 64K carrying different transport currents. It was found that the stabilizer-free 2G tape maintains the unique characteristics previously measured in non-stabilized tape of increasing minimum propagation zone, (MPZ), with transport current and higher quench energy at lower temperatures. The minimum propagation zone length is investigated at low temperatures and in a background field of ~ 0.1T. The change in MPZ size is discussed in the context of the temperature dependent physical properties including the E-J characteristics. The results add more detail to help understand the unique characteristics of increasing MPZ with transport current and lower temperatures.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
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    ABSTRACT: Superconducting switches may find application in superconducting magnet systems that require energy extraction. Such superconducting switches could be bypass-switches that are operated in conjunction with a parallel resistor or dump-switches where all of the energy is dissipated in the switch itself. Bypass-switches are more suited to higher energy circuits as a portion of the energy can be dissipated in the external dump resistor. Dump- switches require less material and triggering energy as a lower switch resistance is needed to achieve the required total dump resistance. Both superconducting bypass-switches and superconducting dump-switches can be ther- mally activated. Switching times that are comparable to those obtained with mechanical bypass-switch systems can be achieved using a co-wound heater that is powered by a ca- pacitor discharge. Switches that have fast thermal diffusion times through the insulation can be modelled as a lumped system whereas those with slow thermal diffusion times were modelled with the full heat diffusion equation. Superconducting switches can be formed of either high temperature superconductors (HTS) or low temperature superconductors (LTS). Switches based on HTS materials allow operation at higher temperatures where the cost of cooling is less. Extracting the magnet energy and depositing the heater energy at higher temperatures will also reduce the load on the overall cryogen ic system during switching and energy extraction. For magnet circuits that are based on high temperature superconductors the switch must also be formed of HTS material. Due to the approximately T^3 dependence of specific heat capacity, switches that operate at higher temperatures have slower heat diffusion times and require higher triggering energies than those operating at low temperature. HTS based dump-switches and HTS based bypass-switches were tested in liquid nitrogen to show that the required switching time could be achieved at these high temperatures. The design and optimisation of superconducting switches that were formed of various superconducting materials were performed for example magnet circuits to provide reference designs of switches. These example circuits were based on selected Large Hadron Collider 600 A circuits that had a stored energy of 5.5 kJ. Superconducting switches may also find application in magnet circuits with higher transport currents and higher energies. The scaling and suitability of the reference designs to higher energy circuits was also described.
    01/2009;
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    ABSTRACT: Conventional superconducting switches for power applications, which operate at liquid helium temperature, generally utilize Nb-Ti superconductor in a cupro-nickel matrix. For superconducting circuits based on High Temperature Superconductors (HTS) that work at higher temperatures, the associated superconducting switches must also be based on HTS. This paper addresses the issues concerning the requirements and the appropriate design of HTS switches, including approaches to fast triggering.
    Journal of Physics Conference Series 02/2008;
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    ABSTRACT: In this paper we report on our new design of a liquid nitrogen/air cooled 100 KW synchronous generator with core-less rotor. This follows our successful completion of 100 KW generators with a 9 wt% Ni steel core operating at 77 K. In the new design, we demonstrate that a coreless rotor using commercial BSCCO tape is a realistic choice while maintaining the cooling at 57-77 K rather than 25-30 K and still achieving reasonable air-gap flux density. This is made possible by a combination of improved HTS wire technology and a careful optimization of HTS winding and flux diverters. The maximum flux density normal to the broad face of the tape is kept below 0.13 T, even when the air-gap density is raised to 0.5 T. The superconducting winding consists of 22 pancake coils with a total length of around 1250 meters of HTS tape. The required temperatures down to 57 K are achieved by a purpose-designed refrigeration system where liquid cryogen is circulated via a network of well insulated pipes and a stationery-rotating liquid coupling junction.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: AC losses of stabilized multifilamentary MgB<sub>2</sub> tapes in applied and self magnetic field were studied systematically at different temperatures, self and external ac fields (les0.3 T), and frequency (3-2000 Hz) to assess the contribution of the magnetic sheath and its interplay with the superconducting filaments. The self-field losses were dominated by the hysteretic losses of the ferromagnetic Ni sheath while the contribution from the superconducting filaments was negligible. Although a considerable loss reduction was obtained with the use of NiCr alloy sheath, the resulting losses were still significantly higher than that of the superconductor as the NiCr sheath was weakly magnetic after thermal processing. In applied ac field, a complex loss behavior was found with loss contributions from the magnetic hysteresis of the sheath, the critical state of the superconducting filaments, and the coupling current across the stabilizing matrix inside the filaments. A long time-constant coupling current was greatly enhanced by the co-existence in the inner matrix of a highly magnetic iron buffer layer and a very low resistive stabilizing copper core. Compared to the normal state losses above the critical temperature T<sub>C</sub>, the overall losses in the superconducting state were increased at low frequencies by the coupling current loss but reduced at high frequencies due to the magnetic shielding by the coupling current.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: The thermal conductivity (TC) of superconducting bulk materials is a key property in determining its stability at the cryogenic conditions and the superconducting behavior in applications as the current leads and the resistive current limiters. The thermal conductivity of bulk MgB<sub>2</sub> materials obtained by the reactive liquid Mg infiltration technique has been measured, in the temperature range from 4.2 K to 300 K, on samples having different grain size and derived from boron powders of different crystallinity and different granulometry. A wide spread of the TC values has been detected for the different samples and a typical maximum of TC at temperatures around T<sub>c</sub> (critical temperature) has been detected only in the materials derived by very pure and macro-crystalline boron. On the contrary, the MgB<sub>2</sub> materials derived from less pure commercial so called "amorphous" boron, in which finer crystalline grains are detectable, show very low values of the thermal conductivity without any maxima in the lower temperatures range. The different behavior can be phenomenologically explained in terms of dependence of the electronic thermal conductivity on the lattice defects and impurities in the samples, with a larger amount of these defects in the microcrystalline samples. An interesting contribution is suggested to be due to the lately described Mg<sub>2</sub>B<sub>25</sub> structure.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: For full cryogenic test of CERN 600 A high temperature superconducting (HTS) current leads prior to integration into the Large Hardron collider (LHC), a dedicated facility has been designed, constructed and operated at the University of Southampton. The facility consists of purpose-built test cryostats, 20 K helium gas supply, helium gas flow and temperature control systems and quench protection system. Over 400 such leads have already been successfully tested and qualified for installation at CERN. This paper describes various design and operation aspects of the test facility and presents the detailed cryogenic test results of the CERN 600 A current leads, including steady state 20 K flow rates.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: Not Available
    Thermal and Thermomechanical Phenomena in Electronics Systems, 2006. ITHERM '06. The Tenth Intersociety Conference on; 01/2006
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    ABSTRACT: This paper reports on the design and construction of the 100 kVA High Temperature Superconducting generator which is successfully built at the University of Southampton. The generator is a 2-pole synchronous machine with a conventional 3-phase stator and a HTS rotor which is constructed from stacks of 9 wt% Ni steel plates. The rotor winding is made of ten single vacuum impregnated pancake coils connected in series and operated in the temperature range 73-77 K. The relatively high operating temperatures are made possible by the use of metallic core as well as flux diverters between the coils. These coils have been successfully manufactured and tested over several cooling cycles. These provide the necessary mechanical support whilst minimizing the heat intake to the rotor core. The coils generate an air gap flux density of about 0.6 T at 77 K, while producing a perpendicular magnetic field of only 0.038 T to board face of the superconducting tape. Furthermore, the design and construction of two novel fiberglass torque tubes is also presented.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: AC loss measurements were carried out on MgB<sub>2</sub> wire and bulk conductors at temperatures between 25 K and 40 K in applied field up to 0.2 T transverse to conductor length. For bulk conductors in the shape of a flat plate (3 mm × 0.5 mm), loss behavior of a thin strip Q∝H<sub>0</sub><sup>4</sup> was observed. A reduction of width to 1 mm leads to a change in loss behavior similar to a round wire with Q∝H<sub>0</sub><sup>3</sup>. For the ex-situ MgB<sub>2</sub> wire of Ni sheath, losses were totally dominated by the hysteresis of ferromagnetic Ni. While the losses in the superconducting core is greatly reduced by the strong magnetic shielding by the Ni sheath, a small increase in the Ni loss was observed upon superconducting transition in the core. This is attributed to the compression of magnetic flux due to the diamagnetic superconducting core. Such an interpretation is supported by measurements of PbBi2223 tape in a cavity of an iron block.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: Pickup coils are widely used for measurements of magnetic susceptibility and hysteretic losses in magnetic and superconducting materials. A general formulation for the calibration of such pickup coils with different geometries is presented. Although the procedure described here is general and can be applied to any coil and sample, special emphasis has been placed on the calibration of saddlelike coils usually used to measure ac losses of long superconductors under ac fields perpendicular to their long dimension. Numerical simulations and experimental measurements have been carried out in order to determine the geometrical limitations of the pickup coil/sample configurations. This is especially important when simple coils of small dimensions and number of turns are used and/or samples of high demagnetization factor are measured. Superconductors of different geometry, in particular, wires and tapes under parallel and perpendicular fields, have been analyzed.
    Journal of Applied Physics 09/2004; · 2.21 Impact Factor
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    ABSTRACT: An integral formulation based on the stream function of sheet currents is applied to finite length superconductors to model the coupling through a normal matrix. This formulation is an extension of Brandt's 2D formulation for modelling a 3D problem. Thin discs and infinite slabs were studied and the critical \dot {B}_{\mathrm {c}} was obtained as a function of applied field. While an excellent agreement for fully penetrated infinite slabs was found with existing theories, original results are presented for partially penetrated slabs as well as for thin discs in a wide range of applied fields.
    Superconductor Science and Technology 01/2004; · 2.76 Impact Factor
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    ABSTRACT: Using 3D finite element method (FEM) modelling, the present work investigates the coupling effect between two finite superconductors through a resistive matrix. This effect, related to the finite length of the conductors, is typically three-dimensional and cannot be analyzed by the widely utilized 2D models. Superconductors are modelled with the non linear power law E=Ec(J/Jc)^n, which has been implemented in the FEM software. The main focus is to demonstrate the feasibility of such calculations and to establish the correlation between the coupling effect and the aspect ratio of the conductor cross-section, in order to extend the existing theory, which is only precise for superconductors of infinite slabs, fully penetrated by the magnetic field. The effect of other parameters such as the conductor length and the gap between the superconducting filaments is also considered. The latter is a parameter which does not feature in the approximate theories.
    Superconductor Science and Technology 10/2003; · 2.76 Impact Factor
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    ABSTRACT: In a two-step heat treatment with one intermediate rolling the final transport critical current, (I<sub>c</sub> at 77 K, self field, 1&inodot;&grave;V/cm), of 37 filament (Bi,Pb)2223/Ag tapes increases linearly with the heating ramp rate: 20 A to 40 A, (∼20,000 Acm<sup>-2</sup>), for 20°C/hour to 400°C/hour. The principle benefit of the rapid ramp rate is likely to come from ramping fast though 800°C to 832°C in the 1st heat treatment. SEM and XRD analysis on the superconductor-Ag interface show a decrease in the amount of secondary phases, Cu<sub>2</sub>O, Bi<sub>2</sub>(Sr,Ca)<sub>2</sub>Cu<sub>1</sub>O<sub>x</sub>, Bi(Sr,Ca)O<sub>x</sub>, with increasing ramp rate and critical current.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
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    ABSTRACT: The ac loss of superconducting composite depends strongly on coupling between superconducting filaments via the resistive matrix. The established technique for loss reduction using twisted filaments relies on the decoupling of the filaments below a critical coupling field Bc, which increases with the reduction of the twist pitch and the matrix conductivity. Although the concept of Bc may be clearly demonstrated using two infinite slabs of finite length, further details on its correlation with the filament/conductor geometry are not yet available. The main obstacle is due to the fact that any accurate analysis of such a problem must be carried out in 3d. In this paper, we describe the initial results from 3d modeling using Cedrat's Flux3D, for which a superconductor module for handling power-law E-J characteristics was developed. Using a simple model of two rectangular superconductors connected through a normal metal, we demonstrate the feasibility for quantitative modeling of their coupling behavior over a wide range of field sweep rates for different conductor geometries. Typical examples were given for cases not addressed by the existing approximate theory, as well as for the evolution of field profile for varying field sweep rate.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor

Publication Stats

256 Citations
64.79 Total Impact Points

Institutions

  • 1989–2011
    • University of Southampton
      • Faculty of Physical and Applied Sciences
      Southampton, England, United Kingdom
  • 1990
    • University of Oxford
      • Department of Materials
      Oxford, ENG, United Kingdom