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A high- T c superconducting (HTS) dynamo enables the injection of large DC currents into a superconducting circuit, without the requirement for current leads. In this work, we attempt to explain the frequency dependence of such dynamos/flux pumps reported in the literature, where it is observed that the rate at which the open-circuit DC voltage increases reduces with increasing frequency, in contrast to the expected linear behaviour. Heat generated in the HTS wire has been the common explanation given to date for this phenomenon. Here we offer an alternative explanation: the interaction between and current flow in the different layers of the HTS wire as the frequency of the dynamo increases. Our claim is based on numerical analysis using a segregated H -formulation finite-element model of the HTS dynamo benchmark problem that is extended to include the full HTS wire architecture and coupled with a thermal model. This framework enables us to efficiently model the relative movement between the rotating room-temperature permanent magnet and the stationary HTS wire and to study the impact of the frequency of rotation and temperature on the open-circuit DC voltage of the dynamo.

The charging/discharging delays in superconducting coils wound without insulation (NI coils) are a major drawback of the technique. While removing the insulation improves safety margins, the increase in the characteristic time constant $\tau _c$ can make a coil unfit for a particular purpose. It is widely accepted for instance that NI coils will not be used in ac applications where $\tau _c\sim 1/f$ . To decrease $\tau _c$ of the NI coils, the same length of superconductor can be wound/connected in parallel rather than in series — decreasing the inductance $L$ , and hence the time constant $\tau _c$ , while maintaining the number of amp-turns $I_{op}N$ . Here we investigate the effect of parallel connecting coils in a magnet using a 2D axially symmetric model which captures all the necessary electromagnetic properties of the HTS NI coils. These properties include: critical current anisotropy $J_c(B,\theta)$ , turn-to-turn conductivity, as well as winding parallelism. Our modeling results show that the parallel connected magnet experiences magnet-wide shielding current effects. Whilst these shielding currents affect field homogeneity — the model enables this effect to be quantified. Furthermore, shielding currents are not an issue when running NI coils in saturated mode. The modeling work presented here provides a simple initial example of how magnet designers may approach designing, optimizing, and operating high current, HTS NI coils.

High-temperature superconducting (HTS) magnets have been widely used in various applications due to their excellent performance. One long-lasting problem, however, is that they have to be powered by electronic power supplies via a pair of thick current leads, which go through room temperature environment into a cryogenic environment. The considerable heat load generated by these resistive current leads at a cryogenic temperature substantially limits the operating current and the energy density of the magnet. In this article, we report a novel mechanism of inductively energizing closed-loop HTS dc magnets. This exploits a newly discovered effect, which appears within a superconducting loop when a global screening current interacts with local screening current. This results in a dc voltage across the superconductor, which enables an alternating superconducting current to be rectified in order to energize a superconducting magnet. Based on this principle, a superconducting transformer–rectifier prototype is realized and demonstrated. Test results show that the prototype can output a dc voltage of up to 25 mV and a maximum direct current over 500 A. It is envisaged that this work will enable future HTS dc magnets to be operated in a closed cryogenic environment, eliminating the need for electronic power supplies and bulky current leads. This would greatly reduce the footprint and power demand of HTS magnet systems and unlock many new opportunities for the applications of this technology.

High- $T_c$ superconducting (HTS) dynamos are simple devices for injecting and sustaining dc currents in superconducting coils/magnets. The simple geometry of these devices consists of a superconducting stator(s) and one or more rotor magnets arranged in identical fashion to a classical alternator. However, unlike the classical alternator, the HTS dynamo gives a self-rectified dc output. This somewhat anomalous result is caused by the non-linear resistivity of HTS materials and the large over-critical eddy currents that flow in the stator. As these over-critical currents must recirculate in the HTS stator, the stator's width becomes a key parameter in the physics of the device. In this work we explore the effect of increasing the stator width through using recent advances in modeling these systems. We find that given enough space in the stator, the total sum of circulating and transport currents do not drive the full width of the stator into the flux-flow regime. Operation of the device in this regime results in a non-linear $I$ – $V$ curve, a marked decrease in the internal resistance at open circuit $R_{oc}$ , a saturation of the open circuit voltage $V_{oc}$ , and a short-circuit current $I_{sc}$ that approaches the in-field critical current of the stator itself $I_{c,min}$ . These behaviors lead to the conclusion that optimal HTS dynamo design should ensure that the stator width be sufficient to avoid current saturation of the superconductor at the target operating current.

In High Temperature Superconducting (HTS) DC magnet applications, the current leads contribute a significant fraction of the cooling load. Higher current operation requires larger current leads and a larger thermal bridge into the cryogenic superconducting components. Removal of current leads into the cryogenic environment would lead to a significant reduction in cooling load and hence reduce ongoing operation costs. Superconducting dynamos are a non-contact method of energising HTS coils, capable of producing significant currents in the connected superconducting circuits that exceed usual current lead arrangements. The use of superconducting dynamos has the potential to reduce the size and mass of a complete magnet system by replacing the power source, current leads, and cryocooler. Previous work on superconducting dynamos has focused on driving larger currents, and understanding the underlying physical principles of operation, but has not investigated the AC losses associated with operation. In this work the losses of a kA class multi-stator squirrel-cage superconducting dynamo are investigated experimentally, and compared to the losses expected in the conventional copper current lead based energisation method. The thermal load associated with conduction cooled copper leads prevents practical application of kilo-amp class dc HTS magnets, however we demonstrated the feasibility of using this superconducting dynamo as an energisation method to enable feasible portable high current HTS magnet systems.

Direct reduction of iron ore pellets using hydrogen gas has the potential to significantly reduce CO2 emissions from the ironmaking process. In this work, green pellets of titanomagnetite ironsand from New Zealand were oxidatively sintered to form titanohematite. These sintered pellets were then reduced by H2 gas at temperatures ≥ 1043 K, and a maximum reduction degree of ~ 97 pct was achieved. Fully reduced pellets contained metallic Fe as the main product phase, but several different (Fe, Ti) oxides were also present as minor inclusions. The phase distribution of these oxides depended on the reduction temperature. With increasing temperature, the relative proportion of pseudobrookite in the final product increased, while the proportion of residual ilmenite and rutile decreased. The reduction kinetics were found to be well described by a pellet-scale single-interface shrinking core model, for reduction degrees up to 90 pct. At temperatures above 1143 K, the rate-limiting step was found to be solely an interfacial chemical reaction process, with a calculated apparent activation energy of 31.3 kJ/mol. For pellet sizes from 5.5 to 8.5 mm, the reaction rate was observed to increase linearly with decreasing pellet diameter, and this linear correlation extrapolated to intercept the axis at a pellet diameter of 2.5 mm. This is interpreted as the minimum length required for a shrinking core interface to develop within the pellet.

This paper reports on a method for strain calibration of fiber optic sensors which have been adhesively bonded to a structural member. Generally, the coefficient of strain transfer ( $k_{\mathrm {ST}}$ ) can be used to calibrate the strain sensitivity of the surface bonded FBG. Here we propose an alternative method to determine $k_{\mathrm {ST}}$ which is based solely on observing the temperature-dependent behaviour of the adhesively mounted FBG sensor. In this paper we prove the validity of our method by comparing $k_{\mathrm {ST}}$ determined via a conventional strain (mechanical) calibration and via our alternative temperature (thermal) calibration. A set of 18 surface bonded FBGs with coefficients ranging from 0.5 to 1 are investigated. The average deviation of $k_{\mathrm {ST}}$ determined via a temperature calibration versus a mechanical strain calibration was ~5%, with a maximum deviation of 13% obtained from the set of bonded sensors measured in this work. This led to an average relative accuracy of < 6%, when the strain sensors were calibrated using our temperature-based calibration method.

For HTS dc magnet applications, a large proportion of the cooling load on the cryocooler is imposed by the external current leads. Removing the need for these current leads would allow these systems to be built using smaller, cheaper cryocoolers and reduce the ongoing operating costs. This can be achieved by using an HTS superconducting dynamo as a fully superconducting dc current source. Superconducting dynamos remove the need for external current leads by providing a wireless method of injecting dc current into a closed superconducting circuit. Most previous work on HTS dynamos has considered devices employing just one coated conductor stator tape, but dynamos employing multiple parallel stator tapes can achieve substantially higher output currents. In this study, we present an HTS dynamo design in which eight parallel HTS stator tapes are arranged in a cylindrical geometry, excited by an axially concentric rotor carrying multiple equally spaced Nd–Fe–B magnets. Here, we focus on the differing behavior observed between the synchronous case when the number of magnet nm is a factor of the stator number ns = 8 (i.e., nm = 1, 2, 4, 8) and the asynchronous case when it is not (i.e., nm = 3, 5). Our results indicate that both synchronous and asynchronous configurations can exhibit output short-circuit currents of up to ∼1.3 kA. In both cases, a relative drop in performance is observed for higher values of nm . We find that the asynchronous magnet arrangement does not greatly improve the dynamo performance compared to the synchronous arrangement. At present, the thermal load associated with conduction-cooled current leads has prevented the practical realization of cryocooled HTS magnets in the kilo-amp (kA) range. However, the dynamo device reported here now offers a practically feasible route to novel kA-class portable cryocooled HTS magnet systems.

High-temperature superconductor (HTS) dynamos provide a means of wirelessly exciting dc currents in HTS coils. By eliminating the need to connect a coil to a room-temperature power supply, an HTS dynamo of comparable performance would present significant improvements to the cost, complexity, and efficiency of HTS-based electromagnetic machines. But due to the lack of a predictive model, development of HTS dynamos must be carried out through empirical investigation. In this paper, we examine the effects of varying either the rotor speed or the number of magnets arranged symmetrically upon the rotor. Both approaches affect the frequency at which magnets cross the HTS stator wire. Here we present experimental results from an axial-flux HTS dynamo, employing an externally mounted rotor carrying m permanent magnets, where 2 ≤ m ≤ 9. We find that the open-circuit dc voltage, short-circuit dc current, and internal dc resistance are all determined solely by the magnet-crossing frequency, regardless of the number of rotor magnets and speed employed to achieve this frequency. For a fixed speed, all three parameters are found to be proportional to m across the full range of conditions studied in this work. However, for a fixed number of rotor magnets, we observe that the open-circuit voltage and short-circuit current both exhibit a similar sublinear dependence on rotor speed. Examination of the transient voltage waveform reveals that this nonlinearity is likely due to a periodic transient heat pulse absorbed by the stator wire each time the magnet traverses it at high speed.

Fluidized bed reduction of iron ore fines is typically inhibited by the onset of “sticking” at temperatures above 973 K, which leads to particle agglomeration and defluidization of the bed. Here, we report the sticking-free fluidized bed reduction of titanomagnetite (TTM) ironsand at 1223 K in Ar-H2 gas mixtures. We show that sticking is prevented by the formation of a protective titanium-rich oxide shell around each particle during the initial reduction stage. This protective shell prevents iron-iron contact between particles throughout the reduction process, enabling metallization degrees of 93 pct to be attained without sticking occurring. Phase evolution during the reaction has also been analyzed using q-X-ray diffraction and scanning electron microscope/energy dispersed spectroscopy. We find that the reduction proceeds through four separate stages. During the initial stage, approximately half of the initial TTM phase is converted to wustite, forming a network of sub-micron wustite channels which interlace the TTM matrix. During this stage, Ti and Al are enriched within the TTM matrix, due to the low solubility of both species in wustite. This enrichment stabilizes the remaining TTM, meaning that wustite is then preferentially reduced to metallic iron in stage 2 of the reduction. In stage 3, the remaining Ti-enriched TTM is reduced directly to metallic iron and ilmenite. The final stage of reduction involves the conversion of ilmenite into rutile and pseudobrookite. Our findings clarify the important role played by titanium species during the reduction of TTM and suggest that New Zealand ironsand can offer significant advantages over conventional hematite ores when used as a feedstock for fluidized bed direct-reduced iron processes.

Dynamic resistance, which occurs when a HTS coated conductor carries a DC current under an AC magnetic field, can have critical implications for the design of HTS machines. Here, we report measurements of dynamic resistance in a commercially available SuperPower 4 mm-wide YBCO coated conductor, carrying a DC current under an applied AC magnetic field of arbitrary orientation. The reduced DC current, I t/I c0, ranged from 0.01 to 0.9, where I t is the DC current level and I c0 is the self-field critical current of the conductor. The field angle (the angle between the magnetic field and the normal vector of the conductor wide-face) was varied between 0° and 90° at intervals of 10°. We show that the effective width of the conductor under study is ∼12% less than the physical wire width, and we attribute this difference to edge damage of the wire during or after manufacture. We then examine the measured dynamic resistance of this wire under perpendicular applied fields at very low DC current levels. In this regime we find that the threshold field, B th, of the conductor is well described by the nonlinear equation of Mikitik and Brandt. However, this model consistently underestimates the threshold field at higher current levels. As such, the dynamic resistance in a coated conductor under perpendicular magnetic fields is best described using two different equations for each of the low and high DC current regimes, respectively. At low DC currents where I t/I c0 ≤ 0.1, the nonlinear relationship of Mikitik and Brandt provides the closest agreement with experimental data. However, in the higher current regime where I t/I c0 ≥ 0.2, closer agreement is obtained using a simple linear expression which assumes a current-independent penetration field. We further show that for the conductor studied here, the measured dynamic resistance at different field angles is dominated by the perpendicular magnetic field component, with negligible contribution from the parallel component. Our findings now enable the dynamic resistance of a single conductor to be analytically determined for a very wide range of DC currents and at all applied field angles. This is the Accepted Manuscript version of an article accepted for publication in 'Superconductor Science and Technology'. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-6668/aaa49e.

The production of large (RE)Ba–Cu–O single grains ((RE)BCO), where RE = Y, Gd or Sm, of complex geometries is presently limited by the intrinsic slowness of the grain growth process. Additionally, the shapes achievable using established melt processing are constrained by the small number of post-processing techniques available. These materials are brittle and hard, which makes machining a difficult task and largely eliminates the possibility of producing highly intricate shapes. An alternative to this slow and inflexible growth process would be to join many small single grains to form one large composite grain, connected by high-performance superconducting joints. A reliable joining technique would also overcome the need for the careful and time-consuming post-growth machining processes. In this work we report on the use of single grain YBCO–Ag as an interface medium to achieve superconducting joints between (RE)BCO bulks. This joining technique is relatively quick and does not require tight process parameter control as there is no need to re-grow the interface joining material. We report on six joints produced from samples cut and joined in a variety of orientations. In addition, a joint was produced using bulk YBCO from two independent single grains. The trapped field properties of the resulting joined sample were measured and the microstructure at the joint was examined. We show that this simple but effective joining technique makes it possible to produce multiple composite grains with comparable superconducting properties to those of a single grain of the same size.

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., 22 pages, 7 figures, 2 tables

High-${T}_{c}$ superconducting (HTS) dynamos are experimentally proven devices that can produce large (more than a kiloamp) 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 a comparison of it with experiment that fully explain 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 a HTS sheet. We first show that our finite-element model replicates all of 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 agree closely 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 a HTS dynamo is governed by the competition between transport and eddy currents induced as the magnet transits across the HTS tape. The eddy currents are significantly higher (approximately 1.5 times) than the local critical current density, and hence experience a highly nonlinear local resistivity. This nonlinearity 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 nonlinear resistivities (which both vary in time and space) is shown to systematically give rise to the key observed parameters of experimental HTS dynamo devices: the open-circuit voltage, the internal resistance, and the short-circuit current. 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 alternative designs for HTS dynamo devices, in which the internal resistance is greatly reduced and the short-circuit current is substantially increased.

Dynamic resistance can be observed in a superconducting tape carrying a DC current which is exposed to an oscillating magnetic field. This effect is attributed to the interaction between the transport current and moving fluxons, and can occur in various superconducting components including high temperature superconducting (HTS) flux pumps, fast-ramping magnets and HTS rotating machines. Although conventionally expressed in terms of a DC ‘resistance,’ the phenomenon is inherently transient in nature, and the voltage drop across the superconductor follows a time-dependent periodic waveform. Here we present experimental measurements of the dynamic resistance of different REBCO tapes carrying a DC current and exposed to an oscillating perpendicular field. Measurements of both the transient voltage waveforms and the time-averaged DC resistances are compared with numerical finite element simulations obtained using the H-formulation. We observe clear variations between the voltage response from different tapes, which can be understood in terms of their differing J c(B, θ) dependence. In particular, a key feature of the experimentally measured waveforms is the emergence of a split ‘double peak’ at higher applied fields. Graphical visualisations of the finite element data show that this coincides with a periodic increase in J c(B, θ) throughout the tape. This occurs during each cycle at those times when the applied field falls below the shielding threshold of the tape (as the penetrating field within the tape then approaches zero). Our findings show that models which assume a constant J c irrespective of local field strength cannot capture the full range of behaviour observed by experiment. This emphasises the importance of employing experimentally measured J c(B, θ) data when simulating transient effects in HTS materials.

Dynamic resistance is observed in type-II superconductors carrying a DC transport current while simultaneously exposed to an alternating magnetic field. The appearance of a non-zero resistance is attributed to the interaction between the transport current and moving fluxons. This effect is relevant to many superconductor applications such as high-temperature-superconductor (HTS) flux pumps, DC/AC magnets, synchronous machines, and persistent current switches. Here, we present a finite element method (FEM) analysis of both the time averaged dynamic resistance and the instantaneous current sharing behaviour in a cable comprised of a stack of four YBCO thin films connected in parallel. Numerical modelling was performed using the H -formulation method implemented in the commercial software COMSOL. The model employs experimentally measured values of the angular dependence of the critical current I c( B, θ ) and the flux creep exponent n ( B, θ ). A single threshold field is observed, above which a finite dynamic resistance is observed in all tapes simultaneously. The time-averaged dynamic resistance of individual tapes tends to be larger for the exterior tapes than the interior tapes, but this difference decreases as the total transport current in the cable increases. We attribute this to shielding currents flowing in the exterior tapes during the majority of the cycle, which displace net DC current into the interior tapes. However, the relative proportion of DC transport current flowing in the exterior and interior tapes is also observed to vary periodically once per half cycle of the applied field. This is due to the periodic trapping of return screening currents in the interior tapes.

The emergence of a potential drop across a current-transporting type-II superconducting loop under a perpendicular oscillating magnetic field is revealed. We have derived analytical formulae to describe the effect under DC transport current in 1D, based on Bean’s critical state model. The analytical formulae are verified by a finite element model. To exploit this effect, we have developed a transformer-like ‘resistive switch’, and experimentally observed a switching effect. This work demonstrates a physically important general insight of the interaction between DC transport currents and time-varying magnetic fields in type-II superconducting loops, which extends beyond the well-known ‘dynamic resistance’ effect. It also provides a useful view on the interaction between a ‘transport-current’ and a ‘screening-current’ in the superconductor. The resulting demonstrated switch has the potential to be used in a variety of applications including superconducting rectifiers, fault current limiters, and superconducting magnetic energy storages.

Direct reduction of iron ore in a fluidized bed reactor is usually limited by the onset of particle sticking at temperatures ≳ 800 °C. This paper examines the phase and microstructural evolution of New Zealand titanomagnetite (TTM) ironsand during fluidized bed reduction by hydrogen gas at temperatures ranging from 750 °C to 1000 °C. All samples were characterized using quantitative X-ray diffraction and scanning electron microscope/energy dispersive spectroscopy. No sticking was observed at any point during this series of experimental reduction reactions. At higher temperatures sticking appears to be prevented by the formation of a Ti-rich oxide shell around each partially-reduced particle. At lower temperatures, no shell is observed to form, but temperatures are not high enough to drive particle sticking/agglomeration. The reduction mechanism appears to vary within different temperature regimes. At high temperatures (950 °C and 1000 °C), the reduction of TTM proceeds via the wustite phase during the initial reduction stage, although the TTM is not completely converted to wustite. The formation of wustite results in an enrichment of Ti within the residual TTM phase. This is observed as an increase in the TTM lattice parameter, which is stretched by Ti substitution into the spinel. For reactions at low temperatures (750 °C and 800 °C), the TTM is directly reduced to metallic iron without any evidence of the appearance of the wustite phase. At ‘intermediate temperatures’ (850 °C and 900 °C), a mixed behaviour is observed with the appearance of low levels of short-lived wustite during the reduction reaction. Overall, these results indicate that the fluidized bed processing of NZ TTM ironsand in hydrogen can be completed over a wide temperature range without incurring sticking.

Dynamo-type high-temperature superconductor (HTS) flux pumps are fully superconducting dc current sources for HTS magnet coils, which eliminate the need for external current leads. Previous work has focussed on dynamos employing a single-coated conductor stator wire, and the maximum output current is found to be limited by the effective internal resistance of the stator wire. It is expected that adding additional stator wires in parallel will increase the maximum output current. However, there have been few experimental reports of this effect, nor of the influence of the rotor magnet configuration in a multiply parallel stator arrangement. In this paper, we report on the design and experimental characterization of an HTS dynamo employing eight parallel HTS stator wires, which are arranged in a cylindrical geometry and excited by an axially concentric rotor carrying one or more permanent Nd-Fe-B magnets. We investigate the effect of changing the number of rotor magnets, and find that the maximum output current is achieved when the number of rotor magnets is less than half the number of stator wires. We have demonstrated output currents of up to 700 A from this device. This is the highest reported output to date from a flux pump operating at liquid nitrogen temperatures. Furthermore, our results indicate that the squirrel-cage architecture used here is capable of achieving a maximum output current of ~ 1.3 kA. At present, the heat load associated with a kA+ conduction cooled current leads prevents such large currents being injected into present-day cryo-cooled HTS magnets. Our results now indicate that this issue could be circumvented through employing an HTS dynamo, opening up new opportunities to consider novel designs of very high-current cryo-cooled HTS magnets.

Increasing the current-carrying capacity of bifilar coils is essential for the high-temperature superconducting (HTS) fault current limiter (FCL) application. We built three bifilar stacks with two, four, and six conductors using 4 mm wide stabilized SuperPower YBCO wires to investigate ac loss dependence of the bifilar stacks on the number of conductors. In the three basic configurations, the upper half conductors carry current in one direction, and the lower half conductors carry current in the opposite direction. We report transport ac loss results on these stacks measured at three different frequencies. The measured loss values were compared with those in stacks with the same conductor number and geometry but producing different degrees of field cancellation. We also explored the dependence of ac loss on a central gap in bifilar stacks composed of four and six conductors and the results were compared with the numerical results obtained from a two-dimensional FEM model. Results show that the bifilar stacks where each turn is stacked with multiple wires is a promising option which compromise both high current-carrying capacity and lower ac loss for HTS FCL application.

Flux pumps provide a contact-free method of energizing dc currents in superconducting coils. Since a predictive quantitative theory for dynamo-type flux pumps does not exist yet, the optimization of device designs must proceed through empirical experiment. In this paper, we investigate the effect of varying the annular width of the iron stator yoke within a high-temperature superconducting dynamo. Reducing the dimensions of this iron yoke reduces the mass of cold iron within the dynamo, and hence has advantages in terms of weight, cryogenic efficiency, and eddy current losses. In this paper, we have investigated the dc output from an experimental high-temperature superconductor (HTS) dynamo in which the annular width of the iron yoke Δ r is varied across a factor of four. Varying this dimension affects the reluctance of the magnetic circuit formed between the rotor and stator. We have shown that the dynamo output is surprisingly insensitive to the size of the iron yoke. The maximum current output ( I sc) is independent of Δ r at all flux gaps. At the closest magnet–stator flux gap, a fourfold increase in Δ r results in only a ∼30% increase in the open circuit voltage ( V oc) and internal resistance ( R d) of the dynamo. Our results indicate that the smallest yoke studied (Δ r = 6 mm) can be utilized with a 25.4-mm Nd-Fe-B rotor magnet, without adverse impact on the electrical performance of this type of HTS dynamo.

Flux pumps can inject large dc currents into superconducting coils, without requiring physical connections between the cryogenic superconducting circuit and the room-temperature environment, thus significantly lowering the heat load on the cryogenic cooling system. Here, we study a dynamo-type flux pump employing a 2G ReBCO coated conductor wire at 77 K with multiple spinning Nd–Fe–B magnets and a soft ferromagnetic iron yoke, with all moving parts situated outside of the cryogenic environment. Dynamo-type flux pumps can be modeled as a dc voltage source with an internal resistance, where all output parameters are functions of the speed (frequency) of the spinning magnets. In this work, we focus on the frequency dependence of the dc output voltage. From instantaneous traces of the generated voltage waveform, we observe three distinct frequency regimes of behavior. We classify them into regions of “low,” “mid,” and “high” magnet frequency, depending on the magnitude of the dc open-circuit voltage and the shape of the voltage waveform. The voltage-generation mechanism is qualitatively understood at “low” magnet frequencies, where the dc voltage increases proportional to frequency. However, at “mid” and “high” magnet frequencies, the dynamo output deviates from this accepted model. In the “mid” frequency region, the dc output voltage is roughly constant with frequency, but is unstable. At “high” frequencies, the dc voltage drops with increasing frequency following an approximately 1/ f dependence. We have also studied the effects of ramping acceleration rate, noting a high degree of reproducibility in the low- and high-frequency regions, but complex unstable behavior in the intermediate region. We discuss our results in the context of self-heating due to EMF-driven eddy currents, leading to partial-quenching of the coated conductor wire. This becomes the dominant limiting factor in flux pump output at high frequencies, eventually suppressing the dc output altogether.

Assembled coated conductors are essential in many high-current high-temperature superconducting (HTS) applications. A promising option is to use Roebel cable, as it offers both low ac loss and mechanical flexibility. In this work, we measured magnetization loss at 77 K in REBCO Roebel cables with varying strand numbers from 6 to 14 to investigate the strand number dependence of ac loss characteristics of the Roebel cables. The 4-mm-wide Roebel strands were punched from nonstabilized 10-mm-wide Fujikura REBCO tapes. The applied field amplitude, frequency, and the field angle (the angle between the magnetic field and the tape wide surface) were varied. Three 10-mm-wide vertical stacks with conductor number of three, five, and seven were prepared as reference stacks using the same source Fujikura tapes. The measured magnetization loss values of the Roebel cables were compared with those of the equivalent stacks as well as with the results of COMSOL modeling of the stacks using the H -formulation. At magnetic-field amplitudes greater than the effective penetration field of the Roebel cables the cables have lower ac loss than equivalent stacks.

© 2017 IEEE. High-Tc superconducting (HTS)-coated conductors are a promising option for the next-generation power devices. However, their thin-film geometry incurs dynamic loss when exposed to a perpendicular external ac magnetic field, which is difficult to predicate and estimate. In this paper, we propose and verify a numerical simulation model to predict the dynamic loss in HTS-thin-coated conductors by taking into account their Jc-B dependence and I-V characteristics. The model has been tested on a SuperPower YBCO-coated conductor, and we observed a linear increase of dynamic loss along the increasing field amplitude after the threshold field. Our simulation results agree closely with experimental measurements as well as an analytical model. Furthermore, the model can predict the nonlinear increase of dynamic loss at high current, while the analytical model deviates from the measurement results and still shows a linear correlation between the dynamic loss and the external magnetic field. In addition, we have used this model to simulate the distributions of magnetic field and current density when dynamic loss occurs. Results clearly show the flux traversing the coated conductor, which causes dynamic loss. These distributions have also been used to analyze the change of dynamic loss when either the transport current or the magnetic field increase individually, while the other factor remains constant. The simulation analysis on dynamic loss is done for the first time in this paper, and our results clearly demonstrate how dynamic loss changes as well as its dependence on transport current and magnetic field. © 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

The reduction of New Zealand titanomagnetite ironsand in a dilute hydrogen–nitrogen gas mixture was studied in situ using neutron diffraction. Neutron diffraction allowed in situ observation of lar...

We present a small-scale pneumo-hydraulic energy storage system (ss-CAES) which can provide frequency support to a power grid, as fast instantaneous reserve. This system is designed to be maintained at rest until called, thus avoiding idling costs. It utilizes a hydraulic drive train which transmits very high torque directly to the shaft of a synchronous generator, thus enabling rapid acceleration of the rotor. When an under-frequency event occurs, the generator must be accelerated from rest and synchronized to the grid. Here, we show that this is achieved by a ballistic synchronization approach which computes and follows an acceleration trajectory which simultaneously synchronizes both phase and frequency. A two-stage strategy is employed which switches from the ballistic acceleration trajectory to a grid-following mode once the synchronization conditions have been met. Computer simulations indicate that this approach enables very rapid synchronization of a model system to the grid in

Electric power systems maintain system security by keeping instantaneous reserves that can substitute for a credible loss of generation. Instantaneous reserves are fast responding generators that maintain electric power system frequency immediately following a sudden mismatch between generation and load, and these are an indispensable system security tool. These are typically offered by partially or unloaded generators which are already synchronised to the AC network. The reserve requirement constrains generator operation, and reduces system flexibility and efficiency. These are important issues in a renewables-based power system. A compressed air energy storage and hydraulic actuation system is proposed to provide instantaneous reserves without constraining existing generators. By rapidly accelerating and synchronising a generator to the AC grid through a pressurised hydraulic drive, the cost of running unloaded or under-loaded spinning generators can be eliminated. This paper presents a novel hydraulic actuation system which is capable of achieving simultaneous frequency and phase matching between the power system and a 100 kW generator within 1 s. The hydraulic drive system, instrumentation, and the nonlinear controller design are described. In simulations, successful synchronisation is achieved under a wide range of system uncertainties.

Wire cost and ac loss are paramount issues for the future uptake of high-temperature superconductor (HTS) ac machines. In particular, new approaches which reduce the total manufacturing cost of HTS coils are critical to the future commercial uptake of HTS technology. Here, we present the results of ac loss measurements of a hybrid HTS coil assembly comprising end windings (EWs) wound with BSCCO wire and a central winding (CW) wound with REBCO wire. As a cost mitigation measure, we have chosen to use REBCO wire which possesses a relatively low self-field $I_{{\rm{c}}}$ at 77 K, equating to approximately half that of the BSCCO wire. Transport ac loss measurements were made on the total coil assembly at both 77 K and 65 K, using a recently developed lock-in amplifier approach. We show the ac losses in the assembly at these temperatures scale with the overall transport critical current threshold of the hybrid coil assembly. The results are then compared with a reference coil assembly having similar geometrical dimensions, and in which both the EWs and CW were wound with identical BSCCO wire. Surprisingly, we observe that the ac loss of the hybrid coil is lower than that of the BSCCO coil, despite the CW of the hybrid coil employing wire a lower overall average $I_{{\rm{c}}}$ . We attribute this effect to the lower ac loss of coated conductor REBCO wire under parallel field, which is the dominant field orientation in the CW region. Based on this observation, we conclude that the hybrid assembly design is a good design strategy for HTS coil assemblies, enabling both wire cost and ac loss to be balanced.

The high-T c superconductor (HTS) dynamo is a type of superconducting flux pump that can inject a dc current into a superconducting magnet coil, without requiring physical connection to an external power supply. Here, we report experimental results from an HTS dynamo employing 46-mm wide coated conductor stator wire. We characterize the output of the device as a function of frequency, and show the effect of slitting the stator wire into two or three parallel equivalent strips. At operating frequencies above ~100 Hz, we observe that the internal resistance and open-circuit voltage ceases to increase linearly with frequency, and that the short-circuit current is greatly reduced. Experimental waveform data of the output open-circuit voltage enables us to identify three distinct and different signature waveforms, which mark the low-frequency, midfrequency, and high-frequency operating regimes. These regimes appear to arise due to the saturation of circulating currents within the stator wire, followed by a rise in temperature of the wire. We conclude that dissipation due to circulating currents plays an important role in depressing the output of this wide stator device at high frequencies. This effect is in marked contrast to results previously reported for a similar HTS dynamo employing narrower stator wires.

A superconducting flux pump based on a high-temperature superconducting (HTS) dynamo has been demonstrated which employs rotating permanent magnets and a ReBCO conductor stator wire. This device is capable of injecting large dc currents (>100 A) into industrial superconducting magnet coils. Dynamic resistance, due to the interaction of the dc current with the ac magnetic field at the stator, provides a performance limit for this type of HTS dynamo exciter. It has been shown for a given device, the output of the HTS dynamo can be described by a simple circuit model linking short-circuit current (I sc ), dynamic resistance (R d ), and open-circuit voltage (V oc ). I sc represents the maximum dc current that can be delivered by the dynamo to a series-connected superconducting circuit and is found to be independent of rotational frequency for a fixed device geometry. We have recently demonstrated that this dynamo effect can be applied to excite a closed superconducting circuit through the cryostat wall, such that all active components of the dynamo are located outside of the cryogenic envelope. Here, we report experimental results which explore the relationship between rotor magnet geometry and ReBCO stator wire width on the output performance of a simple HTS dynamo. We characterize the effect of magnet orientation, aspect ratio and shape, at fixed flux gap, on I sc , R d , and V oc .

Superconducting flux pumps enable dc supercurrents to be injected into a superconducting coil without direct connection to an external current supply. Here, we report experimental data from a dynamo-type flux pump employing a high-T c superconducting stator. This device employs a mechanical rotor that rotates outside the cryogenic envelope, and excites the superconducting circuit through the cryostat wall via a magnetic circuit formed between the rotor and stator. We show that the width of the stator wire employed in this device has a significant effect on its output performance. At low frequencies, both the short circuit current, I sc , and the open-circuit voltage, V oc increase with stator width, and we obtain a maximum value of I sc > 340 A using a stator wire width of 46 mm. However at higher operating frequencies, we observe that I sc reaches a maximum value before dropping with any further increase in stator width. We attribute this to thermally dissipative effects due to circulating eddy currents in these wide superconducting stators. At frequencies above ~400 Hz, we observe a sharp rise in the internal resistance of the stator wire that we attribute to a local quench due to the eddy currents driven by the local electromotive force (EMF) beneath the rotor magnet. This paper emphasizes the importance of frequency dependence when defining optimum operating parameters of a high-T c superconductor (HTS) flux pump. Our results also offer the promise that very high current flux pumps may be developed through maximizing the ratio of the stator wire width to the peak width of the rotor field profile.

High temperature superconductor (HTS) magnet systems usually employ metal current leads which bridge between the cryogenic environment and room temperature. Such current leads are the dominant heat load for these magnet systems due to a combination of electrical resistance and heat conduction. HTS flux pumps enable large currents to be injected into a HTS magnet circuit without this heat load. We present results from an axialtype HTS mechanically rotating flux pump which employs a ferromagnetic circuit and a Cu-stabilized coated conductor (CC) HTS stator. We show the device can be described by a simple circuit model which was previously used to describe barrel-type flux pumps, where the model comprises an internal resistance due to dynamic resistance and a DC voltage source. Unlike previously reported devices, we show the internal resistance and DC voltage in the flux pump are not exactly proportional to frequency, and we ascribe this to the presence of eddy currents. We also show that this axial-type flux pump has superior current injection capability over barrel-type flux pumps which do not incorporate a magnetic circuit.

High-temperature superconducting (HTS) magnet systems conventionally require normal-conducting current leads, which connect between the HTS circuit and an external power supply located at room temperature. These current leads form a thermal bridge across the cryostat wall, and they represent the dominant heat load for many magnet applications. The use of a superconducting flux pump device is an alternative approach to exciting a magnet coil, which can eradicate this parasitic heat load, as such devices do not require direct physical connection to the HTS circuit. However, earlier proposed flux pump designs have required power-dissipating active components to be located within the cryogenic envelope, thus imposing their own parasitic heat load. Here, we report the successful demonstration of a mechanically rotating HTS flux pump, which operates entirely outside of the cryogenic envelope. This prototype device projects flux across a cryostat wall, leading to the injection of a direct current into a thermally isolated closed HTS circuit. This is achieved through the implementation of a flux-concentrating magnetic circuit employing ferromagnetic yoke pieces, which enables flux penetration of the HTS circuit at large flux gaps. We have demonstrated the injection of direct currents of > 30 A into a closed HTS circuit while operating this device across a cryostat wall.

Direct electrochemical reduction of iron ore in concentrated NaOH electrolyte has been proposed as a potential route to substantially reducing the global steel industry’s CO2 emissions. Here, we report the solid-state electro-reduction of sintered pellets formed from titanomagnetite ironsand. This commercial iron ore contains ∼4 wt.% Ti which is directly incorporated within the magnetite lattice. At 110 °C, these pellets are electrochemically reduced and exhibit a well-defined reaction front which moves into the pellet as the reaction progresses. The electro-reduction process selectively produces iron metal, whilst the Ti content is not reduced. Instead, Ti becomes enriched in segregated oxide inclusions, which are subsequently transformed to a sodium iron titanate phase through taking up Na+ from the electrolyte. These inclusions adopt an elongated shape and appear to exhibit locally preferential alignment. This suggests that they may nucleate from the microscopic titanohematite lamellae which naturally occur within the original ironsand particles. The expulsion of contaminant Ti-oxides from the final reduced metal matrix has implications for the potential to development of an industrial electrochemical iron-making process utilising titanomagnetite ore.

We report a facile one-pot solution phase synthesis of one-dimensional Ge1−x Sn x nanowires. These nanowires were synthesized in situ via a solution-liquid-solid (SLS) approach in which triphenylchlorogermane was reduced by sodium borohydride in the presence of tin nanoparticle seeds. Straight Ge1−x Sn x nanowires were obtained with an average diameter of 60 ± 20 nm and an approximate aspect ratio of 100. Energy-dispersive x-ray spectroscopy (EDX) and powder x-ray diffraction (PXRD) analysis revealed that tin was homogeneously incorporated within the germanium lattices at levels up to 10 at%, resulting in a measured lattice constant of 0.5742 nm. The crystal structure and growth orientation of the nanowires were investigated using high-resolution transmission electron microscopy (HRTEM). The nanowires adopted a face-centred-cubic structure with individual wires exhibiting growth along either the 〈111〉, 〈110〉 or 〈112〉 directions, in common with other group IV nanowires. Growth in the 〈112〉 direction was found to be accompanied by longitudinal planar twin defects.

Nowadays, hydraulic drive trains play an important role in different industries. They are employed in power engineering as drive trains in some types of synchronous generation units. This research investigates an experimental setup based on a variable speed hydraulic drive train that rapidly accelerates a synchronous generator for frequency support in power grids. The prototype is composed of a 100kW hydraulic motor, a synchronous generator and a current driven proportional throttle valve for flow control. To safely perform the experimental testing of the designed electro-hydraulic system, a feasible mathematical model must be developed. This requires the full characterisation of the hydraulic drive train. The research presented here shows a simple, yet efficient data acquisition system for the experimental validation of electro-hydraulic systems. Followed by the design of an accurate mathematical model of the complicated hydraulic system.

Dynamic resistance plays an important role in certain high-Tc superconducting (HTS) applications where an HTS coated conductor carries a DC current exposed to an AC magnetic field. Here, we report measurements of the dynamic resistance in a 4 mm-wide YBCO coated conductor under a perpendicular AC magnetic field at 77 K, 70 K, and 65 K. Dynamic resistance was measured at three different frequencies for the reduced current, i (It/Ic0), ranging from 0.04 to 0.9, where It is the DC current level and Ic0 is the self-field critical current of the conductor at each temperature. At all three temperatures, the threshold magnetic field (Bth) values increase with reducing DC current. These results show that, for a given set of applied conditions, dynamic resistance decreases with decreasing operating temperature, which we attribute to the temperature dependent increase in the critical current of the wire. We show that measured Bth values at all three temperatures agree well with the analytical values from nonlinear Mikitik and Brandt equation for i ≤ 0.2 and with a simple linear expression that assumes a current-independent penetration field for i > 0.2. We further show the measured Bth curves at different temperatures normalized by critical current density collapse into one common curve. The above result implies that dynamic resistance in coated conductors at different temperatures under perpendicular AC magnetic fields can be scaled simply using measured Ic0 values at those temperatures and analytical equations.

The use of intermittent sources of energy in modern power grids makes frequency support more challenging. In case of contingent events, fast instantaneous reserve systems should be employed in order to restore the frequency. One of the promising solutions is the use of rapidly accelerated synchronous generators with hydraulic drive-trains. They are driven by energy from compressed gas in pneumo-hydraulic accumulators. To reduce the cost of recharging, nitrogen which is used in these accumulators, can be replaced with air. However, this has the potential to create a dangerous flammable mix of air and mineral oil within the accumulator. This paper investigates the effects of replacing mineral oil with a water-glycol blend in a hydraulic drive-train coupled to a 100 kW, 415 V, 3-phase synchronous generator. We find that water-glycol blends represent a useful approach to reduce the risk of an explosion at facilities where electro- hydraulic drive-trains are employed.

We measured ac loss in Roebel cables consisting of six REBCO strands and ten REBCO strands. The amplitude of the ac transport current and the applied ac magnetic field, and the angle of the applied ac magnetic field to the wide face of the Roebel cable were varied. Measured ac losses of the Roebel cables were compared with those of reference stacks of coated conductors to discuss on the influence of the strand number and the three-dimensional structure of the Roebel cables on their ac loss characteristics.