A detailed comparison of the composition of(RE)-Ba-Cu-O-Ag bulk superconductors

The homogeneity of the microstructure and composition are critical in determining the properties of rare earthbarium‐cuprate,single grain bulk superconductors[(RE)BCO].The magnitude of the trapped magnetic field achieveable in these technologically important materials,in particular,is influenced heavily by the size and distribution of(RE)_(2)BaCuO_(x)(RE‐211)flux pinning inclusions in the bulk microstructure,whereas the size and distribution of silver agglomerates present within the bulk superconducting matrix correlate directly with improved mechanical properties.With careful engineering,these materials have significant potential for application in range of devices related to energy storage,medicine,electro‐magnetic machinery and microelectronic technology.Fabrication of(RE)BCO bulk superconductors typically involves heating a powder compact above its peritectic decomposition temperature followed by slow cooling to facilitate the growth of a single grain.Each(RE)BCO composition has a different peritectic temperature and growth rate,which,therefore,necessitates different requirements in the heating profile.The fabrication temperature and growth rate,for example,may have an effect on the RE‐211 and silver distribution,which may,in turn,affect the superconducting properties of the resulting single grain.In this work we compare the distributions of silver and RE‐211 in the single grain microstructures of YBCOAg,GdBCO‐Ag,EuBCO‐Ag and SmBCO‐Ag bulk superconductors using optical microscopy,scanning electron microscopy and energy dispersive X‐ray spectroscopy.We observe that the distributions are very similar along both the a/b‐axis and c‐axis of these materials.This suggests that factors other than the maximum temperature used to achieve peritectic decomposition and the rate of single grain growth are particularly influential in determining the properties of the as‐processed samples in the top seeded melt growth process.This observation demonstrates there is freedom to use(RE)materials interchangeably between different applications as required,for example,for functional or economic reasons.

Numerical simulation of dynamic loss and total loss in the REBCO tapes under perpendicular AC magnetic fields up to 8 T at 20 K and 50 K

REBCO tapes carry DC current under AC magnetic fields in proposed HTS fusion applications. AC loss will be generated in the process and it is important to understand the AC loss behaviour for safe operation of the fusion magnets. In this work, magnetisation loss (Qm), dynamic resistance (Rdyn), and total loss (Qtotal) in four different REBCO tapes are numerically studied, using the measured and , for the magnetic field amplitude applied perpendicularly up to 8 T at 20 K and 50 K, where represents the magnetic field and field angle () dependent critical current density. The peak of Theva data is different from that of other tapes. We artificially shifted the ab-plane peak of Theva to the left by 25° to match the peak value. The newly shifted data is named as Theva-shift, which was also investigated to study the influence of the Theva peak shift on AC loss. The normalised DC transport current level (i = It/Ic0) ranges from 0.05 to 0.9, where the DC current amplitude and the self-critical current of the tape are represented by It and Ic0, respectively. The simulation results show that the AC losses deviate significantly from the Brandt-Indenbom (BI) equation at high magnetic fields. Jc and instantaneous loss curves for different tapes show correlation at high magnetic fields. The simulation results also show how different characteristics for different tapes influence AC losses. When AC loss values are scaled by the self-field critical current, Qm without current and Qtotal with current in the different tapes show a good agreement. It implies that the temperature dependence of the two types of loss can be calculated from a known loss at one temperature and the self-field critical current.

Flux penetration of an HTS coated-conductor tape by an approaching permanent magnet

Interactions between inhomogeneous magnetic fields and a superconductor lie at the heart of several high‐temperature superconducting(HTS)devices.An example is the situation where the magnetic field from a small permanent magnet(PM)is applied to a coated‐conductor HTS tape,such that flux penetrates the superconductor.The PM field is highly spatially inhomogeneous,such that there is significant variation in the applied field magnitude and direction across the tape.This leads to a varying local critical current density,J_(c)(B,θ),across the tape,and consequently different‘shielding’(magnetic flux penetration)behaviour than is the case for a uniform applied field.Here,we report results from measurements and numerical simulations of the penetrating magnetic field within an HTS coated‐conductor tape which occur as a PM dipole approaches from a distance.Electromagnetic simulations were performed using a finite‐element model based on the H‐formulation,and which incorporated experimentally‐measured anisotropic J_(c)(B,θ)Þproperties from a coated‐conductor tape.This showed good agreement with experimental measurements.The effects of varying key modelling assumptions and parameters were then studied,including changing the widths of the PM and HTS tape and the magnitude of J_(c)(B,θ).The inhomogeneity of the PM field leads to a characteristic gull‐wing distribution of magnetic flux across the superconductor at elevated applied fields.Close approach of the PM to the tape suppresses Jc in the centre of the tape,and this results in the observation of a characteristic maxima for the total shielding currents circulating in the tape.A figure of merit is introduced which uses this effect to provide a threshold definition for‘full flux penetration’of an HTS tape by an inhomogeneous dipole field.

Charging process simulation of a coil by a self-regulating high-T_(c)superconducting flux pump

Self‐regulating high‐temperature superconducting(HTS)flux pumps enable direct current injection into a closed‐loop superconducting coil without any electrical contact.In this work,the process of charging a coil by a self‐regulating HTS flux pump is examined in detail by numerical modeling.The proposed model combines an H‐formulation finite element method(FEM)model with an electrical circuit,enabling a comprehensive evaluation of the overall performance of self‐regulating HTS flux pumps while accurately capturing local effects.The results indicate that the proposed model can capture all the critical features of a self‐regulating HTS flux pump,including superconducting properties and the impact of the secondary resistance.When the numerical results are compared to the experimental data,the presented model is found to be acceptable both qualitatively and quantitatively.Based on this model,we have demonstrated how the addition of a milliohm range,normal‐conducting secondary resistance in series with the charging loop can improve the charging process.In addition,its impact on the charging performance is revealed,including the maximum achievable current,charging speed,and the generated losses.The modeling approach employed in this study can be generalized to the optimization and design of various types of flux pumps,potentially expediting their practical application.

AC loss study on a 3-phase HTS 1 MVA transformer coupled with a three-limb iron core

High-temperature superconducting(HTS)technology provides an alternative approach to achieve compact transformers.Addressing AC loss in the HTS winding is crucial for HTS transformer applications.Most numerical AC loss studies on HTS transformers have neglected the influence of iron cores.This work carries out an AC loss study to explore the impact of an iron core on the HTS windings in a 3-phase HTS 1 MVA transformer coupled with it.AC loss simulations for the transformer winding both with and without the iron core are conducted by adopting the three-dimensional(3D)T-A homogenization method.When the iron core is incorporated,the saturation magnetic fields of iron materials,flux diverters(FDs)with different geometries,and variations in turn spacings in the LV winding composed of Roebel cables are considered to investigate their influence on the AC loss of the transformer winding.The inclusion of the iron core leads to a 1.2%increase in AC loss for the transformer winding while simulating at the rated current.We attribute this slight difference to the non-inductive winding structure of the transformer winding,where a strong magnetic field generated in the space between the LV and HV windings effectively shields the influence of the iron core.

Progress in the development of the 122-type IBS wires

Profit from their low anisotropy and high upper critical field,iron-based superconductors(IBSs)have great potential for high magnetic fields applications.Significant progress has been made for the current density improvement of(Sr,Ba)0.6K0.4Fe2As2(122-type)IBS wires over the past fourteen years.In this article,an overview of the preparation procedures and key issues affecting the critical current performance of 122-type IBS wires is given,to provide a fundamental understanding of what controls the transport critical current density of 122-type IBS wires.Additionally,the recent developments for 122-type IBS wires are presented and future prospect and challenges in the development of 122-type IBS wires for practical applications are discussed.

Cu/Ta sheaths for iron-based superconductors:First experimental findings in Ca/K-1144 wires

Iron based superconducting wires(IBSCs)produced by the Powder in Tube(PIT)method rely on the use of silver sheaths as chemical buffer between the outer metal and the superconducting core.The adoption of silver entails however some limitations,such as the viable temperature range when coupled with copper,and the incompatibility with calcium-based IBSCs already at 600℃,driving the research towards other wires architecture.Taking inspiration from the low temperature superconductors field,we decided to evaluate the adoption of tantalum as diffusion barrier in a layered Cu/Ta architecture,choosing a Ca/K-1144 IBSC as case study considering the high reactivity issues already reported in the case of silver sheaths for this compound.Squared wires were produced through a groove rolling lamination process coupled with a thermal treatment at 800℃.The microstructural analyses show the absence of interdiffusion between the different parts of the wire,and the magnetic characterization shows performance in line with similar polycrystalline manufacts,with margin of enhancement to be pursued via the optimization of the mechanical process and other experimental variables.The reported results suggest thus the effectiveness of tantalum as diffusion barrier for Ca/K-1144 PIT wires.

Hope and challenge of ternary hydrogen-based superconductors under pressure

In recent years,compressed hydrogen-based compounds such as SH3,CaH6,and LaH10 have continuously refreshed the superconducting critical temperature(Tc)record with Tc above 200 K,keeping the hope for room-temperature superconductivity alive.Despite being a long way off,a plethora of unprecedented ternary hydrides at high pressures offers a new opportunity to search for a room-temperature superconductor with improved superconductivity.The advancement of theoretical and experimental techniques,such as the combination of machine learning methods and crystal structure prediction,as well as the fine processing of diamond anvils,is a step toward achieving room-temperature superconductivity in the near future.

Characterization of a Raman-based distributed fiber optical temperature sensor in liquid nitrogen

Distributed Temperature Sensor(DTS)based on Raman scattering have a promising application in temperature monitoring and quench detection of high‐temperature superconducting(HTS)fiber optic cable.In this paper,a series of mechanical property tests were carried out on fiber optical cables with different encapsulation materials to select the best material for the engineering environment.This was followed by loss testing of fiber optic cables with different outer diameters under liquid nitrogen(LN_(2))and the optimal outer diameter to meet measurement accuracy and engineering practicability was determined.Finally,the temperature measurement of fiber optic cables laid on HTS fiber optic cables were calibrated using thermocouple as a reference.The results of this research results provide a suitable fiber optic cable for engineering applications.At the same time,it provides a reference for the selection of optical cable structure parameters in HTS optical cable projects.

Recent progress on high-temperature superconducting filters

Since the discovery of high temperature superconducting(HTS)materials in 1987,the performance of microwave filters such as insertion loss,selectivity,and out-band rejection has been significantly improved by utilizing HTS films.Nowadays,HTS filters have been widely used in wireless communications,space exploration,satellite communications,and several other fields.HTS filters with a special performance,such as wideband,dual-band or multiband,frequency tunable,and high power-handling capability have been designed and fabricated in recent years to meet the increasing requirements of practical applications and to further extend the application areas.This paper reviews the various types of above-mentioned HTS filters as well as the developments of novel design methodologies.

Oxidation in Ca/K-1144 iron-based superconductors polycrystalline compounds

Iron‐based superconductors(IBSCs)are a class of material under investigation for the development of superconducting wires in the low‐temperature‐high magnetic fields power application.Among the various families of IBSCs,the 1144 CaKFe_(4)As_(4) compound is a promising material able to achieve outstanding superconducting properties with a cheap and simple chemical composition.Oxidation,in these compounds,is considered an obstacle for high intergranular critical current density,J_(c,GB).A study devoted to the evaluation of oxidation phenomena and their effects on the superconducting properties is thus needed in order to fully understand the involved mechanisms.From the evaluation of polycrystalline samples obtained by a mechanochemically assisted synthesis route,a degradation of the critical temperature and critical currents has been observed concurrently with oxygen accumulation at grain boundaries in open porosities.However,the crystalline structure at an atomic level seems not affected,as well as intragranular superconducting properties assessed by means of calorimetric methods.These results suggest that loss of superconducting properties in Ca/K‐1144 compounds following oxidation is significantly associated with the worsening of grain connectivity.

Short circuit analysis of a fault-tolerant current-limiting high temperature superconducting transformer in a power system in presence of distributed generations

Power transformers are key elements for the safe and reliable delivery of electrical energy generated by renewable energy resources to consumers via transmission lines.Fault‐tolerant current‐limiting High Temperature Superconducting(FTCL HTS)transformers are type of superconducting transformers that tolerate fault for seconds and limit the fault current without the threat of burnout or delamination of tapes and deformation of windings.In this paper,the fault performance of a FTCL HTS transformer in a standard IEEE power system is investigated.The studied transformer is a 50 MVA 132 kV/13.8 kV transformer where both windings are made up of HTS tapes.The understudied power system consists of two microgrids with distributed generators.Part of the power in microgrids is supplied by the upstream grid which is connected to the microgrids through the HTS transformers.Two fault scenarios have been considered in this power system,in each one of these scenarios,a fault happens in one of the microgrids.Two considered fault scenarios have an approximate fault current of 18x to 23x of the rated current in the secondary windings.Results showed that insulated windings in FTCL HTS transformers could substantially reduce the peak temperature of the HTS windings,compared to bare windings.Afterwards,post‐fault loading is imposed on the HTS windings,to observe their performance against the current increase after fault clearance.In this case,for the first scenario of the faults,the FTCL HTS transformer could tolerate 192%of post‐fault overloading,while this number for the second fault scenario is 170%.Finally,the impact of post‐fault loading on the full recovery time was discussed.

Erratum to“Review on thermal-related measurement methods for superconducting devices and prospect for high-speed maglev transportation application”[Superconductivity 3(2022)100020]

The publisher regrets to publish this article without“Declaration of Competing Interest”.The publisher would like to apologise for any inconvenience caused.The below“Declaration of interests”text should be added.Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Method with reliable accuracy and fast speed for measuring operational current of HTS NI closed-loop coils in steady persistent-current-mode

This study proposes a method for measuring the operational current of high temperature superconducting(HTS)non‐insulation(NI)closed‐loop coils,which operate in the steady persistent‐current‐mode(PCM).HTS NI closed‐loop coils are promising for many easily‐quenching direct‐current(DC)applications,where their performance is determined by magnetomotive forces,total number of turns,and dimensions.As the primary interface parameter in an application system,the operational current must be accurately and rapidly measured.Generally,this is achieved by dividing the measured magnetic field by the coil constant.However,even if the influence of the screening current induced field(SCIF)is not considered,existing methods for the coil constant may be disturbed by the performance and location of Hall sensors,or experience a long measuring period.Therefore,a relatively accurate and fast method is proposed in this study,which is based on adjusting the output current of the adjustable power supply and monitoring the coil voltage as an indicator.The proposed method was validated through experiments and simulations using an equivalent circuit model coupled with a finite element method(FEM)model,and its current accuracy can be equivalent to the resolution of the employed power supply.It was demonstrated that this method reduced the requirements for Hall sensor’s performance and location,and has a more reliable accuracy in contrast to the simulation method.Compared to the experimentally conventional method,the proposed method presents a significantly faster speed.The impact of the SCIF was considered and proven to be negligible for the tested pancake coils.Even for coils whose coil constant vibrates owing to the SCIF,this method can be adapted to directly measure various operational currents.Furthermore,it was demonstrated that the measurement error can be influenced by the current discrepancy among turns when the coil is not in the steady PCM,and a procedure for reducing this error was proposed.

Review on thermal-related measurement methods for superconducting devices and prospect for high-speed maglev transportation application

High‐temperature superconducting(HTS)bulks can not only be self‐stable when levitated above a permanent magnet(PM)but also can be used as quasi PM with higher magnetic energy product due to their magnetic flux pinning characteristics.Therefore,HTS bulks have wide application potentials in maglev trains,maglev bearings,flywheel energy storage,drug delivery,and high field magnets.In the external magnetic field of common application scenarios,HTS bulks have no external input current,so it is difficult to achieve the overall quench.However,local quenching in the bulk is still possible in the harsh fluctuating external field environment.Although it is difficult to reach the total quench,its critical parameters like Jc will inevitably deteriorate,which may collapse the application system.Therefore,in contrast to superconducting wires and tapes that are more concerned with quench detection,HTS bulks with a 3D volume effect are more focused on internal sensitive temperature locations,the impacts of volume and scale,and the coupling influence on application parameters such as magnetism and force.Therefore,for efficient thermal‐related measurement of HTS bulk applications,this paper investigates and discusses 12 commonly‐used temperature measurement or quench detection methods in all superconducting application fields.These methods primarily refer to the current quench detection technologies used in HTS tapes and wires.From the standpoint of practical temperature measurement requirements of HTS bulks and technological limitations of maglev application scenarios,working characteristics and service conditions of the 12 methods,and 4 temperature detection methods are selected through a comprehensive understanding and comparison of basic principles.They are expected to be used in real‐time monitoring and early warning schemes for onboard superconducting levitation devices of HTS maglev transportation or other applications in the future.

Phase evolution mechanism study and fabrication of PbMo_(6)S_(8) superconducting materials with two-step sintering process

PbMo6S 8 superconducting materials are considered to have great potential for practical applications at low temperatures and high fields due to their high upper critical field,low anisotropy,and low preparation cost.In this work,PbMo_(6)S_(8) bulks were prepared through a solid‐state sintering process using PbS,Mo,and MoS_(2) as raw materials.The phase evolution mechanism during the sintering of PbMo_(6)S_(8) was studied in detail.It was found that during sintering at 750℃ for 24 h,both the S and Pb atoms diffuse into the Mo and MoS_(2) particles,leading to the formation of the PbMo_(6)S_(8) phase.After sintering at 950℃ for 72 h,a high superconducting phase content was obtained in the bulk;however,numerous pores remained.Therefore,in order to obtain a higher density for the bulk,a two‐step sintering process was developed.Based on this technique,PbMo_(6)S_(8) bulks with a higher bulk density and a higher superconducting phase content were obtained.This study provides an effective method for the fabrication of high‐quality precursor powders,which can be the foundation for the future fabrication of PbMo_(6)S_(8) superconducting long wires or tapes for practical applications.

Numerical modelling of high-temperature superconducting dynamos:A review

The high‐temperature superconducting(HTS)dynamo enables injection of large DC currents into a superconducting coil,without the need for thermally‐inefficient current leads.Because of this important advantage,there is significant interest in using such technology to energise superconducting coils in superconducting rotating machines and NMR/MRI magnets.Despite the extensive experimental work carried out over the past decade,there was–until very recently–considerable confusion and debate regarding the physical origin of the HTS dynamo’s DC output voltage.Numerical modelling has played a key role in elucidating the underlying physics of such devices and several different numerical models have now been developed as useful and cost‐effective tools to not only explain and further examine experimental results,but also optimise and improve dynamo designs.This review summarises all of the developments in this important area over recent years,including modelling the open‐circuit voltage behaviour in 2D and 3D,the definition of a new benchmark problem for the HTS modelling community,investigating key dynamo parameters,modelling dynamic coil charging behaviour and calculating losses.A view towards the future is provided,including the outstanding challenges and the developments required to address these.

Enhancing in-field performance of GdBCO coated conductors by cooperative irradiation with Ti ions and protons

Irradiation can accurately manipulate defects and adjust pinning landscapes within REBa_(2)Cu_(3)O_(7-δ) (REBCO, RE: rare earths) coated conductors (CCs). This study reports a productive method to dramatically boost the in-field critical current density (J_(c) ) for GdBCO CCs using cooperative irradiation with Ti ions and protons. Remarkably, the in-field J_(c) of commercial CCs can be almost doubled at a wide range of temperatures and magnetic fields. Defects of various sizes induced by cooperative irradiation are more uniform distribution through the entire GdBCO film to improve the vortex pinning characteristics, thereby enhancing the in-field performance of the GdBCO CC. This method highlights how combining different particle irradiation types can tailor defect size and distribution, optimizing pinning landscapes for commercial REBCO CCs.

Superconducting fault current limiter(SFCL)for fail-safe DC-DC conversion:From power electronic device to micro grid protection

In DC micro grids and networks,DC-DC power converters having a large number of semiconductor-based power electronic devices are usually adopted to interconnect the renewable sources and flexible loads.Most of the semiconductor-based devices suffer from poor fault withstanding abilities,but conventional power electronic protection schemes have the bottlenecks of the time-delay,self-malfunction and mis-judgement.This paper presents a novel solution using the superconducting fault current limiter(SFCL)to protect a power electronic device and extend the usage to a micro grid.This SFCL is actually a self-triggering,recoverable,and passive current limiter,which does not involve any additional circuit hardware and software.Experimental investigations and simulation analyses clarify the feasibility of using this superconductor-based protection scheme to implement the self-acting fail-safe protection of DC-DC converters.Further system-level simulations explore the SFCL to suppress the over-current and stabilize the bus voltage of a photovoltaic based DC micro grid,particularly facing millisecond-level transients and faults.Our experimental and theoretical investigations lay some technical bases to establish a superconductor-semiconductor-coupled interdisciplinary application from the view from the applied superconductivity,to power electronics,and to micro grids.

40-meter-long REBCO tapes with critical current over 4,000 A/12 mm at 4.2 K and 13 T by advanced MOCVD

REBa_(2)Cu_(3)O_(7−δ)(REBCO,RE=rare earth)tapes doped with 5%and 15%Zr have been scaled up to lengths more than 40 m in a pilot‐scale advanced metal organic chemical vapor deposition(A‐MOCVD)tool.The precursor compositions used for the long tapes were guided by a study of the influence of(Ba+dopant)/Cu content on the critical current density(Jc)of 5 and 15 mol.%Hf‐and Zr‐added tapes at 4.2 K and 13 T.The 40‐mlong tapes exhibited a critical current(Ic)over 4,000 A/12 mm at 4.2 K and 13 T as well as over 1,400 A/12 mm at 20 K and 20 T.The critical current densities of a 40‐m‐long tape doped with 5%Zr at 4.2 K measured at the National High Magnetic Field Laboratory(NHMFL)were>10 MAcm^(−2)and>5 MAcm^(−2)at 14 T and 30 T,respectively,which are over three times those of commercial REBCO tapes.The infield Jc of 5%Zradded 40‐m‐long tapes was similar to those of previously‐reported high‐performance short samples made with 15%Zr or Hf.These results demonstrate the excellent potential of A‐MOCVD for manufacturing high Ic REBCO tapes for use in ultrahigh‐field magnet applications.