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The effect of compaction of ex-situ MgB2-powder, i.e. cold and hot uniaxial pressing, Spark Plasma Sintering (SPS) and of a post annealing on microstructure and superconducting properties of ex-situ bulk samples is studied. Additionally the impact of powder milling and the thermal stability of MgB2 are discussed. The magnetic critical current density can be enhanced by using of high energy milled powder and annealing at temperatures larger than 850 °C. The best properties were achieved by SPS processing of milled powder.

We present a new high-deposition-rate coating technique that allows us to produce at room temperature long thick films of MgB 2 on flexible steel substrates. Such a technique might give rise to new tapes with higher filling factors compared to the standard processed tapes. With the so-called aerosol deposition technique, MgB 2 films were prepared on Hastelloy substrates with commercially available ex situ-prepared MgB 2 powder (T C,onset of 38 K, JC of 3.8·10 3 A/cm 2 at 4 K and 1 T). Microscopic analyses yield nanocrystalline dense films with high film stresses. The as-deposited films have so far a superconducting transition temperature T C0 of 18.1 K and a critical current density J C up to 5·10 3 A/cm 2 at 4 K and self-field obtained.

High temperature superconducting (HTS) rotating machines show several significant advantages compared to machines built in conventional technique. Former experiments on rotating electric synchronous machines like motors and generators in the power range up to several MW confirmed the well-known benefits of HTS machines like smaller size, less weight and last but not least, a significantly increased efficiency. Especially the increased efficiency of large HTS-Generators up to several hundreds of MW-as operated by utilities-promises an efficient use of fuel and energy sources. This will allow reduced carbon dioxide emissions and becomes more and more important. The development of HTS rotor technology for generators in the range of hundreds of MW bears new challenges. HTS generator windings capable to withstand large centrifugal forces and carrying large currents are required. Also a special cooling system for the rotating winding and further components with novel design will be necessary and have to be developed. In a future HTS generator based on such components, these will have to operate very reliably, so a facility is needed to be able to perform tests. Therefore it is essential to design and build a specific test rig for investigation and test of the required new components. The present paper deals with design and development of such a test rig that allows component tests under realistic conditions, using radii in the range of 0.45 m as in a generator application at rotational speed of up to 3000 rpm, and at low operating temperatures of about 30 K. Additionally a 3 kA high current power supply for HTS test objects like coils or contacts is projected. Aspects of rotor dynamics and fatigue strength analysis had to be considered.

We investigate the use of 2nd-generation High-Temperature Superconductors (2G-HTSs) in the rotors of electrical motors and generators. For these devices the conductor must be wound into robust impregnated coils, which are operated in vacuum at temperatures around 30 K, in strong magnetic fields of about 2T. Differences in thermal contraction between the coil former, conductor constituents, impregnation resin, bandage and heat-sink materials (assembled at room temperature) cause mechanical stresses at operating temperature. Rotating-machine operation adds Lorentz forces and challenging centripetal accelerations up to thousands of g. Second generation-HTS conductors withstand large tensile stresses in axial direction and compression in normal direction. However, shear stresses, axial compression, and tension normal to the conductor can cause degradation in superconducting properties. Such stresses can be mitigated by correct choice of materials, coil lay-out and manufacturing process. A certain stress level will remain, which the conductor must withstand. We have manufactured many impregnated round and race-track coils, using different 2G-HTS conductors, and tested them at temperatures from 25 K to 77 K. Degradation of the superconductor in early coils was traced to the mentioned differences in thermal contraction, and was completely avoided in coils produced later. We will discuss appropriate coil-winding techniques to assure robust and reliable superconductor performance.

We have manufactured and tested new HTS coil configurations in order to extend the range of rotating machines to which HTS tapes can be applied. The rotors of large utility generators require operating currents of several kA, which can be achieved by connecting several HTS tapes in parallel in a transposed cable. Design of a generator poses questions on winding technique for the coils, cooling technique at the required operating temperatures around 30 K, and V-I relation and stability of such coils. These questions were addressed by winding a test coil using 31 m of transposed cable comprising 500 m of BSCCO tape and testing it at 30 K. Smaller generators coupled to gas turbines have to work at many thousands of rpm. The high g-forces and need for compactness make it necessary to place the rotor coils as close as possible to the shaft, which requires coils with 3D-bent heads, like in accelerator dipoles. We have produced this type of coils with BSCCO tapes for the first time. After developing a winding technique using simple tooling, 700 m of BSCCO tapes were used to manufacture two 3D demonstrator coils, which were tested at 30 K as well. We describe winding methods, cooling technique and test results for both types of coils.

Future magnetic-resonance imaging (MRI) systems for medical diagnosis may work on the principle of pre-polarization. This requires a strong local magnetic field which can be ramped up and down in milliseconds. Such fields can be generated efficiently by compact coils made with high-temperature superconductors (HTS). Copper conductors would be too bulky; low-Tc superconductors would give high AC loss at 4 K. We have designed, constructed and tested an HTS double-pancake coil which generates 0.3 T of pulsed field in a 200 mm inner bore. An operating temperature of 30 K was chosen as a compromise between efficient cooling and optimum use of the HTS tape. The coil is cooled by a single GM refrigerator via a heat-pipe system filled with liquid Neon. The heat pipes and supporting structure were designed for minimum eddy-current loss in the pulsed field. The design process had to account also for AC loss in the superconductor, heat transfer from the coil to the cooling fluid and Lorentz forces on the HTS tape. Initial experiments validated the winding process for the mechanically weak Bi-2223/Ag tape, and the glued connections under thermal contraction of the materials. The production process of the coil aimed at ensuring optimum thermal contact between winding and heat pipes. The critical current at 77 K proved that there was no degradation of the HTS material. In tests at 30 K, the coil carried 250 A, 1 Hz pulsed current during up to 20 minutes. Thus the coil fulfilled the design specs

A reliable method for persistent-current mode operation of HTS magnets is still not available. As an alternative solution to external high-current power supplies we have developed a cryogen-free operating transformer-rectifier type HTS flux pump. Using this device only small ac currents have to be supplied into the cryogenic system. Both thermally triggered HTS thin film switches as well as cryogenic MOSFET switches have been used to operate a conduction-cooled 0.5 T HTS magnet coil. This paper presents results of experimental test and modeling of the flux pump operation.

International audience; A European consortium has progressed towards elementary and assembled conductors viable for AC applications in the project ACropolis. The main goal was to achieve low AC losses and high critical current densities in tapes developed for AC coils, transformers and cables. In order to verify the performance of tapes an extensive characterization programme was carried out. The dependence of critical current and AC losses on the parameters of the tape manufacturing process were scrutinized. Furthermore, SEM-EDX and DTA/TG analyses were performed to study the material microstructure, and the diffusion coefficient of oxygen through different matrix metals was measured. The homogeneity of the tapes was verified with magnetic knife and Hall sensor experiments. Transport current measurements were performed at different mechanical stress conditions and also the distribution of critical current along long tapes was determined with a quasi-continuous four-point measurement. The results confirmed that the low AC loss conductors can be produced on an industrial scale and they are competitive in power engineering applications.

High-temperature superconducting (HTS) transformers and reactor coils promise decreased weight and volume and higher efficiency. A critical design parameter for such devices is the AC loss in the conductor. The state of the art for AC-loss reduction in HTS power devices is described, starting from the loss in the single HTS tape. Improved tape manufacturing techniques have led to a significant decrease in the magnetization loss. Transport-current loss is decreased by choosing the right operating current and temperature. The role of tape dimensions, filament twist and resistive matrix is discussed and a comparison is made between state-of-the-art BSCCO and YBCO tapes. In transformer and reactor coils the AC loss in the tape is influenced by adjacent tapes in the coil, fields from other coils, overcurrents and higher harmonics. These factors are accounted for by a new AC-loss prediction model. Field components perpendicular to the tape are minimized by optimizing the coil design and by flux guidance pieces. High-current windings are made of Roebel conductors with transposed tapes. The model iteratively finds the temperature distribution in the winding and predicts the onset of thermal instability. We have fabricated and tested several AC windings and used them to validate the model. Now we can confidently use the model as an engineering tool for designing HTS windings and for determining the necessary tape properties.

We present results on DC current carrying capability and AC transport current losses of cabled conductors assembled from multifilamentary (PbBi)2Sr2Ca2Cu3O10+x tapes. The DC critical current of cabled conductors lies systematically below the sum of critical currents of the tapes. By considering the self field a model for the critical current of cabled conductors is derived which reproduces also the dependence of critical currents on external magnetic fields. For the AC self field transport current losses an empirical description based on the Norris model is derived leading to a scaling of the normalised loss.

Recent progress in developing large area HTS-coated stainless steel tapes is reported. YBCO films have been grown on IBAD-YSZ buffered 0.1 mm-thick steel tapes using a high-rate pulsed-laser-deposition technique which allows a deposition rate up to 70 nm /spl middot/ m/sup 2//h. The coated conductors (CC) are provided with an optimized shunt layer and current contacts which reduce the generation of Joule's heat. An advanced technique for critical current I/sub c/ measurements was developed. The effects of self-field and time relaxation of current in helically wound tapes are analyzed. In long tapes of 10 m /spl times/ 4 mm, the highest I/sub c/ of 78 A at 77 K was observed. Critical currents of 67-75 A were reproducibly achieved in (8-10) m /spl times/ 4 mm coated tapes. For shorter tapes with a length of 0.2-1 m, critical currents of 317-391 A/cm width of the tape were observed. Fifteen wider CC tapes of 0.6 m /spl times/ 43 mm with I/sub c/ = 700 A have been manufactured and employed in newly developed fault current limiters with a nominal current of 3 kA. Critical current densities J/sub c/ = (1.3-2.5) MA/cm/sup 2/ at 77 K were observed in YBCO films. The CC tapes exhibit favorable behavior under axial stress and sufficiently small ac losses.

High-temperature superconducting (HTS) transformers promise decreased weight and volume and higher efficiency. A 1 MVA HTS railway transformer was built and tested at Siemens AG. This paper deals with the prediction of ac loss in the BSCCO/Ag-tape windings. In a railway transformer the tape carries ac current in alternating field, the temperature differs from 77 K, tapes are stacked or cabled and overcurrents and higher harmonics occur. In ac-loss literature these issues are treated separately, if at all. We have developed a model that predicts the ac loss in sets of BSCCO/Ag-tape coils, and deals with the above-mentioned issues. The effect of higher harmonics on the loss in HTS tapes is considered for the first time. The paper gives a complete overview of the model equations and required input parameters. The model is validated over a wide range of the input parameters, using the measured critical current and ac loss of single tapes, single coils and sets of coils in the 1 MVA transformer. An accuracy of around 25% is achieved in all relevant cases. Presently the model is developed further, in order to describe other HTS materials and other types of applications.

Superconducting transformers are an important innovation for future power transmission and transportation systems. Powerful, lightweight, energy-saving and environmentally friendly they offer enormous benefits compared to their conventional counterparts. Siemens is developing a 1-MVA demonstrator transformer for laboratory testing, exhibiting innovative features like horizontal design, cabled-conductor windings and a closed cooling cycle with sub-cooled nitrogen. Being one of the most promising applications Siemens has started a programme towards the development of on-board transformers for electrical rail vehicles. This paper summarises world-wide efforts in the development of superconducting transformers and reports on the progress achieved at Siemens.

In long-length HTS coated metallic tapes a shunt layer is required to subdue the influence of the hot spots as well as to equalize the potential between the HTS film and the metallic tape in the case when the transport current is near to its critical value. Both a modelling and measurements of current transfer between current contact, shunt layer and HTS film have been performed to optimize the architecture of the coated tapes. The model of current interchange allows to determine analytically the total power losses as well as the surface density of the power losses at the contact areas. Expressions derived for power dissipation lead to a criterion for the contact resistance, R s , and the length of the current contact, L, of the given HTS/shunt/contact architecture. For a combination of YBCO and Au layers, for example, the R s must be

Multifilamentary Bi-2223 tapes for AC applications in HTS transformers are studied. By twisting the filaments in tapes with AgMg sheath and Ag matrix the parallel field AC loss at power frequencies is lowered by more the 50% for the optimum twist length of 10 mm. The critical current density of these conductors corresponds to >80% of the non-twisted tape. In order to achieve a larger loss reduction tapes with Ag-alloys and ceramic barriers are studied.

Magnetisation loss is an important factor in the design of superconducting transformers and motors. In these devices the tapes are usually placed face-to-face. Then the magnetisation loss is influenced by the mutual magnetic shielding between adjacent tapes. The shielding is investigated by measuring the magnetisation loss in stacks with various numbers of Bi-2223 tapes, exposed to a 48-Hz perpendicular magnetic field at 77 K. In a stack the penetration field is increased and the magnetisation loss below penetration is greatly decreased, compared to the behaviour of a single tape. The loss at high magnetic-field amplitudes is unaffected. The measured loss is compared to the loss calculated with two different models. The effect of shielding is qualitatively well described with an analytical model. However, predictions made with a numerical model display a better quantitative agreement with the measurement results.

Transformers, motors and magnets based on Bi-2223 tapes have an optimum working temperature where the sum of investment and exploitation costs has a minimum. The investment cost is lower at temperatures below 77 K due to the higher critical current of the superconductor. The exploitation cost is mainly determined by the ac loss, which is dominated by the magnetization loss. The magnetization loss of a Bi-2223 tape at 48 Hz is measured for various temperatures between 65 and 77 K. The temperature dependence of the magnetization loss is fully explained by the change in critical current and agrees with the critical-state model. At high magnetic-field amplitudes the magnetization loss normalized with the critical current is independent of the temperature. Nevertheless, at lower temperatures the exploitation cost is higher due to the higher cooling penalty factor. A relation for the optimum temperature is developed, based on realistic expressions for the cooling penalty factor and the critical current as a function of the temperature. The influence of the other relevant parameters is summarized into a single dimensionless number. It is concluded that the optimum temperature of a power device based on Bi-2223 tapes is in the range 65-75 K.

When a multi-filamentary superconductor is exposed to an alternating external magnetic field, coupling-current loss is a significant part of the magnetization loss. The coupling-current loss at low magnetic-field frequencies is determined by the effective transverse resistivity of the inner filamentary region. The effective resistivity is calculated for various shapes and structures of the inner filaments. Bi-2223/Ag tapes typically have filaments with a high aspect ratio, causing an anisotropic transverse resistivity. When there is no contact resistance between the filaments and the matrix material, the presence of the filaments decreases the effective resistivity. The decrease is greatest for overlapping filaments whose wide sides are oriented parallel to the external magnetic field. Good insulating barriers result in a high contact resistance from the filaments to the matrix. Then the effective resistivity is greatly increased, especially for overlapping filaments that are oriented perpendicularly to the magnetic field. The results are relevant for the calculation of the coupling-current loss in Bi-2223 tapes with twisted filaments.

At high amplitudes and frequencies of the external magnetic field, the superconducting filaments in a composite conductor are fully coupled. This causes a high magnetisation loss. Methods to decouple the filaments are well known for the low-Tc superconductors and have recently been applied also to high-Tc composites. In this paper, expressions are derived for the magnetic-field amplitude and frequency where full coupling sets in, as well as for the twist length required for decoupling the filaments in a general rectangular composite. A comparison is made with other expressions found in the literature. The filaments in a typical Bi-2223/Ag tape in parallel magnetic fields are decoupled just by twisting. In perpendicular fields a high transverse resistivity is required to decouple the filaments and decrease the magnetisation loss.

In AC power-engineering applications, the energy dissipation in the superconductor is dominated by the magnetization due to alternating fields. To reduce this type of loss, conductors are being developed with twisted filaments and an increased matrix resistivity. The magnetic AC loss has been well described for low-T/sub c/ (wire) conductors. In Bi-2223 tapes the picture is different due to strong anisotropy, granularity, flux creep and large aspect ratio of the tape. The magnetic AC loss is investigated at power frequencies in various Bi-2223 tapes (twisted and nontwisted) and with different materials for the matrix (Ag, Ag alloys and ceramic barriers). When the field is parallel to the tape plane, the filaments in twisted tapes can be decoupled and the AC loss is decreased even when the matrix is silver. In tapes with ceramic barriers between the filaments, first indications of filament decoupling are observed also in perpendicular field. Compared to a round wire, there are essential differences between the AC loss mechanisms occurring in a long twisted tape and those in a short piece of nontwisted tape.

The dynamic resistance in a slab-like superconductor is calculated, taking into account a field-dependent critical current density. In superconductors carrying DC transport current in an AC external magnetic field, the dynamic resistance causes a transport loss which depends on the amplitude and frequency of this field as well as on the transport current. This resistance is calculated analytically in a critical-state model applied to a superconducting slab in a parallel field. The field has a general periodic time dependence and for the superconductor a relation between critical current and momentary magnetic field as in the Kim model is assumed. The dynamic resistance appears only at field amplitudes larger than the so-called threshold field, which depends on the transport current. The model predictions are compared with experimental results obtained with a Bi-2223/Ag tape at liquid-nitrogen temperature. At small field amplitude and at low transport current, the derived model predicts the observed dependence of dynamic resistance on field amplitude, field frequency and transport current. For a larger field amplitude and simultaneous high transport current, the resistance is found to be larger than the model predicts. This is probably due to the not completely slab-like geometry and/or to a different field dependence of the critical current density in a high AC field.

In AC power-engineering applications, a large part of the AC loss in the superconductor is due to magnetization by the external field. This magnetic AC loss has been well described for the low- T c conductors. In Bi-2223 tapes the picture is different due to strong anisotropy, granularity and flux creep. Magnetic AC loss in various twisted and non-twisted Bi-2223 tapes has been measured at power frequencies by a pickup method. The results are compared to theoretical models of magnetization loss. When the field is parallel to the tape plane, the filaments in twisted tapes can be decoupled and the AC loss is decreased even when the matrix is pure silver. The extra effect of higher-resistance matrix materials is studied. In perpendicular field it is more difficult to decouple the filaments, due to the particular tape geometry. Contrary to a wire, there are essential differences between the AC loss mechanisms in a long twisted tape and those in a short piece of non-twisted tape. Finally, the dynamic resistance caused by the AC magnetic field is examined.

To replace conventional normal conducting solutions in electrotechnical devices, high-Tc superconductors must offer distinct economical and technical benefits in terms of lower overall loss, volume and weight. Based on AC loss theory we design appropriate 50 Hz reference conductors for cables, transformers and other applications, calculate admissible limits for the conductor variables filament diameter, twist and matrix resistance and compare this to the present state of Bi-2223-tape conductors and AC loss measurements. Further the influence of perpendicular AC field components on losses is addressed. High current devices will require multistrand conductors, where nonuniform current distribution due to unbalanced magnetic coupling may result in partial saturation and enhanced losses. As an example we discuss the multilayer HTSC-cable and present a solution based on a ‘zero flux condition' for azimuthal and axial magnetic fields.

For the most common AC application frequencies, the main component of the AC losses in multifilamentary Bi(2223) tapes are caused by hysteresis- and coupling losses. These losses can be reduced enhancing the matrix resistivity and applying a twist to the filaments. We report on the AC loss properties of 37-filament tapes with AgAu (8 wt.%) matrix, and novel 19-filament tapes with SrCO 3 barriers between the filaments. We performed transport AC loss and magnetic AC loss measurements in parallel and perpendicular magnetic fields. Both kinds of tapes were also prepared with filament twists below a twist pitch of 20 mm. The influence of the different tape modifications on the AC loss behaviour is presented and compared with theoretical models to understand the effect of the resistive matrix. In the case of magnetic AC loss measurements, reduced AC losses due to decoupled filaments were observed for the twisted tapes with a resistive matrix in low parallel fields.

The ac loss in high-Tc superconducting tapes with twisted and non-twisted filaments has been studied by a magnetic method. A brief overview of the theoretical background and the experimental set-up is presented. Measurements were made at 77 K in a magnetic field of 50 Hz frequency and 0.001-0.7 T amplitude. Application of dc transport current made it possible to distinguish between the loss components, arising from intra-grain and from filament currents. The magnitude of the filament loss component indicates that the filaments are fully coupled, which agrees with theory. In other measurements, the orientation of the external field with respect to the tape was varied. Knowledge of the ac loss in parallel and in perpendicular field is sufficient to predict the ac loss for any intermediate orientations of the field.

It has been demonstrated that Fiber Bragg Grating Sensors can measure at very low temperatures in Helium cryostats. The intrinsic temperature sensitivity is very low but can be significantly enhance by the thermal expansion of polymer carriers. Temperature and mechanical strain changes can be measured quite accurately.

The occurrence of power dissipation in superconductors exposed to time varying (AC) magnetic fields or currents is fundamentally related to the existence of vanishing resistance under static (DC) conditions. Even if this AC loss cannot be completely avoided it has to be minimized to tolerable levels in commercial power engineering devices by optimization of the conductor design.

International audience; A European consortium has progressed towards AC coil, transformer and cable applications made of low AC loss Bi-2223 conductors. Internally assembled ring-bundle-barrier conductors were used in AC coils and an externally assembled conductor was realized for transformer and reactor windings. For the design and optimization of HTS AC coils, a numerical tool for the prediction of AC losses in coils and coil sets had been developed. The demonstrations with air-core coils and iron-core reactors showed that the numerical tool can already be confidently used in the design and optimization of HTS AC coils at nominal operation conditions. A series of three power cable prototypes was manufactured to estimate the influence of optimized low AC loss tapes on the total losses of a cable. In order to estimate the impact of the twist pitch on the losses we also manufactured cables with short and long twist pitches. The advantages and drawbacks of low AC loss tapes in power applications are discussed on the grounds of the test results.

In an increasing number of electric power grids the share of distributed energy generation is also increasing. The grids have to cope with a considerable change of power flow, which has an impact on the optimum topology of the grids and sub-grids (high-voltage, medium-voltage and low-voltage sub-grids) and the size of quasi-autonomous grid sections. Furthermore the stability of grids is influenced by its size. Thus special benefits of HTS applications in the power grid might become most visible in confined power grids.

For the AC application of multifilamentary BSCCO-tapes at 50-60 Hz in power cables and transformers, the main components of the AC losses are hysteresis losses in the filaments and coupling current losses in the normal conducting matrix. Coupling losses can be reduced by enhancing the matrix resistivity and by applying a twist to the filaments. The authors developed a novel multifilamentary BSCCO tape in an Ag matrix with resistive SrCO/sub 3/ barriers between the filaments to enhance the transverse resistivity. AC loss measurements on two different types of conductor designs are shown in this paper. In the first series of conductors the barrier forms something like a honeycomb structure in the cross-section between the filaments (DCB tapes). From this conductor type, a series with Ag and AgAu(8 wt.%) matrix was prepared. The AgAu was chosen to increase the matrix resistivity compared to Ag. In the second series of tapes the conductor structure is similar to a Rutherford cable (RBB tapes). Six bundles of 7 filaments are stacked round a central SrCO/sub 3/ core and deformed to a tape. All types of tapes have been twisted with a twist length down to 7.5 mm. In the AC transport current loss measurements, they find a loss reduction in the RBB tapes due to the fact that the filaments in these tapes are nearly fully transposed. In external AC fields, loss reductions in parallel as well as in perpendicular fields are found in both conductor series due to the decoupling of filaments.

The shielding of a DC magnetic field has been demonstrated to be complete at 4.2 K, at least up to 2 T, inside the 18 mm bore of a thick MgB2 bulk cylinder, 70 mm tall. No hysteresis has been detected along the cycle of the field application and removal, and no flux jumps have been detected either. The MgB2 cylinder was manufactured by the Reactive Mg Liquid Infiltration process, an 'in situ' technology able to give very dense MgB2 manufacts. According to the previously measured Jc(B, T) characteristics of a MgB2 bulk superconductor of the same origin and quality, a critical current density of about 30 kA cm − 2 can be assumed at the fields of 2–3 T and at the temperature of 4.2 K of the reported shielding experiment. Accordingly, we estimate that the thickness of the MgB2 cylinder filled by the shielding currents is about 5.75 mm, lower than the present cylinder thickness of 8.77 mm.