Since the discovery of superconductivity in MgB2 many efforts have been undertaken to improve the current carrying capacity of mono‐ or multifilament MgB2 conductors. However, even though MgB2 conductors can be produced in geometries which easily allow twisting or cabling, the reduction of ac losses has often played a minor role, even though many technical superconductor applications like transformers, rotating machinery, and ramped magnets require conductors with low AC losses. In this paper we present short cables together with measured AC‐losses applying a simple cabling technique. Coupling losses of the cables with strands having a single component stainless steel (SS) sheath are negligible. The lower apparent measured losses of cables with Nb/Cu/SS sheaths may be explained by magnetic shielding of the Nb layer and by a systematic problem of the magnetization method for samples containing diamagnetic and ferromagnetic components.

High temperature Superconducting cables for transport currents well above 1 kA, assembled from a number of coated conductor tapes, are required for application in transformers, generators and, with further increased currents, for future fusion reactor coils. At present the assembling of meander shaped tapes by the Roebel technique seems to be the most promising way of manufacturing high current cables. With short cable samples it was previously shown that the problem of the in the plane inflexibility of the tapes can be overcome by this technique.For some ac applications the losses are however still too high. Since the hysteresis losses in the tapes are dominant in the frequency range below 100 Hz, the main emphasis should be directed to reduce this loss component. Several routes to achieve this goal are investigated: striations in the individual tapes reduce the effective width of the superconducting layer and hence the hysteresis losses, but the problem of twisting of these striations is not yet solved.A second route is the reduction of the meander shaped Roebel tape width to a value in the region of 1 mm which allows the assembling of a correspondingly larger number of tapes, e.g. 30 or 40. This would lead to a cable approaching a square cross section with reduced losses and, as another important advantage, with a certain in-plane flexibility. The critical current degradation of tapes with 1 and 2 mm width is investigated and ac-loss results of cables with 2 mm meander tapes are presented. (C) 2009 Elsevier B.V. All rights reserved.

Assembling coated conductors (CC) into flat ROEBEL bars (RACC-cable) is a practical method to reach high transport currents in a low AC loss cable design which is suitable for application in windings. Electrical machinery as large transformers and generators/motors need a few kA transport current. The aim of the presented work was demonstrating the possibility of a strong increase of the transport current of such RACC-cables. So far 1 kA was achieved We present a changed cable design with 3-fold layered strands, an unchanged transposition pitch of 18.8 cm which finally leads to 45 coated conductors in the cable. A 1.1 m long sample (equivalent to 6 transposition lengths) was prepared. Cu stabilized coated conductors purchased from SuperPower were used formatting the ROEBEL strands and assembling the new cable. The new cable reached a record transport current of 2628 A at 77 K in self field (5 muV/cm criterion). A special feature of the cable was the use of 3 slightly different current carrying (plusmn10%) batches of strand material. Although current sharing and redistribution effects could be observed, the behavior of the cable was found to be absolutely stable under all operation conditions. The self field degradation of the critical currents, being of the order of 60% at 77 K could be modeled satisfactory by means of a Biot-Savart-Law approach.

The application of RE-BCO coated conductors (cc's) in high field magnets as well as in electrical machinery decisively depends on the ability to produce conductors with high current densities and small losses. The production process of the conductors restricts the thickness of the superconducting layer and therefore the current. It is possible to enhance the current carrying capability by combining several conductors to a cable. The ROEBEL bar concept is the most suitable solution because of reduced ac losses. Cc's are suitable for shaping ROEBEL strands and to perform the cabling. We employed different scan techniques to get an overview of the conductor properties. The magnetoscan, which was successfully applied to cc's, was optimised for scanning a prototype of such a RACC-cable. The resulting map of the magnetic field reflects the superconducting properties of the conductor and allows analysing the defect structure and the critical currents. Due to the complex configuration of the RACC-cable some considerations concerning the effects at conductor edges and the transitions between the strands were necessary. The field map of the ROEBEL-cables can be used to optimise them.

ROEBEL-type assembling of coated conductors (CC) has recently been introduced as a practicable method to prepare high current flat cables from CC in a low AC loss structure with the feature of fully transposed strands. Up to now we have prepared and analyzed four such RACC cables with 11-16 strands with quite different properties, using two different industrial CC-tapes, SuperPower MOCVD-CC and THEVA TCE-CC. Compared to the sum of single-strand I, values all cables show a current degradation of about 30% or more. The reason can be attributed to the quite inhomogeneous and significantly high self field. The quality of the CC, in particular the homogeneity of the superconducting layer, the amount of applied stabilisation and the feed-in-lengths of the currents, determines the maximum achievable transport current. We present investigations on the conductor homogeneity and transport currents, the influence of strand punching on the strand currents and correlate the cable properties with modelled self field effects in the cables. Main goal of the actual work is to study possibilities of interstrand connections which serve for current redistribution, but have to be balanced against raising coupling losses.

RBCO (R = Y or rare earth element) coated conductors (CC) are the most promising HTS materials for future high field coils operated at moderately high temperature (40-50 K). Coils are planned for the second generation of fusion reactors (DEMO, "DEMOnstrator") and beyond. A ROEBEL bar concept for a high current (kA-class) low AC loss cable is the most suitable assembling technique for conductors in magnet windings due to the flat rectangular cross section. The presented RACC-cable technique (RACC=ROEBEL assembled coated conductors) works with pre-shaping of tapes into strands with the ROEBEL specific meander geometry. The usually very good bending properties of the CC support the assembling procedure of the RACC-cable. We report on a 16 strand RACC-cable with 19 cm transposition length made from CC material from the commercial supplier SuperPower which reached 1020 A transport critical current at 77 K (J eng = 11.3 kAcm -2 ). The basic properties of the virgin YBCO tapes and the shaped strands like orientation and field dependent transport currents, current homogeneity and bending effects, were investigated and correlated with the measured properties of the RACC-cable. Calculation of the self field effects by means of a model adapted to the specific RACC-cable geometry and in particular taking into account the current distribution in the cable, explained the 30% current reduction in the cable quantitatively.

Low ac-loss HTS cables for transport currents well above 1kA are required for application in transformers and generators and are taken into consideration for future generations of fusion reactor coils. Coated conductors (CC) are suitable candidates for high field application at an operation temperature around 50–77K, which is a crucial precondition for economical cooling costs. We prepared a short length of a Roebel bar cable made of industrial DyBCO coated conductor (Theva Company, Germany). Meander shaped tapes of 4mm width with a twist pitch of 122mm were cut from 10mm wide CC tapes using a specially designed tool. Eleven of these strands were assembled to a cable. The electrical and mechanical connection of the tapes was achieved using a silver powder filled conductive epoxy resin. Ac-losses of a short sample in an external ac field were measured as a function of frequency and field amplitude in transverse and parallel field orientations. In addition, the coupling current time constant of the sample was directly measured.

Low AC loss high transport current HTS cables (> 1 kA) are required for application in transformers, generators and are considered for future generations of fusion reactors coils. 2G coated conductors are suitable candidates for high field application at quite high operation temperatures of 50-77 K, which is crucial precondition for economical cooling costs. As a feasibility study we present the first ROEBEL bar cable of approx. 35 cm length made from industrial DyBCO coated conductor (THEVA GmbH, Germany). Meander shaped ROEBEL strands of 4 mm width with a twist pitch of 180 mm were cut from 10 mm wide CC tapes using a specially designed tool. The strands carried in average 157 Amps/cm-width DC and were assembled to a subcable with 5 strands and a final cable with 16 strands. The 5 strand cable was tested and carried a transport current of > 300 Amps DC at 77 K, equivalent to the sum of the individual strand transport critical currents. The 16 strand cable carried 500 A limited through heating effects and non sufficient stabilisation and current sharing. A pulse current load indicated a current carrying potential of > 1 kA for the 16 strand cable.

The measurement of energy dissipation in superconducting high Tc tapes in an external ac-field is of major concern for their technical application at 50 Hz. The widely used technique is based on the measurement of the magnetization loop which is proportional to the dissipated energy. Since the proportionality constant depends on the geometry of the sample and of the pick-up coils used, calibration of the system is necessary. A convenient way is the calibration with a calorimetric measurement which however leads, at 77 K, to the problem of long thermal time constants. It requires furthermore a special preparation of the sample which must be thermally insulated from the cooling bath. This paper gives a simple relationship between the sample geometry and the calorimetric calibration factor, empirically deduced from measurements on tapes and stacks of tapes with rectangular cross sections. The accuracy of the method is within about ±10 %, which is sufficient for most technical applications.

The application of High Temperature Superconductors (HTS) in fusion research has, until now, been limited to the use in current leads. Studies for a use in bus bar systems show a technological potential, too. The applicability of HTS in fusion magnets is presently not feasible but would be desirable for use in future fusion reactors, e.g., DEMO and beyond. The main driver to use HTS materials for fusion coils is to operate the magnets either at high magnetic fields, e.g. 20 T, or at medium or high operation temperatures (50 – 65 K). As the low temperature superconductor (LTS) layouts for a Tokamak or a Stellarator has been optimized by balancing the requirements for current distribution, AC losses and cooling capability, the situation is different in case of HTS, especially for YBCO coated conductors. While transient cooling is not important due to the much higher heat capacity of the materials, AC losses, thermal stability, hot spot temperature and current distribution will play a challenging role due to the structure of the coated conductor tape. This contribution will give an overview about status, promises and challenges of HTS conductors on the way to an HTS fusion magnet system for DEMO and beyond.

The copropagation of two waves in an ultralong semiconductor optical amplifier (SOA) is considered in theory and experiment. One wave is a modulated signal, whereas the other one is unmodulated (continuous wave). The theory bases on a comprehensive traveling-wave model and predicts an exponential improvement of the signal extinction ratio (ER) of the modulated signal, caused by the presence of the unmodulated signal. Conditions for achieving this two-wave competition (TWC) effect are as follows. The SOA is operated under saturation, both waves are copolarized, they have comparable gain and their spectral correlation is between certain limits. The TWC effect is due to nondegenerate four-wave mixing (FWM) in the saturated part of a long SOA and is expected to have a high-speed potential. In order to check the theoretical predictions, 4-mm-long SOAs are developed and experimentally investigated under the given conditions. The measured ER improves by 1.3 dB for a 5-GHz sinusoidal signal, which compares well with the 2 dB theoretically predicted for this configuration. FWM is identified also experimentally as the basic mechanism. Variation of wavelength detuning, pump current, modulation frequency and ER of the injected signal are used to determine optimum conditions for the given device.

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.

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.

The phase dynamics that occur in bulk InGaAsP-InP semiconductor optical amplifiers (SOAs) in response to picosecond pulse excitations at 10 and 40 GHz are studied experimentally and numerically for various amplifier lengths. The time dependencies of the phase changes and of the absolute gain of the amplifier are measured simultaneously. The total phase shifts induced by 1.5-ps pulses at 10 GHz are higher than pi in SOAs with active region lengths between 0.5 and 1 mm and exceed 2pi in a 1.5-mm-long amplifier. Phase shifts above pi are measured at 40 GHz in 1.5-and 2-mm-long SOAs. The dependence of the total phase shift on the amplifier bias current and length and on pump pulse energy is investigated. Numerical simulations based on a comprehensive time-domain SOA model allow us to confirm the experimental results for a wide range of amplifier parameters. In particular, SOAs with lengths up to 5 mm have been modeled, and the calculations suggest that the maximum phase shifts occur in amplifiers of approximately 2-mm length. The phase dynamics measurements are illustrated at the example of an optical time division multiplexing add-drop multiplexer, based on a SLALOM switch, gated by 10- or 40-GHz control pulses. We. find that simultaneous good dropping and clearing is possible if the length and the operating conditions of the SOA in the switch are chosen such as to induce a full pi phase shift.

The authors report an all-optical add-drop multiplexer based on gain-transparent operation of a semiconductor optical amplifier in a novel geometry. Error-free operation at 160 Gbit/s is demonstrated for all channels.

The design and fabrication of a monolithically integrated balanced photodetector module is reported and its application in a single polarisation 160 Gbit/s OTDM transmission experiment with RZ-DPSK modulation format. The 2.5 dB improvement in receiver sensitivity enabled transmission over a record fibre link of 410 km using exclusively EDFAs.

We measured 160 Gb/s eye diagrams with an optical sampling system using a semiconductor optical amplifier (SOA) based interferometric gate. Gain-transparent operation provided high input to output linearity and a large wavelength range for operation.