Showing papers in "Superconductor Science and Technology in 2007"

High-temperature superconductor fault current limiters: concepts, applications, and development status

Abstract: The application of superconducting fault current limiters (SCFCLs) in power systems is very attractive because SCFCLs offer superior technical performance in comparison to conventional devices to limit fault currents. Negligible impedance at normal conditions, fast and effective current limitation within the first current rise and repetitive operation with fast and automatic recovery are the main attributes for SCFCLs. In recent years there has been a significant progress in the research and development (R&D) of SCFCLs. This paper gives an extended review of different SCFCL concepts, SCFCL applications and the R&D status. Within the first part of this paper the most important SCFCLS and, to a limited extent, non-superconducting fault current limiter (FCL) concepts are explained and compared. The second part reviews interesting SCFCL applications at the distribution and transmission voltage level and the third part shows in detail the R&D status. It can be summarized that SCFCLs are, at present, not commercially available but several successful field tests demonstrated the technical feasibility of SCFCLs. First distribution level applications are expected soon. Considerable economical and technical benefits can be achieved by applying SCFCLs at the distribution and transmission voltage level.

Development of an edge-element model for AC loss computation of high-temperature superconductors

Abstract: This paper presents a new numerical model for computing the current density, field distributions and AC losses in superconductors. The model, based on the direct magnetic field H formulation without the use of vector and scalar potentials (which are used in conventional formulations), relies on first-order edge finite elements. These elements are by construction curl conforming and therefore suitable to satisfy the continuity of the tangential component of magnetic field across adjacent elements, with no need for explicitly imposing the condition . This allows the overcoming of one of the major problems of standard nodal elements with potential formulation: in the case of strong discontinuities or nonlinearities of the physical properties of the materials and/or in presence of sharp corners in the conductors' geometry, the discontinuities of the potentials' derivatives are unnatural and without smoothing artifices the convergence of the algorithm is put at risk. In this work we present in detail the model for two-dimensional geometries and we test it by comparing the numerical results with the predictions of analytical solutions for simple geometries. We use it successively for investigating cases of practical interest involving more complex configurations, where the interaction between adjacent tapes is important. In particular we discuss the results of AC losses in superconducting windings.

Magnetic properties and critical currents of MgB2

Abstract: This review focuses on the superconducting properties of MgB2 that are relevant for power applications. The reversible mixed state parameters are the most important, since they define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current. They also determine the flux pinning energy, the pinning force and the elastic properties of the flux line lattice and, therefore, strongly influence the critical current densities. The magnetic properties of magnesium diboride are anisotropic and influenced by the two different energy gaps of the σ- and π-bands. Whereas the transition temperature could not be enhanced significantly during the past five years, the upper critical field was considerably increased by impurity scattering or doping. Flux pinning is very weak in MgB2 single crystals and was only improved by irradiation techniques so far. In polycrystalline samples, grain boundary pinning seems to play the dominant role. High critical currents close to the theoretical limit were found in c-axis oriented thin films. The anisotropy of the upper critical field strongly reduces the critical currents in untextured MgB2 at high magnetic fields, where the supercurrents become highly percolative, since not all grains are superconducting anymore. The performance of polycrystalline wires and tapes was significantly improved during the past few years by increasing the upper critical field and by reducing its anisotropy. Pinning seems to be nearly optimized in many forms of this material, but the connectivity between the grains might be further improved.

Prospects for MgB2 superconductors for magnet application

Abstract: Superconducting magnets have a variety of industrial, medical and research applications. This review discusses the prospects for MgB2 superconductor for practical magnet applications vis-?-vis the intermetallic low (LTS) and high temperature superconductor (HTS) cuprates. It has high TC (39?K), high JC (105?106?A?cm?2 at 4.2?K and in the self-field) and HC2 (15?20?T at 4.2?K) in wire/tape geometry, with great scope for further improvement in the coming years, making it a promising candidate for practical applications. The superconducting properties of MgB2 differ from those of LTS and HTS in many ways. Besides the unusually high TC, MgB2 has a large coherence length, low anisotropy and transparent grain boundaries. The most important difference between MgB2 and other practical superconductors is that it has two superconducting gaps originating from two different bands. Tuning the scattering rates between the two bands improves the superconducting properties and the practical applicability of MgB2. The different methods of fabrication of MgB2 conductors are described and compared. Fabrication of long length MgB2 conductors is relatively easy and less expensive as compared to HTS and the method allows the use of a variety of sheath materials with suitable barriers or reinforcement. The conductors have much better mechanical properties for practical applications. The critical issues and the challenges to be addressed for realization of MgB2 superconductors as the first choice for high field magnet applications are discussed. At present MgB2 is most suited for 20?25?K operation in fields of 1?2?T.

Delamination strength of YBCO coated conductors under transverse tensile stress

Abstract: We present a new experimental technique to measure the delamination strength under transverse tensile stress of YBa2Cu3O7−δ coated conductors for electric power applications. The delamination strength, defined as the tensile stress at which the ceramic layers delaminate from one another, is measured at 76 K for different sample configurations. The delamination strength is reduced by as much as 40% when the conductor is slit to smaller width, a standard fabrication process, and this reduction is due to damage to the ceramic layers near the edges of the conductor. We found that the delamination strength of slit coated conductors can be raised significantly by reinforcing the conductor by laminating it with copper strips and adding solder fillets at the edges. In relatively strong conductors, where the delamination strength is as high as 15 MPa, the critical current does not degrade before actual delamination. This fact greatly simplifies sample characterization of practical high-strength conductors, since only mechanical measurements need to be made. The critical current does, however, degrade significantly as a function of transverse stress before delamination in weak conductors that have relatively low delamination strength below 15 MPa. We discuss how a soft metallic layer in YBCO coated conductors may limit the transverse stress that the superconducting layer experiences in applications.

Limiting factors of normal-state conductivity in superconducting MgB2: an application of mean-field theory for a site percolation problem

Abstract: Normal-state conductivity in polycrystalline MgB2 bulk samples having a systematically varied packing factor was studied The packing factor dependence of phonon term resistivity Δρ(T) = ρ(T)−ρ0 was found to be well explained by the three-dimensional site percolation model The low packing density of the samples and the wet impurity phases at grain boundaries are suggested to be the main causes of poor electrical connectivity in MgB2 Our model enables quantitative evaluations of the intrinsic resistivity inside the grains, the fraction of the active grains that can carry current and the anisotropy of the grains in polycrystalline samples The model predicts that the anomaly suppressed connectivity in rather weak-link-free MgB2 can be understood under a scenario of a percolation problem

HTS machines as enabling technology for all-electric airborne vehicles

Abstract: Environmental protection has now become paramount as evidence mounts to support the thesis of human activity-driven global warming. A global reduction of the emissions of pollutants into the atmosphere is therefore needed and new technologies have to be considered. A large part of the emissions come from transportation vehicles, including cars, trucks and airplanes, due to the nature of their combustion-based propulsion systems. Our team has been working for several years on the development of high power density superconducting motors for aircraft propulsion and fuel cell based power systems for aircraft. This paper investigates the feasibility of all-electric aircraft based on currently available technology. Electric propulsion would require the development of high power density electric propulsion motors, generators, power management and distribution systems. The requirements in terms of weight and volume of these components cannot be achieved with conventional technologies; however, the use of superconductors associated with hydrogen-based power plants makes possible the design of a reasonably light power system and would therefore enable the development of all-electric aero-vehicles. A system sizing has been performed both for actuators and for primary propulsion. Many advantages would come from electrical propulsion such as better controllability of the propulsion, higher efficiency, higher availability and less maintenance needs. Superconducting machines may very well be the enabling technology for all-electric aircraft development.

AC losses in a finite Z stack using an anisotropic homogeneous-medium approximation

Abstract: A finite stack of thin superconducting tapes, all carrying a fixed current I, can be approximated by an anisotropic superconducting bar with critical current density Jc = Ic/2aD, where Ic is the critical current of each tape, 2a is the tape width, and D is the tape-to-tape periodicity. The current density J must obey the constraint , where the tapes lie parallel to the x axis and are stacked along the z axis. We suppose that Jc is independent of field (Bean approximation) and look for a solution to the critical state for arbitrary height 2b of the stack. For c<|x|

Microstructures and critical current densities of YBCO films containing structure-controlled BaZrO3 nanorods

Abstract: BaZrO3 nanorods are known to be effective pinning centers as c-axis-correlated pinning centers. Furthermore, BaZrO3 nanorods in REBa2Cu3Oy (RE: rare-earth element) films are formed by self-assembled stacking of BaZrO3 using a target mixture of a superconductor and BaZrO3 for pulsed-laser deposition, which is a very easy fabrication technique. The density of BaZrO3 nanorods in YBa2Cu3Oy (YBCO) films can be controlled by varying the BaZrO3 content in a target. The BaZrO3 addition has two functions for superconductivity; one is the improvement of pinning forces due to the addition of pinning centers and the other is Tc degradation. The optimum BaZrO3 addition for Jc improvement in magnetic fields is found to be around 3 wt% because of a trade-off between the two functions described above. Furthermore, the length of BaZrO3 nanorods is found to be controlled using two types of target: pure YBCO and a mixture of YBCO and BaZrO3. Varying the BaZrO3 nanorod length has an effect on the pinning mechanism. In particular, magnetic field angle dependences of Jc are varied from c-axis-correlated pinning to nearly random pinning by changing the nanorod length. The magnetic field at the crossover of the pinning mechanism seems to be adjusted by the BaZrO3 nanorod length.

Measuring transport AC losses in YBCO-coated conductor coils

Abstract: Several AC power applications of YBCO-coated conductors (CCs) involve superconducting tapes wound in coils. In such a configuration the superconducting tape is arranged as closely packed turns, which strongly interact due to the generated magnetic field. This has a strong influence on the AC losses, which are different from those of an isolated tape, and need to be precisely quantified in order to predict and reduce the refrigeration requirements of applications. In this paper we experimentally evaluate the transport AC losses in a pancake coil composed of 25 turns of superconducting tape. We describe in detail the measuring technique utilized, pointing out the issues in this kind of measurement. We also present preliminary results of AC loss computation by finite-element modelling.

Grain boundaries and pinning in bulk MgB2

Abstract: We report the grain size dependence of critical current and grain boundary pinning in bulk MgB2. By combining polarized optical microscopy and electron backscatter diffraction, we obtain evidence of special grain boundaries with a high density of dislocations that are able to provide high critical current in MgB2 polycrystals. We argue that reduction of grain size to the nanoscale level is sufficient to provide the critical current densities required for large-scale applications at the boiling temperature of liquid hydrogen.

Study of MgO formation and structural defects in in?situ processed MgB2/Fe wires

Abstract: We fabricated MgB2/Fe wire by a powder-in-tube (PIT) technique, using an in situ process. All wire samples were sintered for 30 min at different sintering temperatures ranging from 650 to 1000 °C. We found strong correlations among crystallinity, critical current density (Jc), irreversibility field (Hirr), upper critical field (Hc2), and microstructures for all MgB2/Fe wires. We observed that the sample with the lowest sintering temperature, ~ 650 °C, had a larger lattice strain, Jc, change in resistivity Δρ(ρ300 K–ρ40 K), Hirr, and Hc2, but a lower density and residual resistivity ratio (RRR). Based on the relationships between all these superconducting and microstructure parameters, grain boundaries are likely to be acting as the predominant pinning centers for MgB2, so grain growth of MgB2 corresponds to a reduction of effective pinning. It should be noted that changes in the MgO fraction within the MgB2 matrix were almost independent of the sintering temperature. This indicates that most MgO may be coming from the starting material.

SQUID-based instrumentation for ultralow-field MRI

Abstract: Magnetic resonance imaging at ultralow fields (ULF MRI) is a promising new imaging method that uses SQUID sensors to measure the spatially encoded precession of pre-polarized nuclear spin populations at a microtesla-range measurement field. In this work, a seven-channel SQUID system designed for simultaneous 3D ULF MRI and magnetoencephalography (MEG) is described. The system includes seven second-order SQUID gradiometers characterized by magnetic field resolutions of 1.2–2.8 fT Hz−1/2. It is also equipped with five sets of coils for 3D Fourier imaging with pre-polarization. Essential technical details of the design are discussed. The system's ULF MRI performance is demonstrated by multi-channel 3D images of a preserved sheep brain acquired at 46 µT measurement field with pre-polarization at 40 mT. The imaging resolution is 2.5 mm × 2.5 mm × 5 mm. The ULF MRI images are compared to images of the same brain acquired using conventional high-field MRI. Different ways to improve imaging SNR are discussed.

Current distribution and ac loss for a superconducting rectangular strip with in-phase alternating current and applied field

Abstract: The case of ac transport at in-phase alternating applied magnetic fields for a superconducting rectangular strip with finite thickness is investigated. The applied magnetic field is considered to be perpendicular to the current flow. We present numerical calculations assuming the critical-state model of the current distribution and ac loss for various values of aspect ratio, transport current and applied field amplitude. A rich phenomenology is obtained due to the highly nonlinear nature of the critical state. We perform a detailed comparison with the analytical limits and we discuss their applicability for the actual geometry of superconducting conductors. A dissipation factor is defined, which allows a more detailed analysis of the ac behaviour than the ac loss. Finally, we measure the ac loss and compare it with the calculations, showing a significant qualitative and quantitative agreement without any fitting parameters.

A new method of determining the critical state in superconductors

Abstract: A new numerical method for solving the critical state, based on the force?displacement curve of the flux lines, is described. The equation can be expressed in terms of the vector potential and can be solved by commercial finite element programs. It gives the critical state directly as a first approximation but a flux flow resistivity can easily be added. It avoids some of the numerical problems that occur if an E?J curve of the form is used with large values of n. It is particularly advantageous for problems involving trapped flux where a power law leads to decay at a rate dependent on the power assumed and the very low electric fields mean that the E?J curve is probably exponential. The flux flow resistivity can be included and since the relevant power law is 1/n rather than n the solutions are more stable. Multiphysics packages, such as FlexPDE, which allow simultaneous equations to be solved, can find the temperature distribution by adding the heat flow equation.

The effects of sintering temperature on superconductivity in MgB2/Fe wires

Abstract: We studied the effects of sintering temperature on the phase transformation, lattice parameters, full width at half-maximum (FWHM), strain, critical temperature (Tc), critical current density (Jc) and resistivity (ρ) in MgB2/Fe wires. All samples were fabricated by the in situ powder-in-tube method (PIT) and sintered within a temperature range of 650–900 °C. It was observed that wires sintered at low temperature, 650 °C, resulted in higher Jc up to 12 T and lower Tc. The best transport Jc value reached 4200 A cm−2 at 4.2 K and 10 T. This is related to the grain boundary pinning due to small grain size. On the other hand, wires sintered at 900 °C had a lower Jc in combination with a higher Tc.

Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields

Abstract: We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt (1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor (the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n.

Insertion of nanoparticulate artificial pinning centres in YBa 2Cu3O7-x films by laser ablation of a Y 2O3-surface modified target

Abstract: YBa2Cu3O7?x+Y2O3 (YBCO+Y2O3) mixed films were prepared on SrTiO3/MgO substrates by pulsed-laser deposition from a YBCO target with a thin Y2O3 sector on the top. The pinning properties for the mixed YBCO+Y2O3 thin film were strongly enhanced, especially at the temperatures lower than 77?K: the maximum global pinning forces FP for the Y2O3 doped sample are 7.8?GN?m?3 near 2?T at 77?K, 54.5?GN?m?3 near 4?T at 65?K, and 189?GN?m?3 near 9?T at 40?K. In the Y2O3-added samples, except for the plane configuration, the angular dependence of Jc showed a plateau, due to the flux pinning by isotropic pinning centres. Y2O3 nanoparticles randomly dispersed inside the YBCO matrix were consistently observed in the cross-sectional TEM images.

Finite-element modelling of YBCO fault current limiter with temperature dependent parameters

Abstract: In this paper, we present a numerical model which takes into account both the thermal and the electromagnetic aspects of the over-critical current regime for high-temperature superconducting (HTS) materials. The electromagnetic and thermal equations have been implemented in finite-element method (FEM) software in order to obtain a novel model for investigating the behaviour of the materials when the current exceeds Ic and the superconductor material goes to the normal state. The thermal dependence of the electrical parameters, such as the critical current density Jc, has been introduced. This model has been used to analyse the behaviour of strip lines of a YBCO/Au fault current limiter (FCL) on a sapphire substrate. Simulations with currents exceeding Ic have been performed, showing that the current limitation phase can be correctly reproduced. Such a model can be used to study the influence of the geometry on the performance of the FCL. It can replace experiments with currents exceeding Ic which may damage or destroy HTS samples and devices.

Introduction of c-axis-correlated 1D pinning centers and vortex Bose glass in Ba?Nb?O-doped ErBa2Cu3Oy films

Abstract: We report the effectiveness of barium niobate as a needle-type vortex pinning center, as well as a change in the vortex state in Ba–Nb–O (BNO)-doped ErBa2Cu3Oy thin films. BNO doping was effective for improving critical current properties for a magnetic field direction parallel to the c axis of a matrix film and induced the generation of columnar defects with diameters of 5–25 nm. The irreversibility lines were also changed by the BNO doping and were similar to those of heavy-ion-irradiated YBa2Cu3Oy crystals in which the vortex Bose glass state was induced. It is suggested that the characteristic field relevant to crossover from the Bose glass to collective pinned glass depends on the BNO doping level. There is a possibility that Ba-based perovskite-type oxides promote the formation of nanorods universally and these nanorods play the role of one-dimensional c-axis-correlated pinning centers.

Pinning size effects in critical currents of superconducting films

Abstract: The thickness dependence of the critical current density Jc(d) in films due to the two-dimensional–three-dimensional (2D–3D) pinning crossover at low magnetic fields is addressed, taking into account the spatial correlation of pinning centres, the combined effect of bulk and surface pinning, and the effect of thermal fluctuations. The whole Jc(d) curve for both the 3D and 2D pinning regimes, and the crossover thickness dc are calculated using a dynamic approach for a random pinning potential characterized by the Gaussian correlation functions. It is shown that the spatial correlation of pinning centres can significantly increase dc as compared to uncorrelated point pins, and the competition between pinning and thermal fluctuations gives rise to a nonmonotonic dependence of Jc(d). The account of multiscale spatial correlations in a uniform pinning nanostructure can result in behaviour of Jc(d) similar to that observed on YBa2Cu3O7−δ. The ultimate limit of Jc is discussed.

Enhanced high-field performance in PLD films fabricated by ablation of YSZ-added YBa2Cu3O7−x target

Abstract: YBa2Cu3O7?x+Y2O3-stabilized ZrO2 (YBCO+YSZ) mixed films were prepared on SrTiO3/MgO substrates by pulsed-laser deposition from a YBCO target with a thin YSZ sector on top. The Jc at 77?K for the mixed YBCO+YSZ thin film was enhanced. The Jc at 77?K for the pure film and the mixed YBCO+YSZ thin film were 1.46 and 1.95?MA?cm?2 (self field) (and 0.09 and 0.28?MA?cm?2 (, B = 5?T)), respectively, even when both films were prepared using the same deposition conditions. The maximum pinning force of a doped film (77?K, ) was 15.9?GN?m?3, which is comparable to the value reached by NbTi at 4.2?K. In the YSZ-added sample, the angular dependence of Jc showed a broad peak at due to the flux pinning by the c-axis correlated defects. Elongated nanorods grown parallel to the c axis were consistently observed in the cross-sectional TEM images.

Systematic study of a MgB2 + C4H6O5 superconductor prepared by the chemical solution route

Abstract: We evaluated the additive effects of malic acid (C4H6O5), from 0 to 30 wt% of the total MgB2, on the lattice parameters, lattice strain, amount of carbon (C) substitution, microstructures, weight fraction of MgO, critical temperature (Tc), critical current density (Jc), and irreversibility field (Hirr) of a MgB2 superconductor. The calculated lattice parameters show a large decrease in the a-axis lattice parameter for MgB2+C4H6O5 samples from 3.0861(6) to 3.0736(1) A, with even a 10 wt% addition. This is an indication of C substitution into boron sites, with the C coming from C4H6O5, resulting in enhancement of Jc and Hirr. Specifically, the Hirr of the MgB2+C4H6O5 samples prepared by the chemical solution route reached around 7 T at 20 K, with a Tc reduction of only 1.5 K. In addition, the self-field Jc of the MgB2+C4H6O5 samples was only slightly reduced at an additive level as high as 30 wt%. However, residual oxygen after evaporation processing contributed to a large amount of MgO in our MgB2+30 wt% C4H6O5 samples. These problems can be further controlled by the amount of C4H6O5 additive or different evaporation temperatures.

Correlated enhancement of Hc2 and Jc in carbon nanotube doped MgB2

Abstract: The use of MgB2 in superconducting applications still awaits the development of a MgB2-based material where current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc, is in the range 2.5–10 at.% depending on field and temperature. Record values of the upper critical field, Hc2(4 K) = 41.9 T (with extrapolated Hc2(0)≈44.4 T), are reached in a bulk sample with 10 at.% DWCNT content. The measured Hc2 versus T dependences for all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich and co-workers.

Enhancement in the high-Tc phase of BSCCO superconductors by Nb addition

Abstract: Pure and Nb2O5 added Bi1.6Pb0.4NbxSr2Ca2Cu3O? superconductors (x varies up to 0.30) were synthesized by the solid state reaction method. Structural and superconducting properties of the samples were investigated. The fraction of the high-Tc phase (2223) increases with Nb addition up to x = 0.10, for which a nearly single 2223 phase was obtained. The critical temperature of the samples increases from 96 to 104?K with doping up to x = 0.20 and a further increase in Nb decreases the Tc sharply. Similarly, there is a considerable enhancement in the critical current density (jc) of the samples in the same doping range. Both AC susceptibility and transport measurements showed that the optimum Nb concentration is x = 0.20. The out-of-phase component of the AC susceptibility measurements showed that inter-grain coupling was enhanced up to the same doping level. Nb additions above x = 0.30 decrease the fraction of the high-Tc phase and degrade the superconducting properties. Present measurements indicate that x = 0.20 is the optimum Nb concentration in the BSCCO superconductor to enhance phase purity, inter-grain coupling and superconducting parameters (Tc and jc).

Development of a laser scribing process of coated conductors for the reduction of AC losses

Abstract: Expectations are high for using coated conductors for electric power applications not only because of better cost performance but also higher Jc–B properties compared with Bi2Sr2Ca2Cu3O10 (BSCCO) tapes. Furthermore, the coated conductors could also reduce AC losses by the use of various methods in post-treatment. When a YAG laser was used for scribing the superconductive layer, an AC loss reduction due to a decrease in the width of the superconductivity layer could be confirmed in short samples. In the case of AC applications using long tapes, high resistance between scribed filaments is necessary. However, the resistance between the filaments scribed by a laser was as low as 10−3 Ω cm−1. The reason for the low resistance was the existence of dross in the filament spacings, which was confirmed by a cross-sectional compositional mapping observation. Although the superconductivity layer was divided by the laser, the dross of the metal substrate made a bridge over the superconductivity filaments. Post-annealing in an O2 atmosphere to oxidize the dross was carried out in order to increase the resistance. Consequently, the resistance between the filaments was improved to be as high as 50 Ω cm−1. We succeeded in controlling the resistance value between filaments in a range five orders of magnitude higher than that of the as-scribed YBCO film, through oxygen atmosphere post-annealing.

The excellent superconducting properties of in situ powder-in-tube processed MgB2 tapes with both ethyltoluene and SiC powder added

Abstract: We found that the addition of ethyltoluene to the starting powder of in situ processed powder-in-tube (PIT) MgB2/Fe tapes is more effective in enhancing Jc values than other hydrocarbon additions such as benzene, naphthalene and thiophene, in spite of the smaller amount of carbon substitution for the boron site. This suggests that the dominant mechanism of Jc enhancement for ethyltoluene-added tape is different from carbon substitution for boron. The addition of both ethyltoluene and SiC nanopowder to the starting powder is much more effective in increasing Jc values. This is because both mechanisms of Jc improvement—one comes from the addition of ethyltoluene and the other comes from the carbon substitution for boron by the SiC addition—work together. The highest Jc values at 4.2 K reached 320 A mm−2 in 10 T and 140 A mm−2 in 12 T for 10 mol% ethyltoluene and 10 mol% SiC-added tape.

Cryocooled wideband digital channelizing radio-frequency receiver based on low-pass ADC

Abstract: We have demonstrated a digital receiver performing direct digitization of radio-frequency signals over a wide frequency range from kilohertz to gigahertz. The complete system, consisting of a cryopackaged superconductor all-digital receiver (ADR) chip followed by room-temperature interface electronics and a field programmable gate array (FPGA) based post-processing module, has been developed. The ADR chip comprises a low-pass analog-to-digital converter (ADC) delta modulator with phase modulation–demodulation architecture together with digital in-phase and quadrature mixer and a pair of digital decimation filters. The chip is fabricated using a 4. 5k A cm −2 process and is cryopackaged using a commercial-off-the-shelf cryocooler. Experimental results in HF, VHF, UHF and L bands and their analysis, proving consistent operation of the cryopackaged ADR chip up to 24.32 GHz clock frequency, are presented and discussed.

Assessment of conductor degradation in the ITER CS insert coil and implications for the ITER conductors

Abstract: Nb3Sn cable in conduit-type conductors were expected to provide an efficient way of achieving large conductor currents at high field (up to 13 T) combined with good stability to electromagnetic disturbances due to the extensive helium contact area with the strands. Although ITER model coils successfully reached their design performance (Kato et al 2001 Fusion Eng. Des. 56/57 59–70), initial indications (Mitchell 2003 Fusion Eng. Des. 66–68 971–94) that there were unexplained performance shortfalls have been confirmed. Recent conductor tests (Pasztor et al 2004 IEEE Trans. Appl. Supercond. 14 1527–30) and modelling work (Mitchell 2005 Supercond. Sci. Technol. 18 396–404) suggest that the shortfalls are due to a combination of strand bending and filament fracture under the transverse magnetic loads. Using the new model, the extensive database from the ITER CS insert coil has been reassessed. A parametric fit based on a loss of filament area and n (the exponent of the power-law fit to the electric field) combined with a more rigorous consideration of the conductor field gradient has enabled the coil behaviour to be explained much more consistently than in earlier assessments, now fitting the Nb3Sn strain scaling laws when used with measurements of the conductor operating strain, including conditions when the insert coil current (and hence operating strain) were reversed. The coil superconducting performance also shows a fatigue-type behaviour consistent with recent measurements on conductor samples (Martovetsky et al 2005 IEEE Trans. Appl. Supercond. 15 1367–70). The ITER conductor design has already been modified compared to the CS insert, to increase the margin and provide increased resistance to the degradation, by using a steel jacket to provide thermal pre-compression to reduce tensile strain levels, reducing the void fraction from 36% to 33% and increasing the non-copper material by 25%. Test results are not yet available for the new design and performance predictions at present rely on models with limited verification.

Comparison between nano-diamond and carbon nanotube doping effects on critical current density and flux pinning in MgB2

Abstract: Doping effects of nano-diamond and carbon nanotubes (CNTs) on critical current density of bulk MgB2 have been studied. CNTs are found prone to be doped into the MgB2 lattice whereas nano-diamond tends to form second-phase inclusions in the MgB2 matrix, leading to a more significant improvement of Jc(H) by doping by nano-diamond than by CNTs in MgB2. TEM reveals tightly packed MgB2 nanograins (50–100 nm) with a dense distribution of diamond nanoparticles (10–20 nm) inside MgB2 grains in nano-diamond-doped samples. Such a unique microstructure leads to a flux pinning behaviour different from that in CNTs-doped MgB2.