Test of 6-kVA three-phase flux transfer-type current-limiting transformer

A 6-kVA three-phase model of the flux transfer-type current-limiting transformer was developed and tested. In this device, the winding loops of YBCO superconducting tapes couple magnetically two independent iron cores: the primary-side iron core and the secondary-side iron core. The former and the latter are equipped with copper primary and secondary windings, respectively. Because the magnetic fluxes linked to the superconducting winding loops must be kept constant, the magnetic flux is transferred by the superconducting YBCO loops between the two iron cores in order to couple magnetically the primary and secondary coils. While the YBCO loops are superconducting, 100% of the magnetic flux is transferred and the device shows the similar function as usual transformers. Once the YBCO loops become normal by a fault current in any of the windings, the power transfer between two iron cores is limited and the current in the secondary winding is limited naturally on a result of decoupling the iron cores

A Survey of Air-to-Ground Propagation Channel Modeling for Unmanned Aerial Vehicles

In recent years, there has been a dramatic increase in the use of unmanned aerial vehicles (UAVs), particularly for small UAVs, due to their affordable prices, ease of availability, and ease of operability. Existing and future applications of UAVs include remote surveillance and monitoring, relief operations, package delivery, and communication backhaul infrastructure. Additionally, UAVs are envisioned as an important component of 5G wireless technology and beyond. The unique application scenarios for UAVs necessitate accurate air-to-ground (AG) propagation channel models for designing and evaluating UAV communication links for control/non-payload as well as payload data transmissions. These AG propagation models have not been investigated in detail when compared to terrestrial propagation models. In this paper, a comprehensive survey is provided on available AG channel measurement campaigns, large and small scale fading channel models, their limitations, and future research directions for UAV communication scenarios

Socially and biologically inspired computing for self-organizing communications networks

The design and development of future communications networks call for a careful examination of biological and social systems. New technological developments like self-driving cars, wireless sensor networks, drones swarm, Internet of Things, Big Data, and Blockchain are promoting an integration process that will bring together all those technologies in a large-scale heterogeneous network. Most of the challenges related to these new developments cannot be faced using traditional approaches, and require to explore novel paradigms for building computational mechanisms that allow us to deal with the emergent complexity of these new applications. In this article, we show that it is possible to use biologically and socially inspired computing for designing and implementing self-organizing communication systems. We argue that an abstract analysis of biological and social phenomena can be made to develop computational models that provide a suitable conceptual framework for building new networking technologies: biologically inspired computing for achieving efficient and scalable networking under uncertain environments; socially inspired computing for increasing the capacity of a system for solving problems through collective actions. We aim to enhance the state-of-the-art of these approaches and encourage other researchers to use these models in their future work

Enhanced Pinning Properties of Laser-Irradiated Bulk MgB2 Superconductors

In order to investigate the effect of laser irradiation on the superconducting properties of MgB2, a pellet-shaped sample was produced and cut into two pieces. The dot-shaped molten patterns, with the distance of 100 µm between them, were formed on the surface of one of the pieces by using an Nd:YVO4 laser. The grains of the irradiated sample were significantly grown due to the laser-induced sintering. The modified surface appeared to have dense and compacted grains as a result of increasing interconnectivity among the superconducting grains. The critical current densities at 5 K in a field of 3 T were found to be 5.38 × 104 and 8.96 × 104 A cm?2 for the reference and the irradiated samples, respectively. The increment of critical current density for the irradiated sample is presumably connected with the changing of pinning mechanism after the laser irradiation. For the irradiated sample which showed a higher value of critical current density at 5 K under high fields, ?? pinning was obtained as a dominant pinning mechanism, while the normal point pinning was dominant for the reference one. These results point out that the superconducting properties of bulk MgB2 can be changed and improved via laser irradiation in a short time. © 2016, Springer Science+Business Media New York

Effect of laser irradiation on superconducting properties of bulk MgB2 sintered at different temperatures

The surfaces of MgB2 bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850 °C, after hot pressing at 200 °C, were irradiated with the same irradiation dose by using an Nd:YVO4 laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB2 superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650 °C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850 °C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB2 in different ways depending on sintering temperature of the superconductor. © 2017, Springer Science+Business Media New York

Effect of laser irradiation on activation energy, irreversibility field and upper critical magnetic field of bulk MgB2 superconductor

In this study, the role of laser irradiation on the upper critical field (Hc2), the irreversibility field (Hirr) and the activation energy (U0) of bulk MgB2 superconductor was first time investigated with the aid of magnetoresistivity measurements conducted at different applied magnetic fields in a range of 0–15 kOe as a function of temperature from 25 to 40 K. For this aim, a disk shaped MgB2 superconductor was produced and cut into two pieces. One of the pieces was irradiated under high vacuum conditions by using an Nd:YVO4 laser. The obtained results showed an increase in the critical temperature (Tc) by about 1 K after irradiation. Also, the values of Hc2(0) were respectively found to be around 143 and 151 kOe while the values of Hirr(0) were about 122 and 130 kOe for the reference and the irradiated samples. Using the thermally activated flux flow model the dependence of U0 on magnetic field was determined. The maximum U0 values of 0.43 and 0.83 eV in zero magnetic field were respectively obtained for the reference and the irradiated samples. Also, the magnetic field dependence of the critical current density Jc(B) values at different temperatures were slightly increased after the irradiation. These results point out that the superconducting properties of bulk MgB2 can be improved via laser irradiation. © 2016, Springer Science+Business Media New York

Effect of Er doping on the superconducting properties of porous MgB2

MgB2 bulk sample with porous structure was produced by using the in-situ solid-state reaction method under argon (Ar) atmosphere of 10 bar. Elemental Er in powder form was doped into MgB2 with different compositions (Mg1-xErx)B2, where x = 0.00, 0.03 and 0.05, in order to investigate the effect of rare-earth (RE) element Er on the structural and electromagnetic properties of porous MgB2. The Er-doped samples result in small grain size structure compared to the undoped one. The lattice constants a and c of the doped samples, determined from X-ray diffraction (XRD) analysis, increase with the increasing Er content, and consequently the superconducting transition temperature (Tonsetc) of MgB2, determined from resistivity measurements, is slightly suppressed. Also, the upper critical field (Bc2), the irreversibility field (Birr) and the critical current density (Jc) values are significantly enhanced in the doped samples. For the best sample (x = 0.03), at 15 K under a magnetic field of 4 T, the Jc value reaches 2.4×104 A cm-2, which is higher than that of the porous sample by an order of 103, and the Birr value at 20 K reaches 9.7 T. These results imply that the RE element Er fills the pores, enhances the density and the grain connectivity. Hence, the superconducting properties of the porous MgB2 sample improve by Er doping