Since a superconductor has no resistance below a certain temperature and can therefore save a large amount of energy dissipated, it is a 'green' material by saving energy loss and hence reducing carbon emissions. Recently the massive manufacture of high-temperature superconducting (HTS) materials has enabled superconductivity to become a preferred candidate to help generation and transportation of cleaner energy. One of the most promising applications of superconductors is Superconducting Magnetic Energy Storage (SMES) systems, which are becoming the enabling engine for improving the capacity, efficiency, and reliability of the electric system. SMES systems store energy in the magnetic field created by the flow of direct current in a superconducting coil. SMES systems have many advantages compared to other energy storage systems: high cyclic efficiency, fast response time, deep discharge and recharge ability, and a good balance between power density and energy density. Based on these advantages, SMES systems will play an indispensable role in improving power qualities, integrating renewable energy sources and energizing transportation systems. This thesis describes an intensive study of superconducting pancake coils wound using second-generation(2G) HTS materials and their application in SMES systems. The specific contribution of this thesis includes an innovative design of the SMES system, an easily calculated, but theoretically advanced numerical model to analyse the system, extensive experiments to validate the design and model, and a complete demonstration experiment of the prototype SMES system. This thesis begins with literature review which includes the introduction of the background theory of superconductivity and development of SMES systems. Following the literature review is the theoretical work. A prototype SMES system design, which provides the maximum stored energy for a particular length of conductors, has been investigated. Furthermore, a new numerical model, which can predict all necessary operation parameters, including the critical current and AC losses of the system, is presented. This model has been extended to analyse superconducting coils in different situations as well. To validate the theoretical design and model, several superconducting coils, which are essential parts of the prototype SMES system, together with an experimental measurement set-up have been built. The coils have been energized to test their energy storage capability. The operation parameters including the critical current and AC losses have been measured. The results are consistent with the theoretical predictions. Finally the control system is developed and studied. A power electronics control circuit of the prototype SMES system has been designed and simulated. This control circuit can energize or discharge the SMES system dynamically and robustly. During a voltage sag compensation experiment, this SMES prototype monitored the power system and successfully compensated the voltage sag when required. By investigating the process of building a complete system from the initial design to the final experiment, the concept of a prototype SMES system using newly available 2G HTS tapes was validated. This prototype SMES system is the first step towards the implementation of future indsutrial SMES systems with bigger capacities, and the knowledge obtained through this research provides a comprehensive overview of the design of complete SMES systems.

While magnesium diboride (MgB2) was first synthesized in the 1950s, MgB2’s superconductive properties were not discovered until 2001. It has the highest superconducting transition temperature of all the metallic superconductors at ~39 K at atmospheric pressure. MgB2 is also unique in that it has a two superconductive gaps, a pi gap at 2 meV and a sigma gap at 7.1 meV. There are a theoretical models discussing the inter- and intra- gap scattering of the superconductivity of MgB2 and the Josephson transport of MgB2 Josephson Junctions. The focus of this work is to further the study of all-MgB2 Josephson junctions and quantum interference device technology. This work discusses the transport in all-MgB2 Josephson junctions and designing, fabricating, and measuring multi-junction devices. The junctions studied include all-MgB2 sandwich-type Josephson junctions (one with TiB2 normal conducting barrier and another with an MgO insulating barrier). The junction MgB2 films were deposited by hyprid physical-vapor deposition and the junction barrier were deposited by sputtering. The junctions were patterned and etched with UV photolithography and argon ion milling. With the TiB2 barrier we studied Josephson transport by the proximity effect. With these junctions, we also observed complete suppression of the critical current by an applied magnetic field showing for the first time a leakage free barrier in an all-MgB2 Josephson junction with a single ultrathin barrier. We also studied junctions utilizing MgO barrier deposited by reactive sputtering which gave a larger characteristic voltage of 1-3 mV compared to TiB2 barriers. By connecting several SQUIDs with varying loop areas we developed of two types of superconducting quantum interference filters (SQIFs). The first SQIF designed with 21 SQUIDs connected in parallel and the SQUID loops are sensitive to magnetic fields applied parallel to the substrate. The SQUID loop areas were designed to vary in such a way that the voltage modulation gave a unique peak corresponding to the absolute value of the applied magnetic field. The SQIF shows an antipeak height of 0.25 mV with a transfer function of 16 V/T at 3 K. The lowest noise measured for this SQIF is 110 pT/Hz1/2. The second SQIF is designed with 17 SQUIDs in parallel and the SQUID loops are sensitive to magnetic field perpendicular to the substrate. This SQIF has shown improved voltage modulation with a peak height of 1 mV and a transfer function of 7800 V/T. The noise sensitivity was measured at 70 pT/Hz1/2. The sensitivity of the SQIF shows MgB2 potential superconductor to improve performance of current superconductive electronics. Utilizing known all-MgB2 junctions and SQUID parameters two rapid single flux quantum (RSFQ) circuits were designed and tested. A toggle flip flop (TFF) operating as a frequency divider was developed. The TFF design consisted of a Josephson transmission line, a splitter, and an interferometer (a DC SQUID). The TFF utilized an improved designed, compared to previous all-MgB2 TFFs, and showed operation up to 335 GHz at 7 K and operation up to 30 K. A low frequency set-reset flip flop (SRFF) was also developed to demonstrate RSFQ digital logic. The SRFF design includes a DC-SFQ converter, a Josephson transmission line, and an inductively coupled readout SQUID. The SRFF demonstrates proper digital logic by toggling between a high and low voltage state with a sequential set and reset input. While these developed devices are not close to the potential that MgB2 allows, they do show the promise MgB2 based devices have in making more sensitive and faster superconductive logic devices. / Physics

In this thesis, superconducting nanostructures for quantum detection of electromagnetic radiation are studied. In this regard, electrodynamics of topological excitations in 1D superconducting nanowires and 2D superconducting nanostrips is investigated. Topological excitations in superconducting nanowires and nanostrips lead to crucial deviation from the bulk properties. In 1D superconductors, topological excitations are phase slippages of the order parameter in which the magnitude of the order parameter locally drops to zero and the phase jumps by integer multiple of 2\pi. We investigate the effect of high-frequency field on 1D superconducting nanowires and derive the complex conductivity. Our study reveals that the rate of the quantum phase slips (QPSs) is exponentially enhanced under high-frequency irradiation. Based on this finding, we propose an energy-resolving terahertz radiation detector using superconducting nanowires. In superconducting nanostrips, topological fluctuations are the magnetic vortices. The motion of magnetic vortices result in dissipative processes that limit the efficiency of devices using superconducting nanostrips. It will be shown that in a multi-layer structure, the potential barrier for vortices to penetrate inside the structure is elevated. This results in significant reduction in dissipative process. In superconducting nanowire single photon detectors (SNSPDs), vortex motion results in dark counts and reduction of the critical current which results in low efficiency in these detectors. Based on this finding, we show that a multi-layer SNSPD is capable of approaching characteristics of an ideal single photon detector in terms of the dark count and quantum efficiency. It is shown that in a multi-layer SNSPD the photon coupling efficiency is dramatically enhanced due to the increase in the optical path of the incident photon.

No / A new dual-log spiral geometry is proposed for microstrip resonators, offering substantial advantages in performance and size reduction at subgigahertz frequencies when realized in superconducting materials. The spiral is logarithmic in line spacing and width such that the width of the spiral line increases smoothly with the increase of the current density, reaching its maximum where the current density is maximum (in its center for ¿/2 resonators). Preliminary results of such a logarithmic ten-turn (2 × 5 turns) spiral, realized with double-sided YBCO thin film, showed a Q.-factor seven times higher than that of a single ten-turn uniform spiral made of YBCO thin film and 64 times higher than a copper counterpart. The insertion loss of the YBCO dual log-spiral has a high degree of independence of the input power in comparison with a uniform Archimedian spiral, increasing by only 2.5% for a 30-dBm increase of the input power, compared with nearly 31% for the uniform spiral. A simple approximate method, developed for prediction of the resonant frequency of the new resonators, shows a good agreement with the test results.

The transport critical current density (J(_c)) of a 37 filament Bi-2223/Ag tape has been measured as a function of field and temperature from 4.2 K up to 90 K. Data have been obtained over a large current range from 10 mA up to 100 A and in fields up to 23 T with the tape in 3 orientations with respect to field. These comprehensive data have been used to test the predictions of the flux creep and weak link models used to explain J(_c) in Bi-2223 tapes. The J(_c)(B,T) dependence of optimised Bi-2223 tapes has been calculated using a curved film model. The model assumes perfect grain connectivity and that the local superconducting properties are equivalent to those in the best reported thin films. A comparison between the calculations and measured J(_c)(B,T) dependencies suggest that in high fields at 20 K, J(_c) in presently available industrially processed tapes is only a factor of 8 below the performance of ideal fully optimised tapes. Transport measurements have been made on Bi-2223 single filaments extracted from an alloy sheathed multifilamentary tape in liquid nitrogen at 77 K in fields up to 300 mT with the field aligned parallel and perpendicular to the a-b planes. Further Jc(B,T) data have been taken in a variable temperature insert at temperatures between 60 to 90 K in fields up to 15 T. In a study of the electric field-current density {E-J) characteristics of the c-axis orientated data at 77 K, negative curvature is observed in traces below 280 mT. However, the 280 mT trace exhibits both positive and negative curvature in different current regimes in contrast to the predictions of standard theory. A laboratory scale Bi-2223 superconducting magnet producing a maximum field of 1.29 T at 4.2 K has been designed and fabricated. The magnet comprises 6 resin impregnated double wound pancakes with a 40 mm bore fabricated via the react and wind route. Critical current density measurements have been made as a function of magnetic field, angle and strain at 4.2 K and 77 K on short samples of the constituent tape. The E-J characteristics of all component coils have been measured and a comparison with short sample data shows that minimal additional damage occurred beyond that produced by the bending strain on the tape and the long length variation in J(_c). Sufficient detail is provided for the non-specialist to assess the potential use of brittle superconducting tapes for magnet technology and construct a laboratory scale magnet.

La supraconductivité induite par effet de proximité dans du graphène CVD macroscopique décoré de nanoparticules d'étain (G/Sn) est le thème central de cette thèse. Dans ce manuscrit, deux projets expérimentaux sont présentés. Le premier de ces projets a consisté à développer et étudier un nouveau matériau manipulable et flexible au sein duquel les corrélations supraconductrices s'étendent à des échelles macroscopiques. Ce matériau est un film composite qui résulte de l'empilement de trois ingrédients originaux: un film fin de parylène d'une épaisseur de quelques micromètres, une monocouche de graphène de plusieurs centimètres carrés et un film discontinu métallique obtenu par démouillage naturel de l'étain en surface du graphène.Des mesures de transport à basses températures ont permis de mettre en évidence l'émergence d'un effet de supraconductivité induite à l'échelle macroscopique. Le courant critique de la transition supraconductrice des films composites a pu être contrôlé par une tension de grille avec une sensibilité de cent nanoampères par volt. Le comportement sous champ magnétique transverse des films composites est similaire à celui des supraconducteurs comportant des joints de grains et est caractérisé, en particulier sous faible champ magnétique, par une forte sensibilité de la transition supraconductrice. Une étude approfondie a enfin montré que le matériau hybride G/Sn est un système percolant bidimensionnel qui se comporte, à l'approche de la transition supraconductrice, comme une jonction unique de taille micrométrique.Dans le cadre du second projet, des cavités supraconductrices radiofréquences (RF) ont été développées. Des pistes G/Sn ont été intégrées à ces circuits supraconducteurs dans l'optique de créer des résonateurs dont la fréquence de résonance est contrôlable par une tension de grille. Un procédé d'intégration de pistes G/Sn conjointement à plusieurs cavités à partir d'un seul transfert (pleine plaque) de graphène a été mis au point. Des mesures en transmission dans des conditions cryogéniques ont été effectuées pour caractériser ces dispositifs hybrides et étudier leur comportement dans le domaine RF. / Superconducting proximity effect in macroscopic CVD-grown graphene decorated by tin nanoparticles is the central topic of this thesis. In this manuscript, two experimental projects are presented.The first of these projects consisted in developing and studying a new easy to handle and flexible material in which superconducting correlations extend over macroscopic scales. This material is a composite film made from the stacking of three original components: a few micrometers thin plastic film in parylene, a several centimeters squares layer of graphene, and a cluster of nanoparticles achieved by natural dewetting of tin on the graphene surface.Cryogenic transport measurements highlighted an induced superconductivity on the scale of the studied composite films pieces dimensions, of the order of the centimetre square. The superconducting critical current of the composite films showed gate tunability of about one hundred nanoamperes by volt. The behaviour of the composite films under a transverse magnetic field is similar to the one of granular superconductors and is characterised, especially under weak transverse magnetic field, by a high sensitivity of the superconducting transition. An in-depth study showed that the G/Sn hybrid material is a bidimensional percolating system that, when approaching the superconducting transition, behaves like a single mesoscopic Josephson junction.Within the framework of the second project, superconducting radiofrequency (RF) cavities have been developed. G/Sn patches have been integrated into these superconducting circuits in order to build gate tunable resonators. A process allowing to integrate G/Sn patches jointly to a series of several cavities from a single graphene transfer have been developed. Transmission measurements in cryogenic conditions have been performed to characterise these hybrid devices and to study their radiofrequency response.

En chargeant en courant une inductance supraconductrice, on stock de l’énergie magnétique. Ce principe est appelé SMES pour Superconducting Magnetic Energy Storage. Ce type de dispositifs a une densité d’énergie relativement faible mais peut avoir une densité de puissance élevée. Cette thèse s’inscrit dans le cadre du projet BOSSE, qui vise à mettre au point un démonstrateur de SMES dans la gamme du MJ. Ce SMES sera à la fois plus compacte que ses prédécesseurs et battra le record actuel d’énergie spécifique d’un bobinage supraconducteur en atteignant 20 kJ/kg. Cet objectif sera atteint grâce à l’utilisation de supraconducteurs haute température critique de seconde génération, dits conducteurs « REBCO ».Cette thèse aborde de manière générale la problématique du design de SMES et propose des éléments de réflexion et des solutions pour un pré-design rapide du bobinage d’un SMES. Le design du SMES à haute densité d’énergie du projet BOSSE est détaillé.Des éléments modulaires (galettes de ruban REBCO) du SMES ont été fabriqués et testés en champ propre et sous champ magnétique externe. Les méthodes et les résultats de détection de transition des galettes de l’état supraconducteur vers l’état normal sont présentés. Ces détections ont permis de garantir l’intégrité des galettes REBCO lors de transitions, même à très forte densité de courant (980 A/mm2 dans le conducteur nu).Ce travail est soutenu par la DGA (Direction Générale de l’Armement). / Magnetic energy is stored when a superconducting inductance is fed with current. This principle is called SMES (Superconducting Magnetic Energy Storage). This kind of device has a relatively low energy density but can have a high power density. This PhD work has been conducted in the frame of the BOSSE project with the objective to develop a SMES demonstrator in the MJ range. This SMES will be especially compact and will reach a specific energy of 20 kJ/kg of winding, which is 50 % over the current world record for a superconducting coil. This performance is made possible by the use of 2nd generation high critical temperature superconductors, so-called “REBCO” conductors.This work tackles the general problematic of SMES design and proposes elements of reflection and solutions for fast pre-design of a SMES winding. The design of the high specific energy SMES of the BOSSE project is presented in detail.Modular elements (pancakes of REBCO tapes) of the SMES have been manufactured and tested in self-field and under background magnetic field. During these tests, transitions from superconducting state to normal state have been detected. These early detections have prevented the pancakes to be damaged when transitions occurred, even at very high current density (980 A/mm2 in the bare conductor). The measurement method is presented, as well as the results of the tests.The BOSSE project has been funded by the DGA (French Defence Procurement Agency).

No description available.

Superconducting single-photon detectors (SSPDs) are the leading detectors in terms of high-speed single-photon counting and high detection efficiency (DE). One factor that limits the DE is the critical current Ic, which is the maximum current before the superconductor switches to the normal state. Increasing device’s bias current towards the Ic can improve the DE. However, the device’s Ic is reduced due to constriction and current crowding at the edges of the wire. Typically, this is caused by fabrication defects. Locally suppressing superconductivity at these defects can potentially lessen the occurrence of current crowding. In this thesis, we used the beam from the helium ion microscope (HIM) and measured the Ic to observe the effects of locally irradiating specific areas on a SSPD wire. Due to the HIM’s small spot size and high collimation, we can control the superconducting gap precisely at the center and edges of the wire. Suppressing the edges can potentially reduce current crowding and increase the device’s critical current while suppressing the center can improve detection sensitivity for photons incident at that location. Our results showed that the irradiated devices had reduced Ic compared to unirradiated devices for both cases. We then extend this method of local suppression of superconductivity to explore an alternative method of fabricating SSPDs by directly writing the device on the superconducting thin film. This can enable the fabrication of devices without the use of lithography resist. In our experiment, we fabricated a 3 μm wire using optical lithography that was disconnected at the center and connected it by writing a single 1 μm wire with the He+ ion beam. We measured the Ic for samples with and without the 1 μm wire pattern and observed that the Ic decreased as we increased the ion dose. Overall, this work aims to contribute to the continuing investigation of the detection mechanism for SSPDs and the improvement of nanofabrication methods using the HIM.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / A process was developed to design and manufacture a dc SQUID magnetometer. Superconductor theory is given as a foundation to explain the Josephson junction. This knowledge is applied to explain the ideal and practical dc SQUID. The design of the dc SQUID is done with inductance calculation formulas. Each step of the manufacturing process is discussed in detail. Many improvements have been made to the process to make each step reproducible. The steps in the manufacturing process that were done in-house are: manufacturing an YBa2Cu3O7−d pellet, depositing the thin film with the pulsed laser deposition process, creating a mask with the UV lithography process, wet etching the circuit and depositing silver contact pads with thermal evaporation. The device is packaged on a printed circuit board device holder and shielded with a mu-metal shield. A test setup is developed to test the final device. Each step in the manufacturing process was successful. The final device did not work, due to oxidisation of the YBa2Cu3O7−d thin film.