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Fusion power plants offer the prospect of a new sustainable source of energy for future generations. The design and R&D of future reactors is expected to largely benefit from the experience gained in the design, construction and operation of ITER. However, deploying reliable fusion power plants, requires to overcome the design challenges and to address the remaining readiness gaps. Europe is starting the Conceptual Design Phase for building a superconducting DEMOnstration Fusion Power Plant (DEMO) and starting operations around the middle of the century. The aim is demonstrating the production of 500 MW of net electricity, with a closed tritium fuel cycle and adequate plant availability. For the design of the Superconducting Magnet System several variants of coils have been investigated in the pre-conceptual design phase, which has concluded in 2020. Some of them are very close to ITER design, therefore have a relatively high technology readiness level. The 'alternative' proposed solutions, that are very promising in terms of costs and performances, need a validation to industrial scale in order to be eligible for the down-selection expected in 2024. In particular, the validation regards the design of the layer-wound graded TF winding pack based on React-and-Wind Nb3Sn conductors, and the hybrid, layer-wound graded CS coil. The challenging aspect is that the CS magnet is made of REBCO conductors in high-field, React-and-Wind Nb3Sn conductors in medium field and NbTi conductors in low field. Also innovative techniques need to be studied for insulating critical parts of the coils, as penetrations, discontinuities and joints. The main issues of the technological development are illustrated. A R&D work plan is presented for the Conceptual Design phase to validate the technology within 2024 and demonstrate the operational capabilities by testing superconducting insert coils, that is about 50-m long wound conductors, by 2027

A higherature superconducting (HTS) cable-in-conduit conductor (CICC) suitable for high-field magnet applications and comprised of twisted-stacked coated-conductor tapes arranged around a helically slotted core has been recently proposed and tested, demonstrating full compatibility with existing cabling technologies. To form the desired shape of any coils for high-field magnet applications, any suitable CICC option needs to be bent. For a magnet design, it is then very important to characterize the bending behavior of the CICC and, in particular, to find the smallest bending radius achieved without performance degradation. To this aim, bending tests were carried out on 1-m-long dummy samples of five helical slots in an extruded aluminum core, in which four REBCO tapes and dummy stainless-steel tapes were mounted in each slot. A controlled bending moment has been applied to the HTS CICC samples at room temperature, and each individual superconducting tape has been electrically characterized as a function of bending radius by measuring the critical current and $n$-index values at 77 K in a self-field condition. Results are analyzed and explained with the help of a three-dimensional cable model implemented in ANSYS and analytical calculations. The experimental and numerical results presented in this paper will demonstrate that the twisted-stack slotted-core technology can meet the bending requirements of high-field magnet designs. © 2002-2011 IEEE.

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values Ic at 77 K (liquid N2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance (Rinter) has also been measured and the observed dependence of Rinter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA. © 2014 IEEE.

A detailed study on YBa(2)Cu(3)O(7-delta) (YBCO) films with BaZrO(3) (BZO) inclusions (YBCO-BZO) deposited on SrTiO(3) substrate with different BZO contents is presented. X ray diffraction, SEM imaging and superconductive critical temperature (T(c)) measurements were performed to evaluate film growth quality. Independently from the BZO content, all YBCO-BZO films showed T(c) values close to that of pure YBCO indicating that the T(c) reduction which is usually observed on YBCO-BZO films can be successfully recovered by changing the deposition conditions. All YBCO-BZO films showed an improved critical current density (J(c)) dependence on magnetic field if compared to the typical J(c)(H) of YBCO, resulting in higher irreversibility fields up to 9 T at 77 K. SEM observation on etched samples suggests a direct link between the defect density measured in YBCO-BZO samples and that calculated from the observed in field performances. The analysis of J(c)(B) as a function of applied magnetic field direction revealed that the pinning due to isotropic pinning centers typical for YBCO is coupled with a c-axis correlated pinning in YBCO-BZO which can be ascribed to BZO induced columnar defects. J(c)(B) recorded at several temperatures down to 10 K revealed matching effect features.

We perform measurements of high-frequency (similar to 48 GHz) microwave surface impedance with an applied magnetic field in YBa(2)Cu(3)O(7-delta) ( YBCO) laser-ablated films with various amounts of BaZrO(3) ( BZO) sub-micrometric inclusions, up to 7 mol % concentration. BZO inclusions are very effective in the reduction of the field-induced surface resistance in our experimentally accessible field range [ 0, 0.8] T. At temperatures low enough, the application of a moderate (similar to 0.2 T) field makes the YBCO/BZO films markedly less dissipative than pure YBCO. This result, examined in the light of the very high measuring frequency ( very small vortex oscillation amplitude) shows that BZO inclusions are even more effective pinning centers than columnar defects. We study the dependence of the vortex parameters ( vortex viscosity, pinning constant) on the BZO concentration. We examine the correlation between the reduction of the microwave dissipation and the areal density of BZO-induced defects. We argue that the very improved performances in a magnetic field are due to individual pinning of vortices on BZO inclusions.

Axial flux electrical machines (AFEM) with shaped disk structure increases the power-to-weight ratio using HTS materials. The introduction of permanent magnets on rotor disk and of superconducting coils on stator disk, instead of conventional field and armature winding, respectively, allows the development of lighter, more compact, and more performing AFEM. In this paper, we report the electromagnetic properties of an HTS coil in a double pancake configuration measured at LN2 temperature. Also, simulation studies by means of finite element and constant lumped models are reported. Commercial grade ReBa2Cu3O7-x (Re = Y or rare earths, ReBCO) tape and epoxy resin have been used for coil winding. The combination of experimental and simulation results allow ac losses, in the HTS coils used in an all-superconducting machine design, to be estimated.

In this paper, YBCO-based commercial wire (tape) has been used for fabricating coils in doubled pancake configuration assembly. The tape has been wound on a circular-shaped support; lap joints between coils have been realized with particular attention to resistance minimization. Each high-temperature superconducting coil by itself, and the assembly afterward, has been powered and the current-voltage curves at different temperatures have been measured using a four-probe technique. Critical currents have been computed by using the electric field criteria. Applied magnetic field was both in perpendicular and parallel configuration. A magneto-static finite element model has been developed by using commercial software, in order to check the agreement with experimental results. © 2013 IEEE.

MgO films are widely used as the first buffer layer in IBAD-YBCO coated conductors, whereas there are only a few examples of its use in RABiTS YBCO coated conductors. Nevertheless MgO, though challenging in terms of lattice parameter and epitaxy, can effectively passivate the metal substrate thus enabling the use of Ni-Cu-based alloy tapes. Such substrates develop a very sharp cube texture and at the same time can be safely handled when their thickness is as low as 40 μm. Epitaxial MgO films were deposited by e-beam evaporation at 400 °C on biaxially textured metal substrates. A transient Pd layer as thin as 10 nm was used to alleviate the lattice mismatch between MgO and the substrate and to enhance film adhesion. Due to the reduced diffusion of oxygen in MgO, this template is effective against substrate oxidation even in the extreme conditions typical of the YBCO growth. On such a template, La0.66Sr0.33MnO3 (LSMO) was epitaxially deposited by PLD. Moreover, BaZrO3 (BZO) film was successfully deposited by CSD on MgO single crystal and is expected to epitaxially grow also on MgO film.

We performed an extensive characterization of Co-doped polycrystalline 122 iron pnictide samples, Ba(Fe1-xCox)2As2 with x=0.07, 0.09, 0.1, 0.12, by means of high-field dc magnetization and ac 1st and 3rd harmonic susceptibility response measurements. From M(H) loops through the Bean model formula both the magnetic field and T dependences of the Jc are derived. From the 3rd harmonic response of the ac susceptibility as a function of the dc magnetic field up to 10 T recorded at different temperatures, we determined the irreversibility line, using the onset criterion. From these analyses, 7% and 9% of Co-doped samples showed the best Jc performances.

The interface between the cube textured Ni-W substrate, both bare and Pd-buffered, and the CeO2/YSZ buffer layer structure-required to growth YBCO coated conductors-samples have been subjected to different oxidizing conditions and have been investigated with different techniques: X-ray diffraction (XRD), X-ray absorption and Auger electron spectroscopy. Due to the Ni-Pd interdiffusion, the presence of a Pd over layer significantly modifies the substrate surface composition and the substrate oxidation mechanism. Depending on the Pd layer thickness, the formation of NiO can be completely prevented while the formation of ternary oxides and of W oxide is favored. Moreover, the Pd layer affects the extension of interface between the substrate and the CeO2 layer and the portion of uncontaminated CeO2.

This paper presents the realization of a prototypal embedded system which implements a new control law characterized by a fixed and quantized control set. This effort is motivated by the observation that many industrial plants with high parametric uncertainties, both for constructive simplicity and/or in order to minimize the operation cost, are usually commanded with signals that can only assume a finite number of values. This paper proposes a new multivalued control algorithm which is robust with respect to disturbances and plant's uncertain parameters and consents the plant's output to practically track a given reference trajectory, with preassigned maximum values of the tracking error and its first derivative. Moreover, this paper deals with the digital realization of this new multivalued control law and the key issues associated with its microprocessor implementation. The efficiencies of the methodology and of the design procedure utilized for the realization of the embedded system are shown through a very interesting application: a temperature control system for a ceramic kiln.

Ni48.5Cu48.5Co3 alloy was studied for the realization of cube-textured substrates for YBCO coated conductors. The small amount of Co was added in order to improve the microstructure as well as the mechanical properties of the binary Ni-Cu alloy. Structural, microstructural and morphological characterizations have been carried out in order to evaluate the suitability of this substrate as a template for YBCO coated conductor application. A cube area fraction as high as 97.7% was obtained after annealing for 30 minutes at 1050degC. Further, mechanical, magnetic and electrical properties have been studied and compared with those exhibited by other Ni-based alloys. The standard CeO2/YSZ/CeO2/YBCO multilayer architecture was deposited on Pd-buffered substrate by means of PLD and characterized in order to verify the suitability of this template for YBCO coated conductor application. The obtained YBCO film is adherent, well textured with an out-of-plane distribution of 4.1deg (FWHM) in rolling direction and good superconducting properties with a critical temperature of about 88 K and a critical current density of 1 MA/cm2. The obtained results demonstrate that this alloy tape is a good candidate for the production of YBCO-based coated conductors.

A new modified TFA-MOD method for the YBa2Cu3O7-x (YBCO) deposition, using only barium trifluoroacetate, is presented. The yttrium and copper trifluoroacetates were replaced by the alcoholic solutions of Cu and Y acetates dispersed in propionic acid to prepare a stable coating solution. Due to the smaller amount of evolved hydrofluoric acid, this method permits the shortening of the pyrolysis time by a factor 4, with respect to conventional trifluoroacetate metal organic deposition (TFA-MOD) method. The X-ray photoelectron spectroscopy (XPS) analyses, performed on a pyrolyzed sample, showed the complete decomposition of precursors. The as obtained films exhibit good morphological, structural and superconducting properties with T c(R = 0) greater than 90 K. Transport measurements at different temperatures and applied magnetic fields have revealed promising superconducting properties of the critical current density (J c) vs. the applied magnetic fields and the value of J c at 77 K and zero magnetic field greater than 3 MA/cm2.

Extensive applications of YBCO tapes claim for a comprehensive knowledge of their behavior in every possible operating condition. In particular, one of the main topics to explore is the tape stability against heat and current disturbances, in terms of minimum quench energy and normal zone propagation velocity (NZPV). In this work we present the results of NZPV measured in a 35 cm long YBCO coated conductors manufactured by SuperPower Inc. The tape is cooled by means of a cryocooler and kept under vacuum in order to achieve near-adiabatic conditions. Two independent heaters are positioned on the tape: one on the YBCO side and the other on the substrate side. The quench is monitored by recording the voltage and the temperature evolution along the tape. The NZPV, at 77 K, reaches a value larger than 2.5 cm/s for a bias current I=50 A which represents the 56% of the critical current value. The quench dynamics of the tape are discussed with the help of a simulation program based on the finite difference discretization of the tape with the Fourier's heat equation: the model is able to reproduce the experimental behavior.

The working features of 2nd generation high temperature superconducting tapes related to applications, such as their stability, are becoming of real interest as long tape lengths become available. In this work, we present results on normal zone propagation velocity obtained on a 40 cm long YBCO coated conductor, called 344 superconductors, manufactured by American Superconductor Corp. Although the tape is wound on a 40 mm diameter cylinder support, its transport features are unaffected compared to the straight tape. The quench is induced by an external heater, and the propagation is observed from the increasing voltages along the tape. In the range 75divide80 K, we observed a propagation velocity lower than 2 cm/s at current bias slightly below 80% of the critical current. Our investigation covers different bias current values as well as temperatures, and experimental analysis is also compared to an approximate numerical model.

The realization of high critical current density YBCO-based coated conductors in the RABiTS approach is primarily related to the sharpness of cube texture developed in the substrate. Among the various proposed Ni-based tapes, Ni-W alloys have attracted a particular interest either because of the enhanced mechanical properties and reduced magnetism with respect to pure Ni, or of the sharp and almost pure cube texture that can be induced. The only other remarkable component associated with cube orientation consists of its twins, whose formation is known to be primarily determined by the stacking fault energy, which is a characteristic of the material. Despite this fact, the obtained textures for a given material can significantly vary, suggesting that the deformation regime, as well as the initial bulk conditions, have a role in influencing twins formation. In this contribution, the effect of the strain conditions during mechanical deformation and initial bulk grain size on the final recrystallization texture is evaluated. EBSD and X-ray analyses evidence the influence of the initial grain size on the recrystallization texture, since sharper cube textures with reduced non-cube components develop in finer-grained starting material. Conversely the condition of plane strain deformation plays no role in determining the recrystallization texture.

The addition of artificially induced defects improves the pinning efficiency of magnetic flux vortices in YBa2Cu3O7-delta (YBCO) films. In YBCO films by pulsed laser ablation (PLD) technique this goal can be pursued by the introduction of epitaxial second phase nanoinclusions in the YBCO matrix. In the present work, the effect of BaZrO3 (BZO) inclusions in stoichiometric YBCO films on transport properties has been studied. Composite targets obtained by addition of BZO powder in molar percents ranging from 2.5 to 7 mol.% have been prepared with BZO mean grain size of about I mu m. Structural and morphological characterizations of the films obtained by PLD from these targets have been carried out through X-ray diffractometry and Scanning Electron Microscopy (SEM) analysis. De electrical transport measurements at 77 K have revealed a better magnetic field retention of the critical current density, J(c) being at 3 T the 14.6% and the 16.4% of the zero field values for YBCO-BZO 5% and 7%, respectively. The complex surface impedance has been measured by means of a dielectric resonator at a high microwave frequency (50 GHz). In order to identify the potentially strong pinning induced by the inclusions, the field-dependent complex resistivity has been determined as a function of temperature in a moderate magnetic field (B < 1 T). Very high depinning frequency are estimated in YBCO films with BZO inclusions indicating extremely high vortex pinning and steep potential wells.

We report on the superconducting properties of epitaxial thin films Y/sub 1-x/Ca/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-y/ with 0/spl les/x/spl les/0.2, grown on SrTiO/sub 3/ [001] by Pulsed Laser Deposition (PLD) technique. The partial substitution of Ca/sup 2+/ at the Y/sup 3+/site in YBCO leads to a monotonic decrease of the zero resistance critical temperature, T/sub c/. The oxygen stoichiometry is varied by means of post-deposition thermal treatment varying the oxygen partial pressure. The dependence of T/sub c/ on annealing oxygen pressure can be attributed to an overdoping effect and/or to a cation disorder.

A CeO/sub 2//YSZ/CeO/sub 2/ buffer layer structure was epitaxially deposited by pulsed laser deposition on a Pd seed layer grown on non magnetic Ni/sub 88/Cr/sub 8/W/sub 4/ alloy. Structural analyzes performed by x-ray diffraction and EBSD show a good texture, with typical YBCO FWHM of about 5/spl deg/ and 8.5/spl deg/ for in-plane and out-of-plane orientations, respectively. About 300 nm YBCO films deposited by PLD on such a buffer layer structure, show typical values of critical temperature of about 89 K and a critical current density greater than 0.6 MA/cm/sup 2/ at 77 K and self field.

A sufficient condition for the robust stability of a class of interval matrices is derived using the Lyapunov approach. The matrices considered have elements which are nonlinear functions of a vector of independent and bounded parameters. The robust stability condition requires that a quadratic form be positive definite in a finite number of conspicuous points of an enlarged parameter space. >

The formation and characterization of MgB/sub 2/ thin films is explored, using room-temperature deposition of precursor layers and subsequent annealing in-situ. Precursors were deposited by means of pulsed laser deposition (PLD), electron beam (EB), and sputtering (SD) techniques. Best results were obtained for B/Mg multilayers deposited by EB and annealed at 630/spl deg/C for 10 minutes, showing typical T/sub Con/ and T/sub C0/ values of about 32-34 K and 31-32 K, respectively. Structural and morphological analyses were performed by x-ray diffraction and SEM. A comparison among the different growth techniques is presented. Results are explained taking into account the pressure-temperature phase diagram of the Mg-B system.

The microstructure of Ni-5at%W (Ni-W) and Ni-11at%V (Ni-V) biaxially textured substrates has been investigated using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The correlation between the substrate microstructure and superconducting transport properties of YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) film grown on it has been studied on the YBCO/CeO/sub 2//Ni-W and YBCO/CeO/sub 2//NiO/Ni-V architectures. Our study has ascertained that the in-plane texture of the substrates is one of the most important factors, limiting the critical current density. The Ni-V substrate has a lower percolation area due to the larger number of twinned grains and a broader in-plane angular distribution and, as a consequence, the YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) film grown on it has a critical current density of 0.6 /spl times/ 10/sup 6/ A/cm/sup 2/, depressed by factor 2 with respect to YBCO grown on the Ni-W substrate. For the Ni-V substrate, another limiting factor is its low oxidation resistance. In contrast to Ni-V, the Ni-W substrate has a larger percolation area, mainly due to the absence of twinned grains, and a high oxidation resistance.

A promising approach to improve the engineering current density of coated conductors is to increase the thickness of the YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) layer. In this framework, a study regarding the relationship between the thickness of the superconducting film and its critical current was performed on biaxially aligned YBCO films grown on epitaxial Ce/sub 2/O/NiO structure using Ni/sub 89/V/sub 11/ non-magnetic substrate. The critical current density (J/sub C/) was measured for a series of YBCO films with thickness ranging from 0.2 to 2 /spl mu/m, deposited at constant rate. J/sub C/ values up to 6.4/spl times/10/sup 5/ A/cm/sup 2/ at 77 K and zero magnetic field were obtained for thinner films decreasing three orders of magnitude for the 2 /spl mu/m thick YBCO film. X-ray diffraction and morphological analyses indicate a progressive structural deterioration with the increase of film thickness.

This paper presents the development of the CeO/sub 2/ buffer layers on both the non magnetic Ni-V and the Pd-buffered Ni substrates by pulsed laser ablation (PLD). The CeO/sub 2/ films show strong in-plane and out-of-plane orientations. The SEM studies showed that the CeO/sub 2/ films are both smooth and continuous. The magnetic and structural properties of the new Ni-V substrate are also presented. The results demonstrate that Ni-V is an alternative substrate for the fabrication of textured YBCO conductors.

The JET (Joint European Torus) Tokamak is to be modified to incorporate a divertor. A coil system in the vacuum vessel has been developed which can produce a range of different divertor plasmas. The divertor coils are of conventional construction and are contained in thin Inconel cases. They will be assembled in the vacuum vessel, welded into their cases, and impregnated with epoxy resin. >

Extensive applications of YBa2Cu3O7-x (YBCO) tapes require a comprehensive knowledge of their behavior in every possible operating condition. In this paper, the thermal stability behavior of a conduction-cooled pancake coil wound with commercial YBCO coated conductors is reported from both the experimental and numerical point of view. The coil stability against heat disturbances is studied in terms of minimum quench energy and normal zone propagation velocity (NZPV) for different values of the coil currents at different temperatures. The quench dynamics of the tape is compared between experiment and the numerical analysis, obtaining a reasonable agreement for voltage traces, NZPV, and temperatures. The model allows a wide range of case studies that can be used to suggest appropriate solutions for the design of detection and protection systems.

In linear multivariable systems it is of interest to achieve pole assignment using observers of reduced order. In this correspondence it is shown that for almost all systems with r independent inputs and m independent outputs, pole assignment can be obtained using an observer the order of which is \max(0,n + 1-m-r) or \lceil(n+1-r)/m\rceil-1 , where n is the dimension of the state space and \lceil a \rceil denotes the least integer greater than or equal to number a . The related procedures for desioning the controllers are given.

In the above paper a method is given for determining the output vector feedback, simultaneously computing the coefficients of the residual polynomial, the roots of which are the remaining unassigned poles. This and similar results are well established but not well known in the western literature. In these comments two algorithms are reported for the incomplete pole assignment by output feedback; with the proposed techniques some degrees of freedom can be deliberately introduced in the synthesis to meet other specifications than only exact incomplete pole assignment.