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The European Roadmap to Fusion Electricity (Federici et al., 2018) [1] details the path to complete within the next three decades the DEMOnstration power plant, DEMO, aiming to a net gain of Energy Q=40. The 2018 DEMO baseline considers a 2 GW tokamak device, a target performance of 6 T on the plasma, with a major radius of 9 m. Considering the plasma aspect ratio and elongation, and the radial build of the thermal shield, the outer radius of the TFC inner leg is about 4.3 m. In the current designs, the TFC winding pack operates in a range of B = 12-13 T and is based on low temperature superconductors (LTS). As an alternative, High Temperature Superconductors (HTS) could be employed to extend the coil operation beyond the typical J/T/B range of Nb3Sn. The rationale behind the exploration of a fully HTS or Hybrid HTS/LTS design of the TFC Winding Pack (WP) is twofold: on the one hand, it is possible to maintain the target performance of the machine redesigning the radial build of each magnet as a function of a more efficient cable in terms of critical current density; on the other hand, it is possible to keep the machine power constant, increasing the field on the plasma axis, and consequently define a new radial build. This work presents and describes a combined algorithm implemented to enhance the DEMO magnet systems and optimize the DTT central solenoid, capable of accounting for the main electromagnetic and structural constraints, and consistently reshape the machine.

Among the Iron-Based Superconductors, the 1144 family attracted significant attention due to the intriguing intrinsic properties exhibited by single crystals, such as high critical fields and critical currents. Polycrystalline compounds, however, are influenced by the non-ideal nature of the material. In this work, we report the superconducting properties at high magnetic fields up to 17 T of Ca/K-1144 bulk polycrystalline compounds obtained via a mechanochemically assisted synthesis route. The material exhibits a significant robustness of intrinsic features such as the superconducting transition onset, varying from approximately 35 K in the absence of the field to approximately 33 K at 17 T. Broadening phenomena of both resistive and magnetic transitions testifies however the presence of granularity issues, most likely due to extrinsic factors such as oxidation, porosity and random orientation of the crystallites.

The paper is focused on the electromagnetic analysis of the Toroidal Field (TF) superconducting coils of the Divertor Tokamak Test facility (DTT) when electrical transients occur in the TF coils system: for example, during the operations of the Fast Discharge Units (FDUs) and considering also, the simultaneous occurrence of a fault condition. During the FDU intervention, a transient voltage excitation lasting few microseconds occurs at the TF coil terminals and it electrically stresses the insulations of TF coils itsef. To investigate the voltage distribution across, inside and between different Double Pancakes (DPs) of each TF coil, a lumped parameters circuital model has been developed and implemented in Ansys Simplorer simulation environment. This model includes both the detailed sub-model of each TF coil and FDU. The transient analyses have been carried out for two different scenarios: a reference one and a failure scenario, considering three different fault resistance values and also two different values of the resistance connecting the TF case to ground. In order to verify the correct sizing of the coil insulation and the TF case-to-ground resistance value inserted in the circuit of each TF coil, the voltages of each TF coil (terminal-to-terminal, terminal-to-ground, across of adjacent DPs and so) were computed in the time domain (in the range of milliseconds) for both scenarios. An overview of calculations and simulation results is presented and discussed, pointing out a high sensitivity of fault conditions and of overvoltage values and addressing mitigation strategies.

The knowledge of the strain distribution of Nb3Sn filaments in a CICC cross-section is a key parameter in understanding the performance and its evolution when the cable undergoes electromagnetic cyclic loading. This work focuses on the Wind and React (WR1) rectangular CICC sample designed by ENEA for the European DEMO project, that has exhibited a current sharing temperature (Tcs) above 6.9 K at 13 T and 81.7 kA, during its qualification tests in the EDIPO facility at the Swiss Plasma Center. Compared to the critical current tests on the constituting wires, the observed CICC performance corresponded to an effective strain value of −0.55%. This work aims to interpret this result on the basis of the analysis of the strain distribution inside the cable itself. Susceptibility versus temperature of WR1 constituting free standing strands has been measured after etching of the stabilizing copper, and it has been compared with the susceptibility measured on the cable during the test campaign in EDIPO. A well-known and already assessed method has been used, starting from susceptibility measurements, to compute the Tc distribution in the cable cross-section and, subsequently, to infer from that the thermal strain distribution. The obtained results give an indication that the strain distribution in WR1 cable is narrow, compared to the strain distribution calculated with the same method for most ITER TF and CS conductors.

In the frame of the conceptual design studies for the Toroidal Field (TF) coils of DEMO, a solution based on a layer-wound magnet, rectangular-shaped Cable-in-Conduit conductors and W&R manufacturing approach, is being developed. The feasibility and performance of a large-size Nb3Sn conductor operating at about 82 kA in a 13 T field has been proven in the past. Another key technology to be demonstrated for a layer-wound TF coil, is that of a joint between two different conductor grades, to be possibly manufactured in-line during winding. The proposed joint solution would provide the minimum room occupancy, with the joint embedded within the winding pack volume. A joint between two low-field conductor grades, i.e., constituted of a small number of superconducting strands and a high number of stabilization copper wires, has been designed and manufactured. The two conductor lengths are characterized by a different number of superconducting wires and different outer dimensions. A joint sample has been assembled and instrumented, in the configuration allowing testing at the SULTAN facility of the Swiss Plasma Center. Both DC and AC performance of the joint has been characterized at the DEMO TF operating conditions. The present paper reports the main manufacturing steps for the joint and its main test results. The implications on the performance and design approach of the TF coil are also discussed, based on the outcome of such tests.

Italian Divertor Tokamak Test (DTT) facility is part of the general European programme on the fusion research. Its specific role is to cover the gap on the power exhaust for the future DEMOstration power plant (DEMO). This tokamak will be built and installed in Italy at Frascati ENEA laboratories. The paper describes the Toroidal Field (TF) coils circuit powered by a TF power supply that feeds in series 18 toroidal superconducting magnets and the development of three software models in order to validate the adopted technical solutions in term of maximum voltage and maximum current stress on each power device/component, due to both the operating and fault conditions, and across TF coils.

Superconducting properties of the Fe(Se,Te) system are directly related to its microstructure. However, in the framework of a not well assessed phase diagram, a large spread of chemico-physical and functional properties is reported for compounds characterized by similar overall chemical compositions. This is in particular observed in materials produced by melting routes, where chemical inhomogeneities and multiple phases are commonly obtained. Both composition and morphology of these inhomogeneities seem to play a role in the behaviour of the superconducting phases, pointing out the need to assess the dependence of phase composition and therefore of the material performance on the sample thermal history. In this work, the effect of different post-synthesis annealing treatments is studied. A melting route has been adopted to synthesize polycrystalline samples, subsequently subjected to thermal treatments at different temperatures. Morphology and microstructure were characterized by means of x-ray diffraction and microscopy techniques. The superconducting properties have been evaluated by means of electrical and magnetic measurements. It is observed how microstructure, chemical composition and morphology of the obtained products strictly depend on the annealing temperature. Moreover, a crucial role of the cooling step is evidenced: in fact, a rapid cooling procedure from high temperature leads to the formation of high temperature (Tconset > 19 K) superconducting phases, but the overall bulk superconducting performance is negatively affected. Other annealing treatments can promote instead an enhancement in the critical current at high fields with respect to pristine sample, suggesting that a careful engineering of a post-synthesis thermal step can represent a useful tool to optimize the material properties.

As prototypes of future commercial tokamaks, DEMOs nuclear fusion power plants are expected to be able to produce cost-effective electrical power. In this view, an optimized design becomes crucial in the whole engineering workflow. Up to now, the design of one of the most critical components, the cross-section of each of the toroidal field coils inner leg winding pack, was performed using a sequential trial-and-error procedure. In this work, a novel comprehensive approach is proposed to include all the main design aspects into a unified tool taking advantage of artificial neural networks for faster computation in finding optimal design configurations. This procedure overcomes several difficulties including dealing with both real-valued and discrete design variables, the significant CPU-time of magneto-structural analysis and also guarantees the optimality for the winding pack configuration. The proposed methodology was demonstrated for the 2019 ENEA DEMO configuration which includes 16 toroidal field coils, made-up of 6 Nb3Sn double layers and a Wind and React manufacturing technique.

The paper is focused on the optimal design of Fast Discharge Unit (FDU) for the quench protection of the Toroidal Field (TF) magnets of the Divertor Tokamak Test facility (DTT), an experimental facility under construction in ENEA Frascati Research Centre (Rome, Italy). The FDU is a safety key component that protects the superconducting magnets when a quench is detected through the fast extraction of the energy stored in the magnets by adding a discharge dump resistor (DR) in the TF magnets circuit. A comparison between a fixed DR and a switched variable DR has been implemented by changing resistor parameters and by using multiple current control of the power electronics components (IGCTs). The new configuration allows to reduce the maximum voltage and the thermal stresses both for superconducting magnets and for FDUs (Fast Discharge Units), so reducing the insulation level of all TF (Toroidal Field) coil circuits, including also the power supply, reducing the hotspot temperature on the TF coils and the specific energy through them. Another advantage of the proposed configuration is the reduction of the size of all electrical devices of TF coil circuits so achieving a more effective and reliable design, also decreasing the overall costs.

Iron-based superconductors are an interesting class of materials that have attracted significant interest during the last years, and which could be employed in a variety of potential applications due to their high upper critical field Hc2. In order to exploit the properties of these materials for practical applications, in this paper we explore two different routes for the fabrication of polycrystalline samples belonging to 11-family. In particular, the influence of dedicated heat treatments has been investigated to optimize their overall superconducting properties. Then, we compared the pinning properties of these two types of samples to correlate the fabrication process with the pinning landscape. The final aim is to obtain an increased Tc and Hc2, as well as an enhanced pinning efficacy, which could lead to higher critical current density Jc relevant for practical uses. © 2017 IEEE.

This study is focused on the phase separation phenomena in Fe(Se,Te) materials. In particular, it investigates the effect of a high temperature and long time annealing treatment on morphostructural and superconducting properties of bulk samples. The synthesis of Fe(Se,Te) compounds was carried out through a melting route, and the materials were characterized in their morphostructural and magnetic properties. The obtained compounds possess a high degree of inhomogeneity due to dendritic precipitates of secondary phases in a superconducting tetragonal matrix. The high temperature annealing treatment induces a morphological evolution of the material and leads to the structuration of two distinct superconducting phases.

The superconducting DTT magnetic system needs a quench detection systems (QDSs) fast enough to trigger the dumping of the magnetic energy in case of quench and avoid irreversible damage of the cable systems. With this aim, a primary system based on the detection of the resistive voltage associated with the quench offers the best quench detection guarantees. The tokamak environment is affected by several electromagnetic noises during plasma scenarios so that the resistive voltage detection, during normal operation, can be compromised by the presence of large voltages induced by self and mutual magnetic coupling among coils and with plasma or passive elements: the resulting inductive voltages across a TF coil, or part thereof, could be much higher than the quench voltage thresholds. Voltage compensations techniques, therefore, have necessarily to be foreseen in the QDSs conceptual design, to discriminate the resistive component associated with the quench. We present a reconnaissance of all known electromagnetic noises that could affect a TF coils QDS in DTT: this analysis is conducted by means of analytical calculations, made up with the aim to evaluate and have a prevision of the maximum extent of the voltages induced across TF coils and Double Pancakes (DPs) during, in particular, the Single Null (SN) scenario.

In the effort to unveil the relations between synthesis conditions, morpho-structural features and superconducting properties of (11)-type iron-based superconductors, this study is focused on FeSe0.5Te0.5 materials obtained from high-temperature melts and different cooling rates. Crystalline and chemical composition are evaluated by means of XRD and SEM analyses, highlighting the formation of complex multiphase structures: the obtained results suggest that high-temperature nucleating phases may play a key role in the observed phase separation. Superconducting properties, evaluated by magnetization measurement and dc electrical resistance in different magnetic field conditions up to 18 T, highlight the low homogeneity of the superconducting phases, reflecting the microstructural observation. © 2002-2011 IEEE.

A 4-probe method, which directly measures the transverse resistivity in multi-filamentary superconducting strands, has been developed at ENEA. The technique has been applied to NbTi strands in order to compare the results with the ones obtained through indirect approaches. Samples with different external diameters have been considered, for evaluating the effect of the Cu:non-Cu ratio into the wires. For Nb3Sn strands with either a distributed Nb-Ta diffusion barrier design, or with a single Ta diffusion barrier between the filamentary region and the stabilizing copper shell, a detailed mapping of the transverse resistivity of the matrix has been achieved. The temperature and magnetic field dependence of the transverse resistance has been measured for Nb3Sn samples.

In order to study the effect of bending on strands' performances, a finite differences numerical method has been implemented at ENEA, which discretizes a single strand in several elements representing bundles of twisted filaments. The code has been validated, and a satisfactory agreement has been obtained with experimental measurements carried out on an ITER-type OST strand, highlighting the critical impact of geometric modeling and the potential of the model as a predictive tool. The aim of this work is to verify the agreement between model results and critical current measurements of bent strands, and to help in clarifying some aspects of current redistribution process within the strand cross-section, in view of developing more appropriate theoretical models. Further improvements to the model system are presented: in order to simulate more representative conditions for cables in operating conditions, periodic bending is now introduced. Simulation results are compared with available experimental data on OST Dipole strand, with both twisted and untwisted filaments. Numerical values from the inter-filament resistivity measurements carried out at ENEA, are introduced as computational parameters, being the transverse resistance the main physical factor governing the analyzed mechanisms in multi-filamentary superconductors. In all those cases a satisfactory agreement among measurements and computational results has been found.

In multi-filamentary superconducting strands the transverse resistivity gives a fundamental contribution to the mechanisms governing the coupling losses in AC regime and the current transfer length, for example influencing the performance of wires with bending strain. Nevertheless, the transverse resistance matrix generally used into the simulation codes of superconducting strands is determined as a fitting parameter obtained from the experimental V-I curves. The first direct measurement of the transverse resistance as a function of the temperature in multi-filamentary wires by using a 4-probe method has been performed at ENEA. Here we study the effect of a background magnetic field on the V-I characteristics, obtaining interesting results. Moreover, the resistivity between the bundles and the outer copper layer is measured at different temperatures. These experimental values, together with the inter-bundle resistivity, are inserted as input parameters in the strand current diffusion equations, for the validation of a finite differences numerical model. The agreement between simulated V-I curves and experimental characteristics is excellent.

A cold worked multifilamentary Cu/Nb-47w%Ti wire was annealed at a temperature of 375°C for various times from 2 minutes up to 100 hours. Filaments were extracted from copper matrix by etching. With the attempt to improve understanding of superconducting properties, extracted filaments were tested with a Vibrating Sample Magnetometer (VSM) as a function of field over a wide temperature range. The Ic curves were obtained from the magnetization cycles at different temperatures, and the normalized pinning forces (fp) were plotted as a function of the reduced applied field (b). For each sample, it was found that the fp peak clearly shifts to lower fields as the temperature approaches Tc. Further, the pinning curves also depended on the annealing time. The fp vs. b curves were reproduced with the 2-components model which recently has been developed and successfully validated on a number of optimized NbTi commercial strands. Results are discussed in terms of current pinning models, also in comparison with results of microstructural characterization. © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.