C. Fiamozzi Zignani

The JT-60SA Toroidal Field Conductor Reference Sample: Manufacturing and Test Results

In the framework of the JT-60SA design activities, EU home team has defined a reference layout for the Toroidal Field conductor: it is a slightly rectangular Cable-In-Conduit NbTi conductor, operating at 25.7 kA with a peak field of 5.65 T. ENEA has assigned LUVATA Fornaci di Barga the task to produce the strands and to perform cabling, whereas jacketing and compaction have been carried out in its own labs. The sample, successfully tested at the CRPP SULTAN facility, has been assembled in such a way as to avoid the bottom joint between the two legs, thus using a single conductor length (about 7 m). An ad-hoc developed solution to restrain the U-bent conductor section (where jacket is not present), consisting in a stainless steel He-leak tight box with an inner structure designed in order to completely block the cable, has been also developed and manufactured by ENEA, where the sample has been also assembled. Instrumentation installation and final assembly of the sample have been performed by the SULTAN team. The main aspects of the sample manufacturing and characterization are here presented and discussed.

Successful performances of the EU-AltTF sample, a large size Nb3Sn cable-in-conduit conductor with rectangular geometry

One of the design features which yet offers interesting margins for performance optimization of cable-in-conduit conductors (CICCs), is their geometry. For relatively small size Nb3Sn CICCs, operating at high electromagnetic pressure, such as those for the EDIPO project, it has been experimentally shown that a design based on a rectangular layout with higher aspect ratio leads to the best performance, especially in terms of degradation with electromagnetic loads. To extend this analysis to larger size Nb3Sn CICCs, we manufactured and tested, in the SULTAN facility, an ITER toroidal field (TF) cable, inserted into a thick stainless steel tube and then compacted to a high aspect ratio rectangular shape. Besides establishing a new record in Nb3Sn CICC performances for ITER TF type cables, the very good test results confirmed that the conductor properties improve not only by lowering the void fraction and raising the cable twist pitch, as already shown during the ITER TFPRO and the EDIPO test campaigns, but also by the proper optimization of the conductor shape with respect to the electromagnetic force distribution. The sample manufacturing steps, along with the main test results, are presented here.

Magnetic and Transport Characterization of NbTi Strands as a Basis for the Design of Fusion Magnets

We present the results of an extended measurement campaign carried out on available NbTi commercial strands to be used in the design of fusion reactor magnets, including candidate strands for the ITER high field Poloidal Coils, PF1/6, and for the Toroidal Field Coils of the JT-60SA Tokamak. Magnetic and transport measurements have been carried out at variable temperature and magnetic field. From magnetization cycles we were able to extract information about AC hysteretic losses, and to extend the current density determination to lower fields, thus enabling the optimization of numerical fits in a wider magnetic field range. It has been found that the normalized bulk pinning force of the material, though showing good temperature scaling throughout the explored range, cannot be described by a single function of the type balpha(1 - b)beta. On the contrary, the full summation of two contributions, each dominant in a different magnetic field range, returns a good fit of the data. Extending this 2-components description to an expression for the critical current density, a very good agreement with experimental measurements is obtained over the whole explored B, T range. Collecting a database of available strands performances, especially in the range of relatively high temperatures (T > 5.5 K) and magnetic fields (B ~ 6 T), typical of applications in large coils for fusion reactors, constitutes a sound basis for magnets design, which should be based on strand properties measured in the operation-relevant temperature and magnetic field range.

Variable-temperature characterization of NbTi strands in the low critical-current density range

A facility for the characterization of superconducting strands at variable temperatures has been recently upgraded at ENEA. Measurement of transport properties of superconducting strands can be now carried out in the temperature range from 3.5 K to 15 K, with stability less than 10 mK, and in magnetic fields up to 12.5 T, covering a range of critical currents between a few Amperes, up to about 300 A. In this study we present the results of a measurement campaign carried out on NbTi wires, aimed also at comparing the performances of some available candidate strands, to be used in the design of the Toroidal Field Coils of the JT60-SA Tokamak, as well as in the design of the ITER high field Poloidal Field Coils, PF1 & 6. We measured strands with different diameters, and with Cu-nonCu ratios ranging between about 1.5, up to about 7. The measurements showed the critical behavior of the NbTi in the range of relatively high temperatures (T > 5.5 K) and high magnetic fields (B > 6 T), typical for large coils in fusion reactors, where Cable-in-Conduit Conductors are used and where temperature margins of at least 1K are required for a sound design. The experimental results highlighted the need for an optimization of the numerical fits normally used to predict NbTi critical current densities, starting from measurements performed at 4.2 K. Based on our results, we conclude that fit parameters for each NbTi strand should be inferred from experimental characterizations in the temperature and magnetic field range of interest for the design.

Simulation by Finite Difference Numerical Method of

We report on the simulation of the current distribution in Nb3Sn strand subjected to pure bending strain, obtained by resolving the implicit diffusion equations with finite difference algorithm in Mathworks environment. The critical current dependence on bending, temperature, and magnetic field is modeled by the Improved Deviatoric Scaling Law and is used in the power law electric field dependence across the superconductor. The strand is discretized in elements representing groups of twisted filaments embedded in the stabilization matrix and a distributed constant circuit model is applied for current transfer among filament bundles. The code is preliminarily validated by comparison with analytical solutions for different simplified situations, each one corresponding to a proper boundary condition. Transverse matrix resistivity and twist-pitch values are crucial elements for matching numerical results with experimentally measured critical currents.

{\rm Nb}_{3}{\rm Sn}

For the protection of the LHC superconducting magnets, about 2100 specially developed by‐pass diodes were manufactured by DYNEX SEMICONDUCTOR LTD (Lincoln, GB) and about 1300 of these diodes were mounted into diode stacks and submitted to tests at cryogenic temperatures. To date about 800 dipole diode stacks and about 250 quadrupole diode stacks for the protection of the superconducting lattice dipole and lattice quadrupole magnets have been assembled at OCEM (Bologna,Italy) and successfully tested in liquid helium at ENEA (Frascati, Italy). This report gives an overview of the test results obtained so far. After a short description of the test installations and test procedures, a statistical analysis is presented for test data during diode production as well as for the performance of the diode stacks during testing in liquid helium, including failure rates and degradation of the diodes.

Strand Under Bending Strain

The superconducting wires are generally made of several hundreds or thousands of fine superconducting filaments embedded in a metallic matrix. Several relevant properties of the superconducting wires depend on the transverse resistances between filament bundles. In Nb3Sn wires realized with Bronze Route or Internal Tin technology, the presence of the bronze matrix can determine a significant increase of the transverse interfilament resistance with respect to wires with copper matrix. This increased resistivity in turn plays a role in determining the ac losses, thermal stability, and sensitivity to mechanical bending of the wire. The direct measurements of the transverse electrical resistances give useful information both for stability computations and to analyze the mechanical performance of the wire. The complexity of these measurements is however remarkable, due to the current distribution phenomena that occur among superconducting filaments during these tests. This paper presents the application of a 2D FEM model of the wire cross section and of a 3D electrical circuit model of the wire sample to derive qualitative and quantitative information about the transverse electrical resistance matrix. The paper shows that a detailed qualitative and quantitative description of the measurement results can only be obtained by means of a 3D model, that allows computing the current distribution along and across the sample length during the measurements.

Cryogenic Testing of High Current By‐Pass Diode Stacks for the Protection of the Superconducting Magnets in the LHC

The LHC (Large Hadron Collider), the accelerator being constructed on the CERN site, involves the operation of more than 8000 superconducting magnets of various current ratings. Essential elements for the powering of these magnets are the HTS current leads. These devices provide the electrical link between the warm cables from/to the power converter and the low temperature superconducting bus bars bringing the current from/to the cryo‐magnets. Thus they operate in a temperature range between room temperature and liquid helium temperature. The operation of the LHC will require more than 1000 HTS current leads operating at currents ranging from 600A to 13000A. Cryogenic tests of the series of 13000A and 6000A HTS current leads are made at ENEA in the framework of a CERN‐ENEA collaboration.This report gives an overview of the experimental set‐up built in ENEA. The set‐up was designed following the typical criterion of a scientific experiment but it was dimensioned to satisfy the schedule of an industrial sca...

Analysis of Transverse Resistance Measurements in

Iron-chalcogenide superconductors are appealing for high-magnetic-field applications due to their promising superconducting properties, i.e., extremely high upper critical and irreversibility field.In this paper, the first achievements obtained from sintering polycrystalline samples are presented. The sintering of FeSe polycrystalline powders and pellets was obtained in several steps starting from stoichiometric quantities of freshly polished powders shots; after grinding, powders were loaded into cleaned and dried silica tubes sealed under vacuum for subsequent heat treatments in order to obtain the superconducting phase. Samples were then characterized from the structural, transport, and magnetic points of view. From the structural characterization, it has been discovered that, in addition to the presence of the desired tetragonal FeSe phase, hexagonal phase and several impurities are present in the samples; the lack of homogeneity inside the samples is confirmed by the superconductive characterization: despite the small undeniable enhancement of the results with the application of subsequent HTs and, nearby, a resistive superconductive onset of 12 K, the transition is still broad after the third HT.

{\rm Nb}_{3}{\rm Sn}

The primary issue facing Nb3Sn cable-in-conduit conductors (CICCs) has been degradation, often observed during electromagnetic cycling, in the current sharing temperature, n-index, and critical current, in respect to measured strands values, due to strain effects. In the last years, work performed mostly relating to fusion magnet technology has led to a better understanding of the parameters required to improve the constraints imposed by the brittle nature of the Nb3Sn filaments, such as cables low void fraction and long twist pitch sequence. On the other hand, experimental campaigns on bent Nb3Sn wires, pre-compressed into a stainless-steel jacket, have shown that an appreciable decrease in the n-index values already occurs at the strand level, well below the irreversible mechanical load regime for filament breakage. This result is explained, with the support of simulation results, taking into account the broadening of the critical current distribution on wires cross section due to the presence of the jacket and to bending strain. Considering the CICCs layout impact on their overall performances, the effective resemblance between pre-compressed bent wires and strands inside cables is emphasized, and an innovative interpretation of cabled conductors test results is given, suggesting a tool to predict their performances in terms of the n-index versus critical current relation of constituting strands, characterized under simultaneous pre-compression and bending strain.