Valentina Corato

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.

Direct measurement of interfilament resistance in Nb3Sn strands

In modeling the properties of superconducting multifilamentary strands, transverse resistivity plays a crucial role in the definition of the coupling losses in ac regime, as well as of the current transfer length, that affects the transport properties of Nb3Sn wires subject to bending strain. We present the first direct measurement of the interfilament transverse resistance in superconducting strands from room temperature to 4.2 K. Results have been compared to the transverse resistance of a sample on which the outer copper stabilization layer has been removed by chemical etching, obtaining interesting indication on the preferential current paths within the wire cross section. An excellent agreement between experimental data and theoretical models has been found in describing the whole strand, while improvements are required in modeling the filamentary region alone.

Performance enhancement under bending of Nb3Sn strands with untwisted filaments

We report on the effect of bending strain on the transport properties of multifilamentary Nb3Sn strands having twisted or untwisted superconducting filaments in order to clarify some aspects of the current transfer process within the wire cross section. Critical current measurements were performed on internal tin Nb3Sn strands, which were inserted and compacted inside thin stainless steel tubes before the heat treatment. With the application of a maximum bending strain of 0.5%, the strands with twisted filaments present a degradation of the critical current and of the n-index, while an enhancement of the performances is observed for the untwisted strands. The experimental results are interpreted in relation to the predictions of Ekin’s model.

Improvement of electromechanical properties of an ITER internal tin Nb3Sn wire

The critical current of an internal tin Nb3Sn wire developed by Oxford Instruments, Superconducting Technology for International Thermonuclear Experimental Reactor (ITER) (OST type-I, billet No. 7567) has been studied under axial strain at fields between 12 and 19 T at 4.2 K. Simulating the situation in a cable in conduit, where thermally induced compressive strain is important, a single wire (strand) was jacketed with AISI 316L stainless steel. The reinforced wire shows an important increase in em, the applied strain where Ic reaches its maximum, from 0.25% to 0.57%. In addition the irreversibility limit, eirr, is improved from 0.50% applied strain to >1.10%. It could also be shown that the Ic at zero intrinsic strain is almost identical. This demonstrates that jacketing does not influence the physical parameters of the original wire. Experimental data of the bare wire has been well fitted by different strain functions. However, it was not possible to model the data of the jacketed wire. There are indica...

Test Results of Three Poloidal Field Superconducting Samples in SULTAN

Superconductors for the ITER Poloidal Field Coils are large cable-in-conduit conductors (CICC) made of NbTi strands encased in a round-in-square stainless steel jacket. Three prototype conductor sections for poloidal field coils PF1/6, PF2/3/4 and PF5 have been fabricated in collaboration of the domestic RF, CN and EU agencies and tested in SULTAN Test Facility at the nominal operating conditions. The test aimed to characterize the DC and AC behavior of the conductors. The DC test was focused on the current sharing temperature (Tcs) at the nominal operating current and nominal operating background field. The take-off electric field at the nominal Helium mass flow rate was investigated versus the cable current density over a broad range of field and temperature. The AC loss measurement was performed before any electromagnetic loading and after a number of load cycles in order to define the impact of cyclic loads on the coupling currents constant of the cable. From the test results in SULTAN test facility, the margins in normal operation and the limits of the operation range of the ITER PF conductors are assessed.

Design, Manufacture, and Test of an 80 kA-Class Nb3Sn Cable-In-Conduit Conductor With Rectangular Geometry and Distributed Pressure Relief Channels

Within the frame of the R&D activities carried out in Europe for the toroidal field coils of the nuclear fusion device DEMO, a fundamental milestone was considered to be the demonstration of Nb3Sn cable-in-conduit conductors (CICC) performance in the demanding range of interest for DEMO. Among the different technological solutions envisaged, the present paper deals with a wind & react CICC solution, with rectangular geometry, thick steel jacket, and distributed pressure relief channel, designed to operate at 82 kA in a magnetic field of 13 T and with a current sharing temperature Tcs > 6.5 ,K. The main manufacturing steps of the prototype conductor are described in the present paper, carried out within industrial environment, partly using the facilities and procedures available for the manufacture of ITER conductors. A sample was designed for the EDIPO facility at the Swiss Plasma Center, Switzerland, in the configuration usually adopted for the test of ITER poloidal field conductors, where the two straight conductor legs are part of the same cable length, with a continuous transition through a bottom hairpin-type joint, thus avoiding any resistive connection. The conductor has been characterized in terms of dc performance at relevant operating conditions and the absence of any performance degradation with electro-magnetic load cycles has been verified, thus, qualifying the proposed technological solution. AC losses and thermo-hydraulic tests have also been carried out, providing relevant information for further coil design.

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}

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.