Gennaro Romano

Challenges and status of ITER conductor production

Taking the relay of the large Hadron collider (LHC) at CERN, ITER has become the largest project in applied superconductivity. In addition to its technical complexity, ITER is also a management challenge as it relies on an unprecedented collaboration of seven partners, representing more than half of the world population, who provide 90% of the components as in-kind contributions. The ITER magnet system is one of the most sophisticated superconducting magnet systems ever designed, with an enormous stored energy of 51?GJ. It involves six of the ITER partners. The coils are wound from cable-in-conduit conductors (CICCs) made up of superconducting and copper strands assembled into a multistage cable, inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the toroidal field (TF) and central solenoid (CS) coils require about 600?t of Nb3Sn strands while the poloidal field (PF) and correction coil (CC) and busbar conductors need around 275?t of Nb?Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and has called for a significant worldwide production scale up. The TF conductors are the first ITER components to be mass produced and are more than 50% complete. During its life time, the CS coil will have to sustain several tens of thousands of electromagnetic (EM) cycles to high current and field conditions, way beyond anything a large Nb3Sn coil has ever experienced. Following a comprehensive R&D program, a technical solution has been found for the CS conductor, which ensures stable performance versus EM and thermal cycling. Productions of PF, CC and busbar conductors are also underway. After an introduction to the ITER project and magnet system, we describe the ITER conductor procurements and the quality assurance/quality control programs that have been implemented to ensure production uniformity across numerous suppliers. Then, we provide examples of technical challenges that have been encountered and we present the status of ITER conductor production worldwide.

Improvement of Magnetic Field Behavior of Ex-Situ Processed Magnesium Diboride Tapes

MgB2 tapes have been synthesized through the ex-situ powder-in-tube (PIT) method. This technique involves the cold working of tubes of various metals-in this case Ni-previously filled by suitably reacted MgB powders, followed by proper heat treatments. In particular, it allows the use of different starting powders and the control over their properties. We studied the influence of the starting powders on the superconducting properties of the final conductors, and we tried to improve their behavior in magnetic field by mastering their grain size and by inserting appropriate doping. In order to improve the pinning properties of the tapes, the granulometry has been varied through ball-milling of MgB, and different dopants-such as carbon or SiC nanoparticles-have been introduced either on the precursors or on MgB. Critical current measurements on the tapes are presented, both magnetic from the SQUID and transport in high magnetic field. In particular, IV characteristics have been measured up to 13 Tesla at GHMFL (Grenoble) in order to extract the critical current in two directions, i.e. with the field perpendicular and parallel to the tape surface.

Study of the MgB2 grain size role in ex?situ multifilamentary wires with thin filaments

The MgB2 superconductor has already demonstrated its potential, in particular for DC applications such as magnetic resonance imaging (MRI) magnets, thanks to the low cost of the raw materials and to its simple production process. However, further efforts have still to be made in order to broaden its employment in AC applications such as superconducting fault current limiters (SFCLs), motors, transformers etc. The main issues are related to the reduction of AC losses. Some of these can be faced by obtaining multifilamentary conductors with a large number of very fine filaments and, in this context, the powders granulometry can play a crucial role. We have prepared MgB2 starting powders with different granulometries and by the ex situ powder-in-tube (PIT) method we have realized multifilamentary wires with a number of filaments up to 361 and an average size of each filament lowered down to 30 µm. In particular we have studied the relationship between grain and filament size in terms of transport properties and have shown that the optimization of this ratio is possible in order to obtain suitable conductors for AC industrial applications.

Role of the Grain Oxidation in Improving the In-Field Behavior of

The grain size and the grain boundaries play an important role in determining the pinning properties and the current carrying capability of MgB2. Here we present an experimental study on the effect of grain oxidation on the superconducting properties of MgB2 ex-situ Powder-in-Tube (PIT) tapes. We fabricated Fe-sheathed monofilamentary tapes using a variety of powders synthesized in inert (IA) or oxidizing (OA) atmosphere. In order to understand the role of magnesium oxide (MgO) we studied the effect of the presence of the oxidation layer on the field dependence of the critical current density. To this purpose, we developed a process for the powder synthesis and tube filling and sealing in controlled atmosphere to reduce the oxygen contamination; with this process the connectivity of the grains in the final tapes proved to be increased. Then, we introduced a controlled oxidation step. We will show how the presence of the MgO layer enhances the critical current field dependence. High energy X-ray diffraction measurements were performed on the samples, and the grain size of the unsintered and sintered powders in the tapes was measured by means of Scanning Electron Microscopy (SEM). The pinning force was studied, and different mechanisms were found for the samples prepared in IA and OA. The different behavior shown by the two different typologies of tapes was correlated with the presence or absence of the MgO layer.

{\rm MgB}_{2}

MgB2 monofilamentary nickel-sheathed tapes and wires were fabricated by means of the ex-situ powder-in-tube method using either high-energy ball milled and low temperature synthesized powders. In the first part of the paper it will be shown the effect of milling on tapes realized with powders synthesized at 900degC. The milling time and the milling revolution speed were tuned in order to maximize the critical current density (Jc) in field: the maximum Jc value of 6times104 A/cm2 at 5 K and 4 T was obtained for the tape prepared with powders milled for 144 h at 180 rpm. In the second part of the paper we study the effect of the powders synthesis temperature in a wire configuration. Various synthesis temperature were investigated (730degC-900degC) finding a best Jc value for the wire prepared with powders synthesized at 745degC. We speculate that this optimal temperature is due to the fluidifying effect of unreacted magnesium content before the sintering process which could better connect the grains.

Ex-Situ Tapes

In this paper, we report a new synthesis route to produce boron powders characterized as being amorphous and having very fine particle size. This route has been developed to improve the performances of superconducting MgB2 powders, which can be directly synthesized from this nanostructured boron precursor by following the ex-situ or the in-situ PIT method during manufacturing of tapes, wires, and cables. All the procedure steps are explained, and the chemical-physical characterization of the boron powder, using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques, is reported. Furthermore, a comparison with commercial boron is given. Preliminary results of the magnetic and electrical characterization, such as critical temperature TC and transport critical current density JC t, for the MgB2 tape are reported and compared with those for the tape prepared with commercial boron.

High-Energy Ball Milling and Synthesis Temperature Study to Improve Superconducting Properties of

MgB2 tapes and wires have been synthesized through the ex-situ Powder-In-Tube (PIT) method. We studied the effects of using different metallic materials for the sheaths in new round or square shaped multifilamentary wires; enhanced critical currents in 19 filaments wires are reported. The influence of the starting powders on the superconducting properties of the final conductors was thoroughly investigated in order to improve their behavior in magnetic field. In particular we analyzed the effect of controlling the grain size in the powders by high energy ball milling and by lowering the synthesis temperature. TEM analysis results are shown that highlight the role played by the amorphous layer at the grain boundaries.

{\rm MgB}_{2}

In the present paper we report an in?situ high-energy x-ray diffraction analysis of MgB2 tapes during the preparation process. The experiment was performed in a specifically designed furnace working in a reducing atmosphere, compatible with the Laue diffraction condition. The? MgB2 synthesis was realized starting from MgH2 and amorphous B in powder form as precursors, varying reaction temperature and testing different cooling processes. We analysed both the MgB2 synthesis and the sintering process of tapes prepared with these powders. Phase evolution, micro-?and crystallographic structure were monitored during the different thermal treatments. Among the main results we observed the formation of MgB2 at an extraordinary low temperature (300??C), probably as a result of a solid-state reaction between MgH2 and B. Furthermore, we studied the dependence of the microstructure upon the thermal treatment and its effect on the critical current performance of the superconducting tapes.

Ex-situ Tapes and Wires

In dc and ac practical applications of MgB2 superconducting wires, an important role is represented by the material sheath that has to provide, among other things, a suitable electrical and thermal stabilization. One way to obtain a large-enough amount of low-resistivity material into the conductor architecture is to use it as the external sheath. In this paper, we study ex situ multifilamentary MgB2 wires fabricated with oxide-dispersion-strengthened copper (GlidCop) as the external sheath in order to reach a good compromise between critical current density and thermal properties. We prepared three GlidCop samples with different contents of dispersed submicroscopic Al2O3 particles. We characterized the superconducting and thermal properties, and we showed that the good thermal conductivity, together with the good mechanical properties and a reasonable critical current density, makes the GlidCop composite wire a useful conductor for applications where high thermal conductivity is required at temperatures above 30 K, such as superconducting fault-current limiters.

A Novel Process to Produce Amorphous Nanosized Boron Useful for

The ITER machine will require approximately 275 tons of Nb−Ti strands that will be used in poloidal field (PF) coils, correction coils (CC) and feeder busbars. The performance of all these conductors for the ITER machine is qualified by a short full-size sample (4 m) current sharing temperature (T cs) test in the SULTAN facility at CRPP in Villigen, Switzerland, at the design operating current and peak field. Three ITER domestic agencies participated in PF conductor fabrication (China, the European Union, Russia) while the conductors for feeder busbars and correction coils are entirely produced by the Chinese domestic agency. Each conductor type was qualified by the ITER International Organization after reaching T cs values in excess of ITER specifications. This qualification enabled the launch of procurement and industrial production of the Nb−Ti cable-in-conduit conductors in each domestic agency. In this paper, we summarize the performance of the qualified Nb−Ti samples of the ITER Project, comparing strand performance with conductor performance. The details of the test results will be discussed in terms of dc performance, ac losses and minimum quench energies of each conductor type.