Christophe Verwaerde

Investigation of ITER Use Internal-Sn

By means of a SQUID magnetometer, the irreversibility temperatures and magnetization transitions of an ITER-type internal-Sn Nb3Sn superconducting wire were measured during warming and cooling cycles at a fixed magnetic field. The results obtained show that the irreversibility temperature T*(H) is strongly dependent on A15 phase composition and can be used to optimize the heat treatment process for the internal-Sn Nb3Sn wire. The A15 phase composition in internal-Sn Nb3Sn wire is temperature independent for full-time reaction. From T*(H) and magnetization transition analysis, the heat treatment condition of our ITER use wire is optimized as 675 degC/128 h, which results in the best A15 phase composition suitable for high-field application

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

As the design of the Iseult/INUMAC 11.7 T MRI magnet involves new technical solutions using NbTi conductors (due to its larger dimensions), we developed a conductor R&D plan. Only the main coil conductor of this shielded magnet is covered by this R&D. Three types of design (monolith, cable in channel and cable around a copper core) have been developed by CEA and ALSTOM MSA.The main coil prototype conductors will have a minimum critical current of 1,500 A at 12 T and 2.8 K. Due to the unusual specified temperature, we translated the requirements to industrial standards (2,591 A at 9.5 T and 4.22 K taking into account 5% margin drop for R&D). The main characteristics of the conductor are firstly that it must have very precise and reproducible dimensions (tolerances lower than 15 ¿m) to enable the magnetic field homogeneity. Secondly, it must attain simultaneously a low electrical resistance (20 ¿¿/m at 10 K and 12 T) and a high mechanical strength (¿0.2% > 250 MPa). The aims of the industrial development are to validate the conductor performance obtained in the design conditions, especially critical current and conductor behavior during coil manufacturing. Several lengths have been produced for the 3 design types. The conductor prototypes are fully characterized by mechanical, electrical and geometrical measurements. From these results, several criteria enable selection of the most efficient conductor type.

Superconducting Wire Through Irreversibility Temperature Measurement

Through magnetization measurement with a SQUID magnetometer the heat treatment optimization of an international thermonuclear experimental reactor (ITER)-type internal-Sn Nb3Sn superconducting wire has been investigated. The irreversibility temperature T*(H), which is mainly dependent on A15 phase composition, was obtained by a warming and cooling cycle at a fixed field. The hysteresis width ΔM(H) which reflects the flux pinning situation of the A15 phase is determined by the sweeping of magnetic field at a constant temperature. The results obtained from differently heat-treated samples show that the combination of T*(H) with ΔM(H) measurement is very effective for optimizing the heat reaction process. The heat treatment condition of the ITER-type wire is optimized at 675°C/128 h, which results in a composition closer to stoichiometric Nb3Sn and a state with best flux pinning.

Conductor R&D for the Iseult/INUMAC Whole Body 11.7 T MRI Magnet

Four sets of monoelementary (ME) and two kinds of multifilamentary (MF) internal-Sn Nb3Sn superconducting strands were designed and fabricated, in which various component ratios, different composite configurations, and some third-element additions were arranged. All strands were submitted to a first heat treatment (HT) of 210 °C/50 h + 340 °C/25 h for Cu-Sn mixing, followed by the A15 phase formation HT. The four ME strands were reacted at 675 °C, 700 °C, and 725 °C for 100 and 200 h, respectively, and the two MF strands at 650 °C, 675 °C, 700 °C, and 725 °C for 128 and 200 h, respectively. The analysis of the reacted strands comprised the A15 phase composition distribution by means of X-ray energy-dispersive spectroscopy and the critical temperature Tc by means of superconducting quantum interference device magnetization measurements. The obtained results indicate that, for sufficiently reacted internal-Sn Nb3Sn strands, the final A15 phase composition and Tc are determined by the diffusion and solid reaction mechanism of the A15 phase formation. In particular, the onset Tc values and the average Sn content in a grain do not depend on the reaction temperature, the local compositions in the strand, the composite configuration arrangement, and the third-element addition.

Magnetization study of ITER-type internal-Sn Nb3Sn superconducting wire

Four sets of mono-element internal-Sn (MEIT) wires which have different Sn-Cu ratio and 1 at% Zr addition in one wire were designed and fabricated for investigating A15 phase formation kinetics of ND3 Sn superconductors. All samples underwent a 210degC/50 h + 340degC/25 h thermal duration for Cu-Sn alloying prior to the A15 phase formation heat treatment (HT). Four reaction temperatures of 650degC, 675degC, 700degC, and 725degC were chosen to study the temperature and time influence. All the heat- treated samples were examined by scanning electronic microscope for A15 layer thicknesses that were then plotted versus HT time at different temperatures and were nonlinearly fitted. Superconducting quantum interference device magnetization measurements were used to determine the inductive JC variations for the heat- treated wires. The obtained results demonstrate that the A15 phase growth is promoted by four factors: reaction temperature elevation, reaction time extension, Sn-Cu ratio enlargement, and the alloyed Zr addition. The phase formation kinetics of MEIT Nb3Sn superconducting wires is in agreement with Yn = K(T)t relation and the A15 growth exponent n is affected by HT temperature and Zr alloying.

Characterizations of A15 Phase Composition and

Composites made of niobium filaments within an oversaturated α-CuSn bronze matrix were processed by restack bundle drawing. The differences in mechanical behaviour of the various components entail an heterogeneity of deformation. Such a feature is mostly unfavourable not only for the phase transformations that govern the microstructure and the electromagnetic properties but also for the composite mechanical integrity. The obtained results finally demonstrate the chief importance of the selection of both the design of the composite and the processing steps.

T_{c}

The evolution of the distribution of strains and stresses within the multifilamentary composites designed for an improved bronze route is characterized all along their restack bundling process of fabrication. The results of numerous investigations of the microstructure are briefly compared to those of finite element simulations. The work compares the effects of the difference in mechanical behaviour of the various components to that of their repartition. The importance of the composite lay-out, that is to say of the relative disposition and of the volume of the bronze areas, as well as of the size, morphology and spacing of the filaments, for the Nb3Sn formation is demonstrated. The consequences for both the Nb3Sn grain size and the critical current are finally displayed.