Neutron imaging of sub-10-micrometres spatial resolution has been recently achieved in 2D mode within the framework of the Neutron Microscope project at the Paul Scherrer Institut. Here we report on the development of the PSI Neutron Microscope instrument and the results of the first microtomographic imaging experiment of multifilament superconducting MgB2 wire. The sample of MgB2 superconducting 37 multifilaments embedded in copper-nickel matrix was investigated –in microtomographic mode– with the scientific interest regarding the distribution of boron within the individual superconducting filaments (about 40μm in diameter). The resulting tomographic dataset revealed the distribution of boron within the entire 0.8mm thick multifilamental wire with the isotropic voxel size of 2.6 micrometres.

We have compared the mechanical properties and the degradation of the critical current after uniaxial tensile loading at room temperature (RT) and at 77 K of ex situ and in situ MgB2 wires. The strain that the wires can withstand without degradation is at 77 K substantially higher than at RT. In order to explain the mechanical behavior of the wires, the lattice distortions of the different wire constituents and their texture have been measured simultaneously with the composite wire stress and strain in a high-energy synchrotron beamline. The different MgB2 microstructure in both wire types is revealed in filament cross sections prepared by the focused-ion-beam technique and in fracture surfaces.

Porosity is one of several current limiting mechanisms in MgB2 wires. We have compared the microstructural homogeneity and the porosity distribution in different ex situ and in situ MgB2 powder-in-tube wires. The submicrometer structure was determined using focused ion beam nanotomography. The ex situ wires exhibit an isotropic microstructure, which has been quantified in terms of an identical tortuosity in transverse and longitudinal filament direction. The very homogenous microstructure in the new ex situ wire generation is probably one reason for its strongly improved critical current density. The in situ wire has an anisotropic microstructure with a lower tortuosity in the axial direction. The microstructural inhomogeneity of the in situ filaments makes microstructural characterization and the comparison between materials and superconducting properties particularly challenging.

The elastic anisotropy caused by the texture in the Nb 3 Sn filaments of PIT and RRP wires has been calculated by averaging the estimates of Voigt and Reuss, using published Nb 3 Sn single crystal elastic constants and the Nb 3 Sn grain orientation distribution determined in both wire types by Electron Backscatter Diffraction. At ambient temperature the calculated Nb 3 Sn E-moduli in axial direction in the PIT and the RRP wire are 130 GPa and 140 GPa, respectively. The calculated E-moduli are compared with tensile test results obtained for the corresponding wires and extracted filament bundles.

Some of the bronze-route Nb3Sn wires produced for the ITER toroidal field magnets have values of the Cu residual resistivity ratio (RRR) lower than the specification (RRR > 100) when reacted with the longest ITER heat treatment cycle (cycle A: high-temperature plateau 200 h at 650 °C). As the low RRR value was suspected to be due to the Cr plating, CERN has carried out extensive investigations on two different Cr-plated Nb3Sn wires in order to assess how the RRR is influenced by the Cr plating. Each type of wire was reacted with three heat treatments differing for the duration of the high-temperature plateau (from 100 to 200 h). The presence of Cr in the Cu stabilizer was investigated using a transmission electron microscope together with energy-dispersive X-ray spectrometry. In the most critical wire reacted for 200 h, CrS nanoparticles and Cr in solid solution (0.02 at.%) with Cu were found up to 13 μm from the Cr-Cu interface. At larger distance, we could not quantify the Cr concentrations because of the detector sensitivity limit (about 0.016 at.% Cr in Cu). We provide a model that can explain the measured RRR degradation purely in terms of the Cr contamination that we observed.

The elastic anisotropy caused by the texture in the Nb3Sn filaments of PIT and RRP wires has been calculated by averaging the estimates of Voigt and Reuss, using published Nb3Sn single crystal elastic constants and the Nb3Sn grain orientation distribution determined in both wire types by Electron Backscatter Diffraction. At ambient temperature the calculated Nb3Sn E-moduli in axial direction in the PIT and the RRP wire are 130 GPa and 140 GPa, respectively. The calculated E-moduli are compared with tensile test results obtained for the corresponding wires and extracted filament bundles.

For the LHC upgrade, CERN has launched a large program to develop next generation accelerator magnets based on high- Jc Nb3 Sn Rutherford cables. These magnets are characterized by a magnetic field and/or an aperture significantly larger than that of current Nb-Ti LHC magnets. The increased field/aperture will require coil pre-stresses much larger than 100 MPa. Since Nb3Sn cables are extremely sensitive to strain, critical current measurements under traverse compression are essential to estimate the transport current properties of the conductor within the magnet. To this purpose CERN has developed a sample holder (to be used in the FRESCA test station) that allows testing Rutherford cables under a transverse force of up to 2 MN/m. The new holder can house cable samples up to 1.8 m long and 20 mm wide. The large transverse force is only applied over the sample high field region, which is 70 cm long and over which the FRESCA dipole magnet generates a homogeneous fields of up to 10 T. Recently the critical current of the first cable sample has been measured at different transversal loads ranging from 90 MPa to 155 MPa. The measurement was carried out at 4.3 K on a 10-mm-wide Rutherford cable based on eighteen Powder In Tube (PIT) wires with a diameter of 1.0 mm. In this paper, the results of the test are reported, discussed and compared with recently measured data of the same single wire (1.0-mm PIT) tested under transverse loads.

For the LHC upgrade, CERN has launched a large program to develop next generation accelerator magnets based on high- Jc Nb3 Sn Rutherford cables. These magnets are characterized by a magnetic field and/or an aperture significantly larger than that of current Nb-Ti LHC magnets. The increased field/aperture will require coil pre-stresses much larger than 100 MPa. Since Nb3Sn cables are extremely sensitive to strain, critical current measurements under traverse compression are essential to estimate the transport current properties of the conductor within the magnet. To this purpose CERN has developed a sample holder (to be used in the FRESCA test station) that allows testing Rutherford cables under a transverse force of up to 2 MN/m. The new holder can house cable samples up to 1.8 m long and 20 mm wide. The large transverse force is only applied over the sample high field region, which is 70 cm long and over which the FRESCA dipole magnet generates a homogeneous fields of up to 10 T. Recently the critical current of the first cable sample has been measured at different transversal loads ranging from 90 MPa to 155 MPa. The measurement was carried out at 4.3 K on a 10-mm-wide Rutherford cable based on eighteen Powder In Tube (PIT) wires with a diameter of 1.0 mm. In this paper, the results of the test are reported, discussed and compared with recently measured data of the same single wire (1.0-mm PIT) tested under transverse loads.

The texture of Nb3Sn in recent multifilamentary composite wires has been studied by neutron diffraction, synchrotron x-ray diffraction and electron backscatter diffraction. In powder-in-tube (PIT) type superconductors the Nb precursor filaments exhibit a strong 〈110〉 fiber texture as a consequence of the severe cold drawing process, and a 〈110〉 texture is also observed in the Nb3Sn. In the Nb–Ta precursor of the restacked rod process (RRP) strand there is an additional texture component, and in both Ta-alloyed and Ti-alloyed RRP type conductors the Nb3Sn grains grow with a preferential 〈100〉 orientation.

The critical current density of the Nb3Sn superconductor is strongly dependent on the strain applied to the material. In order to investigate this dependence, it is a common practice to measure the critical current of Nb3Sn strands for different values of applied axial strain. In the literature, several models have been proposed to describe these experimental data in the reversible strain region. All these models are capable of fitting the measurement results in the strain region where data are collected, but tend to predict unphysical trends outside the range of data, and especially for large strain values. In this paper we present a model of a new strain function, together with the results obtained by applying the new scaling law on relevant datasets. The data analyzed consisted of the critical current measurements at 4.2 K that were carried out under applied axial strain at Durham University and the University of Geneva on different strand types. With respect to the previous models proposed, the new scaling function does not present problems at large strain values, has a lower number of fitting parameters (only two instead of three or four), and is very stable, so that, starting from few experimental points, it can estimate quite accurately the strand behavior in a strain region where there are no data. A relationship is shown between the proposed strain function and the elastic strain energy, and an analogy is drawn with the exponential form of the McMillan equation for the critical temperature.

High critical current density Nb3Sn wires (Jc > 2500 A/mm 2 at 4.2 K and 12 T) are the conductors considered for next-generation accelerator magnets. At present, the large magnetization of these strands is a concern within the scientific community because of the impact it might have on the magnet field quality. In order to characterize the magnetic behavior of these wires, an extensive campaign of magnetization measurements was launched at CERN. Powder-in-tube strands by Bruker-EAS and Restacked Rod Process strands by Oxford Superconducting Technology were measured between 0 and 10.5 T at different temperatures (ranging from 1.9 to 14.5 K). The samples, based on strands with different subelements dimensions (35 to 80 μm), were measured with a vibrating sample magnetometer. The experimental data were analyzed to: (1) calculate the effective filament size and the optimal parameters for the pinning force scaling law and (2) define the field-temperature region where there are flux jumps. It was found that the flux-jump can limit the maximum magnetization of the Nb3Sn wires and that the maximum magnetization at higher temperatures can be larger than the one at lower temperatures. In this paper, the experimental results and the analysis are reported and discussed.

High critical current density Nb3Sn wires (Jc > 2500 A/mm2 at 4.2 K and 12 T) are the conductors considered for next-generation accelerator magnets. At present, the large magnetization of these strands is a concern within the scientific community because of the impact it might have on the magnet field quality. In order to characterize the magnetic behavior of these wires, an extensive campaign of magnetization measurements was launched at CERN. Powder-in-tube strands by Bruker-EAS and Restacked Rod Process strands by Oxford Superconducting Technology were measured between 0 and 10.5 T at different temperatures (ranging from 1.9 to 14.5 K). The samples, based on strands with different subelements dimensions (35 to 80 μm), were measured with a vibrating sample magnetometer. The experimental data were analyzed to: (1) calculate the effective filament size and the optimal parameters for the pinning force scaling law and (2) define the field-temperature region where there are flux jumps. It was found that the flux-jump can limit the maximum magnetization of the Nb3Sn wires and that the maximum magnetization at higher temperatures can be larger than the one at lower temperatures. In this paper, the experimental results and the analysis are reported and discussed.

Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature (T-c) in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters T-c and B-c2 in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the T-c distribution for a commonly used reaction schedule (675 degrees C /84 h) and for an optimized heat treatment (625 degrees C /320 h). For the first time, we show that the wire reacted at 625 degrees C /320 h exhibits two separated contributions in the T-c distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower B-c2, and a broad peak due to the fine grains, with a higher B-c2.The kinetics of the Sn diffusion in Nb and the growth rate of the A15 layer were experimentally studied. The influence of Ta doping on the A15 phase formation was analysed by electron microscopy, the growth rate and the grain morphology in binary and Ta-alloyed Bronze route wires with the same filament layout being compared at different stages of the heat treatment. At the end of reaction, the well known microstructure comprising equiaxed and columnar regions was observed in the filaments of both the binary and the Ta-alloyed wires. Based on these observations and from the growth rate analysis we conclude that Ta does not affect the Sn diffusion rate.

Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters Tc and Bc2 in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the distribution for a commonly used reaction schedule (675 /84 h) and for an optimized heat treatment (625 /320 h). For the first time, we show that the wire reacted at 625/320 h exhibits two separated contributions in the distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower Bc2, and a broad peak due to the fine grains, with a higher Bc2. The kinetics of the Sn diffusion in Nb and the growth rate of the A15 layer were experimentally studied. The influence of Ta doping on the A15 phase formation was analysed by electron microscopy, the growth rate and the grain morphology in binary and Ta-alloyed Bronze route wires with the same filament layout being compared at different stages of the heat treatment. At the end of reaction, the well known microstructure comprising equiaxed and columnar regions was observed in the filaments of both the binary and the Ta-alloyed wires. Based on these observations and from the growth rate analysis we conclude that Ta does not affect the Sn diffusion rate.

Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters T c and B c2 in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the distribution for a commonly used reaction schedule (675 /84 h) and for an optimized heat treatment (625 /320 h). For the first time, we show that the wire reacted at 625/320 h exhibits two separated contributions in the distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower B c2 , and a broad peak due to the fine grains, with a higher B c2 . The kinetics of the Sn diffusion in Nb and the growth rate of the A15 layer were experimentally studied. The influence of Ta doping on the A15 phase formation was analysed by electron microscopy, the growth rate and the grain morphology in binary and Ta-alloyed Bronze route wires with the same filament layout being compared at different stages of the heat treatment. At the end of reaction, the well known microstructure comprising equiaxed and columnar regions was observed in the filaments of both the binary and the Ta-alloyed wires. Based on these observations and from the growth rate analysis we conclude that Ta does not affect the Sn diffusion rate.

Obtaining basic knowledge of breakdowns is desirable in many fields of physics; however, often the energy absorbed by a breakdown is not known or can vary by large amounts. We have therefore investigated how processing and breakdown properties scale with available energy for two materials, Cu and Mo, in the energy range of about 1 mJ to 1 J. A central result obtained is that there appears to be an optimum energy for processing and thus the highest possible electric field a processed material can sustain without breaking down depends on energy accordingly; what this implies for radio frequency cavity processing is discussed as well. For Cu, both the local field and the field enhancement factor showed only a weak dependence on energy; the average of the local field over the entire energy range investigated was (9.6 +/- 0.4) GV/m at breakdown. For Mo, the local field increased with increasing energy, while the field enhancement factor remained constant at 34 +/- 2. Finally, a possible explanation of the direct current (DC) processing mechanism-at least in the case of Mo-as an oxide removal process is presented.