René-Louis Flukiger

Effects of neutron irradiation on pinning force scaling in state-of-the-art Nb3Sn wires

We present an extensive irradiation study involving five state-of-the-art Nb3Sn wires which were subjected to sequential neutron irradiation up to a fast neutron fluence of 1.6 × 1022 m−2 (E > 0.1 MeV). The volume pinning force of short wire samples was assessed in the temperature range from 4.2 to 15 K in applied fields of up to 7 T by means of SQUID magnetometry in the unirradiated state and after each irradiation step. Pinning force scaling computations revealed that the exponents in the pinning force function differ significantly from those expected for pure grain boundary pinning, and that fast neutron irradiation causes a substantial change in the functional dependence of the volume pinning force. A model is presented, which describes the pinning force function of irradiated wires using a two-component ansatz involving a point-pinning contribution stemming from radiation induced pinning centers. The dependence of this point-pinning contribution on fast neutron fluence appears to be a universal function for all examined wire types.

Effect of quasi-hydrostatical radial pressure on Ic of Nb3Sn wires

High-performance Nb3Sn conductors are intended to be used in large-scale magnets like the International Thermonuclear Experimental Reactor (ITER) and in the upgrade of the Large Hadron Collider (LHC). Due to the occurrence of high electromagnetic forces, a detailed knowledge of the response of the critical current to the three-dimensional mechanical loads acting on the wires inside the cables is required. A detailed analysis of transverse stress effects on the critical current for powder-in-tube and bronze route Nb3Sn wires is presented. In an earlier publication, we have described the effect of transverse stress exerted on a Nb3Sn wire by means of two parallel plates. In the present paper, we analyse the effect of transverse stress exerted simultaneously by four walls on a wire being confined in a U-shaped groove. In order to get a more realistic picture of the situation of wires embedded in a Rutherford cable, the compression by four walls was also performed after impregnating the wire with epoxy in the same U-shaped groove. The result is very different from the case of pressing by means of two walls: the effect of pressure on Ic is now strongly reduced, which is attributed to the almost hydrostatic pressure in the U-shaped groove. This is further confirmed by the comparison between the effects of axial and transverse loads on the upper critical field and the pinning force. The present data are also compared against the effects of mechanical load on the critical current of cables in large-scale magnets.

Stress distribution and lattice distortions in Nb3Sn multifilament wires under uniaxial tensile loading at 4.2 K

The lattice parameter changes in three types of Nb3Sn superconducting wires during uniaxial stress–strain measurements at 4.2 K have been measured by high-energy synchrotron x-ray diffraction. The nearly-stress-free Nb3Sn lattice parameter has been determined using extracted filaments, and the elastic strain in the axial and transverse wire directions in the different wire phases has been calculated. The mechanical properties of the PIT and RRP wire are mainly determined by the properties of Nb3Sn and unreacted Nb. This is in contrast to the bronze route wire, where the matrix can carry substantial loads. In straight wires the axial Nb3Sn pre-strain is strongest in the bronze route wire, its value being smaller in the PIT and RRP wires. A strong reduction of the non-Cu elastic modulus of about 30% is observed during cool-down from ambient temperature to 4.2 K. The Nb3Sn Poisson ratio at 4.2 K measured in the untwisted bronze route wire is 0.35. The present study also shows that the process route has a strong influence on the Nb3Sn texture.

Variation of the Critical Properties of Alloyed Nb-Sn Wires After Proton Irradiation at 65 MeV and 24 GeV

A recent proton irradiation study on Ta and Ti alloyed industrial multifilamentary Nb3Sn wires has now been extended to fluences up to 1.38 × 1021 p/m2, the Bragg peak region being located outside of the wire. In the present work, magnetization measurements were performed up to 14 K and 10 T, to follow and determine with more precision the development of the upper critical field Bc2 with fluence. It was found that the critical temperature, Tc, decreases linearly with increasing fluences, about 3% up to the highest fluence 1.38 × 1021 p/m2. The transition width does not change after irradiation, thus reflecting a homogeneous damage in analogy to neutron irradiation. Both the critical current density, Jc, and the upper critical field, Bc2, were found to increase in the considered fluence range. It was obtained that the larger enhancement of Jc, (about 45% for Ta alloyed wires and 100% for Ti alloyed wires at 10 T) is not correlated to that of Bc2 (about 5% for Ti alloyed and 10% for Ta alloyed up to the highest fluence). The enhancement of Jc in Ta alloyed wires is very similar for both PIT and RRP processing, thus assigning a major importance to the nature of the additive. The present results after irradiation were analysed applying the two-mechanism model on the volume pinning force, taking into account both grain boundary pinning and point pinning. A comparison between the present results and those achieved after neutron irradiation on the same Nb3Sn wires shows that protons cause considerably higher damage than neutrons: the same effect on Jc and Tc is already observed at fluences being one order of magnitude smaller.

Variation of

The variation of <i>T</i>_c and <i>J</i>_c for a series of binary and ternary alloyed Nb₃Sn wires of the RRP and PIT type has been investigated after fast neutron irradiation (E>;0.1 MeV) at the TRIGA reactor in Vienna up to a fluence of φt = 1.4 × 10²² m^-2. In contrast to Ti alloyed bronze route Nb₃Sn wires, where a maximum is observed at around 0.4 ×10²² m^-2, it was found that the maximum of (<i>J</i>_c/<i>J</i>_c0) versus φ<i>t</i> in the present ternary alloyed wires was still not reached. A comparison with an earlier study on bronze route wires at RTNS-II shows that the variations of <i>T</i>_c versus φ<i>t</i> and <i>J</i>_c versus φ<i>t</i> do not scale: a substantially higher <i>J</i>_c value is now observed for the same <i>T</i>_c value. The degree of atomic ordering being comparable, the shift of the maximum of <i>J</i>_c/<i>J</i>_c0 with fluence is attributed to radiation induced “defect clusters.” Two observations are used for explaining the observed effect: (a) a shift of (<i>J</i>_c/<i>J</i>_c0)_max toward higher fields, observed for binary bronze wires with higher Sn contents and (b) by calorimetry, the average of the <i>T</i>_c distribution in the filaments of RRP and PIT wires is found to occur at values up to 1.5 K higher than in bronze wires. The present data suggest that the effect of “defect clusters” is higher for wires with higher average Sn contents.

{(J_{\rm c}/J_{{\rm c}0})}_{\max}

Superconducting magnets based on Nb3Sn technology are being developed for the LHC high luminosity upgrade. In this context, irradiation induced changes of the superconducting properties of state-of-the-art Ti and Ta doped Nb3 Sn wires are presently studied. During irradiation tests with protons of 65 MeV at the cyclotron of Université Catholique de Louvain (UCL), 1.4 GeV at the CERN ISOLDE facility, and 24 GeV at the CERN IRRAD1 facility, the superconductor samples become radioactive and their handling must follow the legal specifications for radioactive materials. We have estimated the activation and the resulting ambient dose equivalent rate up to a fluence of 1017 p/cm2 with the Monte-Carlo Code FLUKA. The estimates were verified with experimental activation spectra. Samples for magnetization measurements with a mass of approximately 20 mg have to be considered as radioactive, but they do not have the potential to harm the operators handling them at radiation levels below 1 μSv/h. Larger samples (longer wires and sample holder materials like Cu and TiAl6V4) show correspondingly higher ambient dose equivalent rates and activation levels and they must be handled by radiation workers in designated areas, taking into account the standard precautions for work with radioactive materials. The use of radiation shielding during handling is recommended.

of Binary and Ternary Alloyed RRP and PIT

Data on magnetization, magnetic susceptibility and microstructure modification of irradiated Nb3Sn platelets are presented. The irradiation was produced at room temperature by fast protons with the energies of 12.4 and 12.8 MeV with fluencies of 5x1017 and 1x1018 cm-2. Variation of the superconducting transition temperature versus irradiation dose was determined. Temperature dependence of magnetic susceptibility of Nb3Sn platelet with 160 μm thickness demonstrates several steps corresponding to different superconducting transition temperatures. We supposed that there are layers inside the sample with significantly different radiation damage levels caused by particles movement termination (Bragg peak). It was found that after the irradiation a lot of randomly oriented platelet-like Nb-enriched particles of 0.1-0.5 μm size appear in volumes with maximal damages.