[en] Here, a new design for the anode of a time projection chamber, consisting of a charge-detecting "tile", is investigated for use in large scale liquid xenon detectors. The tile is produced by depositing 60 orthogonal metal charge-collecting strips, 3 mm wide, on a 10 cm × 10 cm fused-silica wafer. These charge tiles may be employed by large detectors, such as the proposed tonne-scale nEXO experiment to search for neutrinoless double-beta decay. Modular by design, an array of tiles can cover a sizable area. The width of each strip is small compared to the size of the tile, so a Frisch grid is not required. A grid-less, tiled anode design is beneficial for an experiment such as nEXO, where a wire tensioning support structure and Frisch grid might contribute radioactive backgrounds and would have to be designed to accommodate cycling to cryogenic temperatures. The segmented anode also reduces some degeneracies in signal reconstruction that arise in large-area crossed-wire time projection chambers. A prototype tile was tested in a cell containing liquid xenon. Very good agreement is achieved between the measured ionization spectrum of a ²⁰⁷Bi source and simulations that include the microphysics of recombination in xenon and a detailed modeling of the electrostatic field of the detector. An energy resolution σ/E=5.5% is observed at 570 keV, comparable to the best intrinsic ionization-only resolution reported in literature for liquid xenon at 936 V/cm.

[en] Nb₃Sn strands for high-current, high-field magnets must be cabled before reaction while the conductor is still composed of ductile components. Even though still in the ductile, deformable state, significant damage can occur in this step, which expresses itself by inhomogeneous A15 formation, Sn leakage or even worse effects during later reaction. In this study, we simulate cabling damage by rolling recent high performance powder-in-tube (PIT) and internal tin (IT) strands in controlled increments, applying standard Nb₃Sn reaction heat treatments, and then examining the local changes using magneto-optical imaging (MOI), scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). These combined characterizations allow any local damage to the filament architecture to be made clear. MOI directly reveals the local variation of superconductivity while CLSM is extremely sensitive in revealing Sn leakage beyond the diffusion barrier into the stabilizing Cu. These techniques reveal a markedly different response to deformation by the PIT and IT strands. The study demonstrates that these tools can provide a local, thorough, and detailed view of how strands degrade and thus complement more complex extracted strand studies.

[en] The ITER toroidal field (TF) strand procurement initiated the largest Nb₃Sn superconducting strand production hitherto. The industrial-scale production started in Japan in 2008 and finished in summer 2015. Six ITER partners (so-called Domestic Agencies, or DAs) are in charge of the procurement and involved eight different strand suppliers all over the world, of which four are using the bronze route (BR) process and four the internal-tin (IT) process. In total more than 500 tons have been produced including excess material covering losses during the conductor manufacturing process, in particular the cabling. The procurement is based on a functional specification where the main strand requirements like critical current, hysteresis losses, Cu ratio and residual resistance ratio are specified but not the strand production process or layout. This paper presents the analysis on the data acquired during the quality control (QC) process that was carried out to ensure the same conductor performance requirements are met by the different strand suppliers regardless of strand design. The strand QC is based on 100% billet testing and on applying statistical process control (SPC) limits. Throughout the production, samples adjacent to the strand pieces tested by the suppliers are cross-checked (‘verified’) by their respective DAs reference labs. The level of verification was lowered from 100% at the beginning of the procurement progressively to approximately 25% during the final phase of production. Based on the complete dataset of the TF strand production, an analysis of the SPC limits of the critical strand parameters is made and the related process capability indices are calculated. In view of the large-scale production and costs, key manufacturing parameters such as billet yield, number of breakages and piece-length distribution are also discussed. The results are compared among all the strand suppliers, focusing on the difference between BR and IT processes. Following the completion of the largest Nb₃Sn strand production, our experience gained from monitoring the execution of the QC activities and from auditing the results from the measurements is summarised for future superconducting strand material procurement activities. (paper)

[en] Systematic studies of the intrinsic irreversible strain limit ε_i_r_r_,_0, microstructure, and microchemistry were made on several internal-tin Nb_3Sn pre-production wires, fabricated for the domestic agencies of the USA and China participating in the International Thermonuclear Experimental Reactor. These wires were produced by Luvata, Oxford Superconducting Technology (OST), and Western Superconducting Technologies (WST), and were intended for the tokamak’s toroidal-field coils. The results of this study show that, for a final heat-treatment at 650 °C to form the A15 phase, both ε_i_r_r_,_0 and the de-pinning field B_c_2* improved by increasing heat-treatment duration beyond 100 h for the Luvata wires. On the other hand, we saw no improvement in these two parameters as a function of heat-treatment duration in the OST wires. Furthermore, micro-chemical analysis of OST wires revealed that some Nb_3Sn filaments have a Sn- and Ti-rich phase at the interface between Cu(Sn) matrix and Nb_3Sn in the form of a shell around individual filaments. This phase is far less prominent in the Luvata and WST conductors, and could inhibit diffusion of Sn and Ti into Nb_3Sn filaments during the reaction and may potentially be the reason for the lack of noticeable change in B_c_2* with heat-treatment duration in the OST wires. The increase of ε_i_r_r_,_0 and B_c_2* with heat-treatment duration in the Luvata wires and the lack of increase in the OST wires may suggest a possible correlation between ε_i_r_r_,_0 and the stoichiometry of the A15 composition. Investigation of the samples’ microstructure revealed only a small number of cracked Nb_3Sn filaments despite the significant and permanent degradation of their critical current I_c when subjected to longitudinal tensile strain ε beyond ε_i_r_r_,_0. The scarcity of cracks indicate that I_c(ε) measurements are highly sensitive to crack formation in Nb_3Sn filaments, especially at low electric-field criteria not≦0.1 μV cm"−"1, even when the sizes of the individual filaments are only few micrometers. All the strands contained substantial Kirkendall porosity, but we found that the quantity and distribution of the Kirkendall voids vary significantly with strand design. Luvata wires have the least porosity, followed by WST wires, and then by OST strands. However, even though the presence of cracks in the Nb_3Sn filaments that are in close proximity to Kirkendall voids suggest a correlation between crack initiation and the proximity of the filaments to these voids, the porosity investigation established no definitive relationship between porosity and ε_i_r_r_,_0 in the wires studied. (paper)

[en] The performance of the toroidal field (TF) magnet conductors for the ITER machine are qualified by a short full-size sample (4 m) current sharing temperature (T_cs) test in the SULTAN facility at CRPP in Villigen, Switzerland, using the operating current of 68 kA and the design peak field of 11.8 T. Several samples, including at least one from each of the six ITER Domestic Agencies participating in TF conductor fabrication (China, European Union, Japan, Russia, South Korea and the United States), have been qualified by the ITER Organization after achieving T_cs values of 6.0–6.9 K, after 700–1000 electromagnetic cycles. These T_cs values exceed the ITER specification and enabled the industrial production of these long-lead items for the ITER tokamak to begin in each Domestic Agency. Some of these samples did not pass the qualification test. In this paper, we summarize the performance of the qualified samples, analyze the effect of strand performance on conductor performance, and discuss the details of the test results. (paper)

[en] Scaling of the pinning force (F_p) with temperature, field and strain for Nb₃Sn wires is widely discussed but important issues are still unresolved, for example whether empirical engineering-oriented scaling functions can be replaced by more microscopic, physics-based models, which take account of the fact that F_p becomes zero at an irreversibility field (H*) less than the upper critical field (H_c2). In the present work we compare earlier extensive measurements of the critical current density (J_c) of an ITER-benchmark bronze wire to new measurements of H* and H_c2. Our study was also made in different strain states. We observe that the Kramer extrapolations, which are based on transport data taken up to 13 T, significantly overestimate the low temperature, high field behaviour and also underestimate the critical temperature (T_c) at zero field. An attempt is made to connect measured H_c2(T, ε) data on this practical conductor to microscopic theory. We also discuss the inevitable differences that occur between fully penetrated J_c data which average over all the A15 layers and small current-density probes of the H_c2 transition which are characteristics of the best part of the layer. We connect in this way the large-scale 'average' characteristics of J_c(H, T, ε) to small current 'best' characteristics of H_c2(T, ε)