The fusion power density produced in a tokamak is proportional to its magnetic field strength to the fourth power. Second-generation high temperature superconductor (2G HTS) wires demonstrate ...remarkable engineering current density (averaged over the full wire), JE, at very high magnetic fields, driving progress in fusion and other applications. The key challenge for HTS wires has been to offer an acceptable combination of high and consistent superconducting performance in high magnetic fields, high volume supply, and low price. Here we report a very high and reproducible JE in practical HTS wires based on a simple YBa2Cu3O7 (YBCO) superconductor formulation with Y2O3 nanoparticles, which have been delivered in just nine months to a commercial fusion customer in the largest-volume order the HTS industry has seen to date. We demonstrate a novel YBCO superconductor formulation without the c-axis correlated nano-columnar defects that are widely believed to be prerequisite for high in-field performance. The simplicity of this new formulation allows robust and scalable manufacturing, providing, for the first time, large volumes of consistently high performance wire, and the economies of scale necessary to lower HTS wire prices to a level acceptable for fusion and ultimately for the widespread commercial adoption of HTS.
A study of the quench propagation velocity in coated conductors produced by various manufacturers is presented. Based on an experimental investigation of the thermal and electrical properties of the ...coated conductors, the longitudinal normal zone propagation velocity has been calculated as a function of the temperature and magnetic field, and for different orientations of the field with respect to the tape surface. Contrary to what observed in low-temperature superconductors, we have found that the quench propagation velocity in REBa 2 Cu 3 O 7-x coated conductors is weakly affected by the magnetic field and temperature, and is mainly determined by the operating current. The longitudinal normal zone propagation velocity at a given operating current assumes comparable values in the examined coated conductors, regardless of the differences in the structural, thermal, and electrical properties. The normal zone propagation velocity in the direction perpendicular to the tape surface has also been studied, starting from experimental data of the longitudinal and transverse thermal conductivities of the coated conductors.
Individualized education is emerging as an innovative model for physician training. This requires faculty coaching to guide learners' achievements in academic performance, competency development, and ...career progression. In addition, coaching can foster self-reflection and self-monitoring using a data-guided approach to support lifelong learning.
Coaching differs from mentoring or advising, and its application in medical education is novel. Because of this, definitions of the concept and the constructs of coaching as applied to medical education are needed to accurately assess the coaching relationship and coaching processes. These can then be linked to learner outcomes to inform how coaching serves as a modifier of academic and competency achievement and career satisfaction.
We developed definitions and constructs for academic coaching in medical education based on review of existing education and non-education coaching literature. These constructs focus on 1) establishing relationship principles, 2) conducting learner assessments, 3) developing and implementing an action plan, and 4) assessing results and revising plans accordingly.
Coaching is emerging as an important construct in the context of medical education. This article lays the vital groundwork needed for evaluation of coaching programs aimed at producing outstanding physicians.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Adensecoating on complex substrates is a challenge in physical vapor deposition.•Niobium on copper coatings are investigatedwith different sputtering techniques.•High power impulse magnetron ...sputtering with positive pulse improves film density.•Preliminary validation for superconducting radio-frequency cavities is performed.
The production of a dense and void-free thin film on large and complex substrates is still a challenge in Physical Vapor Deposition. High Power Impulse Magnetron Sputtering (HiPIMS) with voltage inversion (positive pulse) after the main negative pulse is an attractive alternative to a negatively biased substrate to improve the film properties. In this manuscript, the properties of Nb thin-films deposited on flat Cu samples at different incidence angles with respect to the sputtered Nb target are investigated for various coating techniques. In particular, the results obtained using HiPIMS with the application of a positive voltage are compared with those resulting from negatively biased substrates, using as a reference films coated with the consolidated technique of Direct Current Magnetron Sputtering. Images of the film cross section obtained with a Focused Ion Beam - Scanning Electron Microscope enable to assess film morphology and local thickness, which is compared with the value obtained from X-ray Fluorescence measurements. Differences in the film morphology are highlighted for samples placed perpendicularly to the surface of the sputtered target. A significant densification for HiPIMS in the presence of a positive pulse is observed. The application of this approach to the coatings of superconducting radio-frequency cavities is discussed.
High temperature superconductors, especially rare-earth barium copper oxide (REBa2Cu3O7−x or REBCO) tapes are an enabling technology for magnetic fields beyond 23.5 T, the limit of the existing ...technology based on the low temperature superconductors (LTS) niobium-titanium (NbTi) and niobium-tin (Nb3Sn). Due to their higher prices, REBCO tapes are in general only employed as insert coils in an LTS outsert. Due to the tape geometry, the effect of the screening currents is high in REBCO insert coils. The resulting screening current-induced field (SCIF) can distort the shape and magnitude as well as affect the temporal stability of the field after charging. Their influence has to be well understood and it can be a limiting factor in field quality critical applications such as nuclear magnetic resonance magnets. In this work, we present the test results of a layer-wound 4 T REBCO insert coil operated in the background of a 21 T LTS outsert reaching a combined field of 25 T. Using an array of 13 calibrated Hall sensors, we investigate the spatial and temporal effects of the SCIFs at different background fields. Series expansion is employed to separate the contributions of the insert and of the outsert. We present in this work the characterization of the effect of the SCIF on the shape of the field and on its temporal stability in background fields from 0 T up to 19 T. We compare the experimental results with existing simulation models for the validation of the model at high magnetic fields, which had never been done for layer-wound REBCO coils. Based on the experimental data, we demonstrate the necessity of SCIF-aware simulations for REBCO insert coils, regardless of the insert's relative operating current. Before summarizing and concluding the work, we give a short overview of proposed methods for SCIF reduction in REBCO coils from the literature.
Abstract
In the framework of the Future Circular Collider Study, the development of thin-film coated superconducting radio-frequency copper cavities capable of providing higher accelerating fields ...(10–20 MV m
−1
against 5 MV m
−1
for the Large Hadron Collider) represents a major challenge. The method investigated here for the production of seamless niobium-coated copper cavities is based on the electroforming of the copper structure around a sacrificial aluminium mandrel that is pre-coated with a niobium thin film. The first feasibility study, applied to a flat aluminium disk mandrel, is presented. Protective precautions are taken towards the functional niobium film during the production process and it is shown that this technique can deliver well performing niobium films on a seamless copper substrate. This way, the non-trivial chemical treatments foreseen by the standard procedures (e.g. SUBU, EP) for the preparation of the copper surface to achieve the proper adhesion of the niobium layer are also avoided. The only major chemical treatment involved in the reverse-coating method is represented by the chemical dissolution of the aluminium mandrel, which has the advantage of not affecting the copper substrate and therefore the copper-niobium interface.
We evaluated various heat treatments (HT) for maximizing the Nb 3 Sn layer thickness while retaining a refined grain microstructure in low filament count internal-Sn Nb 3 Sn Rod-In-Tube wires with ...internally oxidized nanoparticles. These wires were manufactured in our laboratory using SnO 2 as oxygen source and Nb alloys containing Ta and Zr or Hf. By reacting the wires at 650 °C for 200 hours we obtained relatively thin reaction layers but high layer critical current densities (layer J C ) of ∼3000 A/mm 2 for Hf-containing wires and ∼2700 A/mm 2 for Zr-containing wires, both at 4.2 K and 16 T. Notably, both of these values are over the layer J C threshold of 2500 A/mm 2 , which is estimated to be necessary for attaining the corresponding Future Circular Collider (FCC) target non-Cu J C of 1500 A/mm 2 . Following this heat treatment, the fine-grained Nb 3 Sn area occupies only ∼35% of the filament area for Hf-containing wires and ∼20% for Zr-containing wires. After heat treatments with a reaction step at 700 °C these values increase to 70-80% and ∼60%, respectively, with only a minor increase of the grain size. However, we observed a noticeable decrease in the layer J C for these HT. Magnetic measurements show that the high J C wires exhibit a point defect contribution from precipitates to the pinning force, which is missing in wires with depressed J C values. The higher heat treatment temperatures may have caused excessive coarsening of the oxide precipitates, to sizes unsuitable for flux pinning. Reaction heat treatment temperatures in the range of 650 °C to 700 °C and durations between 50 and 200 hours may provide a better compromise between the Nb 3 Sn layer thickness, its grain size and nanoparticle size.
Internal oxidation can achieve significantly enhanced Jc in Nb3Sn wires, but the mechanism of oxygen transport and oxide precipitation is not fully understood. Our investigation employs X-ray ...Absorption Near-Edge Structure (XANES) spectroscopy, revealing insights into the oxidation of Zr and its interaction with oxygen in different areas of the wire cross section. We discovered that in samples reacted at 650 °C the majority of Zr in the Nb3Sn layer is oxidized as ZrO2, whereas it predominantly remains non-oxidized within the residual alloy. This is an interesting finding especially for samples where oxygen has to diffuse first through the entire layer of Nb alloy to reach the regions where Nb3Sn will form. The onset critical temperature ( Tc ) of the residual Nb alloy was the lowest in such samples, most probably due to a higher content of interstitial oxygen resulting from the diffusion gradient. This report adds to existing indications that ZrO2 precipitates in superconductors employing internal oxidation are only located within the Nb3Sn layer and opens new avenues of research on the formation of this precipitates.
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