Enhanced implosion stability has been experimentally demonstrated for magnetically accelerated liners that are coated with 70 μm of dielectric. The dielectric tamps liner-mass redistribution from ...electrothermal instabilities and also buffers coupling of the drive magnetic field to the magneto-Rayleigh-Taylor instability. A dielectric-coated and axially premagnetized beryllium liner was radiographed at a convergence ratio CR=Rin,0/Rin(z,t) of 20, which is the highest CR ever directly observed for a strengthless magnetically driven liner. The inner-wall radius Rin(z,t) displayed unprecedented uniformity, varying from 95 to 130 μm over the 4.0 mm axial height captured by the radiograph.
Magnetizing the fuel in inertial confinement fusion relaxes ignition requirements by reducing thermal conductivity and changing the physics of burn product confinement. Diagnosing the level of fuel ...magnetization during burn is critical to understanding target performance in magneto-inertial fusion (MIF) implosions. In pure deuterium fusion plasma, 1.01 MeV tritons are emitted during deuterium-deuterium fusion and can undergo secondary deuterium-tritium reactions before exiting the fuel. Increasing the fuel magnetization elongates the path lengths through the fuel of some of the tritons, enhancing their probability of reaction. Based on this feature, a method to diagnose fuel magnetization using the ratio of overall deuterium-tritium to deuterium-deuterium neutron yields is developed. Analysis of anisotropies in the secondary neutron energy spectra further constrain the measurement. Secondary reactions also are shown to provide an upper bound for the volumetric fuel-pusher mix in MIF. The analysis is applied to recent MIF experiments M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014) on the Z Pulsed Power Facility, indicating that significant magnetic confinement of charged burn products was achieved and suggesting a relatively low-mix environment. Both of these are essential features of future ignition-scale MIF designs.
ABSTRACT Gamma-ray binaries consist of a neutron star or a black hole interacting with a normal star to produce gamma-ray emission that dominates the radiative output of the system. Only a handful of ...such systems have been previously discovered, all within our Galaxy. Here, we report the discovery of a luminous gamma-ray binary in the Large Magellanic Cloud, found with the Fermi Large Area Telescope (LAT), from a search for periodic modulation in all sources in the third Fermi LAT catalog. This is the first such system to be found outside the Milky Way. The system has an orbital period of 10.3 days, and is associated with a massive O5III star located in the supernova remnant DEM L241, previously identified as the candidate high-mass X-ray binary (HMXB) CXOU J053600.0-673507. X-ray and radio emission are also modulated on the 10.3 day period, but are in anti-phase with the gamma-ray modulation. Optical radial velocity measurements suggest that the system contains a neutron star. The source is significantly more luminous than similar sources in the Milky Way, at radio, optical, X-ray, and gamma-ray wavelengths. The detection of this extra-galactic system, but no new Galactic systems, raises the possibility that the predicted number of gamma-ray binaries in our Galaxy has been overestimated, and that HMXBs may be born containing relatively slowly rotating neutron stars.
The objectives of this tutorial are as follows: 1) to help students and researchers develop a basic understanding of how pulsed-power systems are used to create high-energy-density (HED) matter; 2) ...to develop a basic understanding of a new, compact, and efficient pulsed-power technology called linear transformer drivers (LTDs); 3) to understand why LTDs are an attractive technology for driving HED physics (HEDP) experiments; 4) to contrast LTDs with the more traditional Marx-generator/pulse-forming-line approach to driving HEDP experiments; and 5) to briefly review the history of LTD technology as well as some of the LTD-driven HEDP research presently underway at universities and research laboratories across the globe. This invited tutorial is part of the Mini-Course on Charged Particle Beams and High-Powered Pulsed Sources, held in conjunction with the 44th International Conference on Plasma Science in May of 2017.
We report the identification from multiwavelength observations of the Fermi Large Area Telescope (LAT) source 4FGL J1405.1−6119 (=3FGL J1405.4−6119) as a high-mass gamma-ray binary. Observations with ...the LAT show that gamma-ray emission from the system is modulated at a period of 13.7135 0.0019 days, with the presence of two maxima per orbit with different spectral properties. X-ray observations using the Neil Gehrels Swift Observatory X-ray Telescope show that X-ray emission is also modulated at this period, but with a single maximum that is closer to the secondary lower-energy gamma-ray maximum. A radio source, coincident with the X-ray source is also found from Australia Telescope Compact Array observations, and the radio emission is modulated on the gamma-ray period with similar phasing to the X-ray emission. A large degree of interstellar obscuration severely hampers optical observations, but a near-infrared counterpart is found. Near-infrared spectroscopy indicates an O6 III spectral classification. This is the third gamma-ray binary to be discovered with the Fermi-LAT from periodic modulation of the gamma-ray emission, the other two sources also have early O star, rather than Be star, counterparts. We consider at what distances we can detect such modulated gamma-ray emission with the LAT, and examine constraints on the gamma-ray binary population of the Milky Way.
We have developed a physics-based transmission-line-circuit model of the Z pulsed-power accelerator. The 33-m-diameter Z machine generates a peak electrical power as high as 85 TW, and delivers as ...much as 25 MA to a physics load. The circuit model is used to design and analyze experiments conducted on Z. The model consists of 36 networks of transmission-line-circuit elements and resistors that represent each of Zs 36 modules. The model of each module includes a Marx generator, intermediate-energy-storage capacitor, laser-triggered gas switch, pulse-forming line, self-break water switches, and tri-plate transmission lines. The circuit model also includes elements that represent Zs water convolute, vacuum insulator stack, four parallel outer magnetically insulated vacuum transmission lines (MITLs), double-post-hole vacuum convolute, inner vacuum MITL, and physics load. Within the vacuum-transmission-line system the model conducts analytic calculations of current loss. To calculate the loss, the model simulates the following processes: (i) electron emission from MITL cathode surfaces wherever an electric-field threshold has been exceeded; (ii) electron loss in the MITLs before magnetic insulation has been established; (iii) flow of electrons emitted by the outer-MITL cathodes after insulation has been established; (iv) closure of MITL anode-cathode (AK) gaps due to expansion of cathode plasma; (v) energy loss to MITL conductors operated at high lineal current densities; (vi) heating of MITL-anode surfaces due to conduction current and deposition of electron kinetic energy; (vii) negative-space-charge-enhanced ion emission from MITL anode surfaces wherever an anode-surface-temperature threshold has been exceeded; and (viii) closure of MITL AK gaps due to expansion of anode plasma. The circuit model is expected to be most accurate when the fractional current loss is small. We have performed circuit simulations of 52 Z experiments conducted with a variety of accelerator configurations and load-impedance time histories. For these experiments, the apparent fractional current loss varies from 0% to 20%. Results of the circuit simulations agree with data acquired on 52 shots to within 2%.
Concern exists about accepting live kidney donation from ‘medically complex donors’—those with risk factors for future kidney disease. This study's aim was to examine variation in complex kidney ...donor use across US transplant centers. We conducted a retrospective cohort study of live kidney donors using organ procurement and transplantation network data. Donors with hypertension, obesity or estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m2 were considered medically complex. Among 9319 donors, 2254 (24.2%) were complex: 1194 (12.8%) were obese, 956 (10.3%) hypertensive and 392 (4.2%) had low eGFR. The mean proportion of medically complex donors at a center was 24% (range 0–65%). In multivariate analysis, donor characteristics associated with medical complexity included spousal relationship to the recipient (OR 1.29, CI 1.06–1.56, p < 0.01), low education (OR 1.19, CI 1.04–1.37, p = 0.01), older age (OR 1.01 per year, CI 1.01–1.02, p < 0.01) and non‐US citizenship (OR 0.70, CI 0.51–0.97, p = 0.03). Renal transplant centers with the highest transplant volume (OR 1.26, CI 1.02–1.57, p = 0.03), and with a higher proportion of (living donation)/(all kidney transplants) (OR 1.97, CI 1.23–3.16, p < 0.01) were more likely to use medically complex donors. Though controversial, the use of medically complex donors is widespread and varies widely across centers.
A retrospective study using UNOS data demonstrated that 24% of live kidney donors had obesity, hypertension, or low estimated glomerular filtration rate at the time of donor nephrectomy.
Structured protocols offer a transparent and systematic way to elicit and combine/aggregate, probabilistic predictions from multiple experts. These judgements can be aggregated behaviourally or ...mathematically to derive a final group prediction. Mathematical rules (e.g., weighted linear combinations of judgments) provide an objective approach to aggregation. The quality of this aggregation can be defined in terms of accuracy, calibration and informativeness. These measures can be used to compare different aggregation approaches and help decide on which aggregation produces the "best" final prediction. When experts' performance can be scored on similar questions ahead of time, these scores can be translated into performance-based weights, and a performance-based weighted aggregation can then be used. When this is not possible though, several other aggregation methods, informed by measurable proxies for good performance, can be formulated and compared. Here, we develop a suite of aggregation methods, informed by previous experience and the available literature. We differentially weight our experts' estimates by measures of reasoning, engagement, openness to changing their mind, informativeness, prior knowledge, and extremity, asymmetry or granularity of estimates. Next, we investigate the relative performance of these aggregation methods using three datasets. The main goal of this research is to explore how measures of knowledge and behaviour of individuals can be leveraged to produce a better performing combined group judgment. Although the accuracy, calibration, and informativeness of the majority of methods are very similar, a couple of the aggregation methods consistently distinguish themselves as among the best or worst. Moreover, the majority of methods outperform the usual benchmarks provided by the simple average or the median of estimates.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Magnetically driven implosions are susceptible to magnetohydrodynamic instabilities, including the magneto-Rayleigh-Taylor instability (MRTI). To reduce MRTI growth in solid-metal liner implosions, ...the use of a dynamic screw pinch (DSP) has been proposed P. F. Schmit et al., Phys. Rev. Lett. 117, 205001 (2016). In a DSP configuration, a helical return-current structure surrounds the liner, resulting in a helical magnetic field that drives the implosion. Here, we present the first experimental tests of a solid-metal liner implosion driven by a DSP. Using the 1-MA, 100–200-ns COBRA pulsed-power driver, we tested three DSP cases (with peak axial magnetic fields of 2 T, 14 T, and 20 T) and a standard z -pinch (SZP) case (with a straight return-current structure and thus zero axial field). The liners had an initial radius of 3.2 mm and were made from 650-nm-thick aluminum foil. Images collected during the experiments reveal that helical MRTI modes developed in the DSP cases, while nonhelical (azimuthally symmetric) MRTI modes developed in the SZP case. Additionally, the MRTI amplitudes for the 14-T and 20-T DSP cases were smaller than in the SZP case. Specifically, when the liner had imploded to half of its initial radius, the MRTI amplitudes for the SZP case and for the 14-T and 20-T DSP cases were, respectively, 1.1 ± 0.3 mm , 0.7 ± 0.2 mm , and 0.3 ± 0.1 mm . Relative to the SZP, the stabilization obtained using the DSP agrees reasonably well with theoretical estimates.
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