Electrothermal instability plays an important role in applications of current-driven metal, creating striations (which seed the magneto-Rayleigh-Taylor instability) and filaments (which provide a ...more rapid path to plasma formation). However, the initial formation of both structures is not well understood. Simulations show for the first time how a commonly occurring isolated defect transforms into the larger striation and filament, through a feedback loop connecting current and electrical conductivity. Simulations have been experimentally validated using defect-driven self-emission patterns.
We report the magneto-Rayleigh-Taylor instability (MRTI) plays an essential role in astrophysical systems and in magneto-inertial fusion, where it is known to be an important degradation mechanism of ...confinement and target performance. In this Letter, we show for the first time experimental evidence of mode mixing and the onset of an inverse-cascade process resulting from the nonlinear coupling of two discrete preseeded axial modes (400- and 550 - μm wavelengths) on an Al liner that is magnetically imploded using the 20-MA, 100-ns rise-time Z Machine at Sandia National Laboratories. Four radiographs captured the temporal evolution of the MRTI. We introduce a novel unfold technique to analyze the experimental radiographs and compare the results to simulations and to a weakly nonlinear model. We find good quantitative agreement with simulations using the radiation magnetohydrodynamics code hydra. Spectral analysis of the MRTI time evolution obtained from the simulations shows evidence of harmonic generation, mode coupling, and the onset of an inverse-cascade process. The experiments provide a benchmark for future work on the MRTI and motivate the development of new analytical theories to better understand this instability.
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.
Advances in rail materials from conventional rail steels to those with higher yield points and the potential of additively manufactured laser clad coatings to improve the durability of railway track ...components presents a new challenge in characterisation. Many of these new and novel materials have either limited test samples available or are more resistant to strain and therefore present challenges in characterisation. The method reported here uses twin disc tests to simulate cyclic loading experienced by rail steel in service. A sample from a single test condition is analysed, measuring the shear yield stress and the accumulated shear strain at multiple depths below the contact surface, from which a Shear Yield Stress – Plastic Shear Strain (SYS-PSS) relationship is extracted. Knowledge of the stress history of a rail sample is not required to apply the method and minimal samples are required, providing a technique which can be used on rail steel samples removed from service.
Questions: (1) Which disturbances (harvesting, browsing, drought and fire) are important for tree mortality in a tropical dry forest (TDF), and do these change with size class (juvenile, sapling, ...adult)? (2) How is mortality and recruitment in a TDF related to soil moisture content (SMC) and does the relationship change for different size classes? (3) Is disturbance-related mortality selectively removing particular species from the TDF? Location: Sonebhadra and Mirzapur districts, Uttar Pradesh, India. Methods: We analysed the structure of a TDF in central India in terms of the tree composition of juveniles, saplings and adults at five distinct sites located along a gradient of SMC, and recorded the numbers of individuals in each size class killed by the four disturbance types over 2 yr. We also recorded total stem density and recruitment at each site. We compared annual mortality index (AMI) and its four disturbance components (harvesting, browsing, drought and fire) and annual recruitment index (ARI) against the mean SMC of each site using GLMM. Results: The impact of all disturbances on total AMI decreased as SMC increased whereas ARI increased as SMC increased. Mortality due to harvesting was substantially higher than other disturbances for adult and sapling trees, whereas both harvesting and browsing were important drivers of mortality for juveniles. There was little evidence that particular species were being deliberately selected for harvesting across sites. Conclusions: Tree saplings and adults in this TDF were mainly killed by harvesting, indicating that anthropogenic impacts on tree mortality are more important than non-anthropogenic impacts in the TDF, and impacts of all disturbances become more severe with increasing water stress. Thus changes in TDF structure due to harvesting are likely to be more rapid in more arid environments.
Novel experimental data are reported that reveal helical instability formation on imploding z-pinch liners that are premagnetized with an axial field. Such instabilities differ dramatically from the ...mostly azimuthally symmetric instabilities that form on unmagnetized liners. The helical structure persists at nearly constant pitch as the liner implodes. This is surprising since, at the liner surface, the azimuthal drive field presumably dwarfs the axial field for all but the earliest stages of the experiment. These fundamentally 3D results provide a unique and challenging test for 3D-magnetohydrodynamics simulations.
Hunting and deforestation are the two biggest threats to vertebrates in Southeast Asia. In the last 50 years, monoculture rubber plantations replaced large areas of tropical rainforests in ...Xishuangbanna, southwest China. We set up camera traps at 109 stations (57 in forest reserves and 52 in rubber plantations) to determine the distribution of mammal species in Menglun, Xishuangbanna. We also interviewed 37 experienced hunters (mean age: 54) in the study area to understand their perceptions of species abundance trends. We used hierarchical multispecies occupancy modelling to determine the effect of distance to village, distance to forest edge and elevation on mammal occupancy after accounting for imperfect detection. We used non‐parametric tests for the rank data to evaluate perceived species trends. Using a combination of historical literature (1954–1985), hunter interviews, direct observations and camera‐trap surveys, we only recorded 56% (15 out of 27) of the medium‐to‐large sized (>1 kg) mammal species known previously from the study region. Interviews suggested that current population densities of all extant mammal species are below historical levels. Our camera traps captured 11 mammal species in the forest reserve and only two species in rubber plantations. Low capture rates (10 captures) in rubber mean we could only determine mammal occupancy in forests. Environmental variables did not affect community level mammal occupancy in forests, but common muntjac (Muntiacus muntjak) and the northern pig‐tailed macaque (Macaca leonina) avoided forest edges. At the time of the survey, hunting was still prevalent in the region. Therefore, conservation interventions that end both hunting and deforestation are urgently needed to stabilize and increase mammal populations in the region.
Using a combination of historical literature, hunter interviews, camera traps and hierarchical Bayesian occupancy modelling, we determined the status of mammals (>1 kg) in a rubber‐forest landscape mosaic in Xishuangbanna, China. Our results show that rubber plantations only sustain two native species, current population densities of all extant mammal species are below historical levels, and several are locally extinct. Conservation management should end both hunting and deforestation to stabilise and increase mammal populations in the region.
This article explores the structure of cultivated religious experience. For the Buddhist philosopher Jñānaśrīmitra (c. 980–1040), the religious experience of the Buddhist yogin (yogipratyakṣa) is not ...spontaneous or sporadic but must be intentionally and rationally cultivated. I argue that Jñānaśrīmitra's picture parallels certain contemporary constructivist accounts of religious experience, according to which the prejudices, expectations, and interpretative structures of the practitioner shape the character of the experience in question. Despite this, however, Jñānaśrīmitra maintains that religious experience is direct, non-conceptual, and ineffable. Even though cultivation begins by focusing on rationally understood, effable conceptual content (the Buddha's teaching of the Four Nobles' Truths, for instance), the yogin's relation to that content is transformed through cultivation (bhāvanā). Cultivation makes awareness-events with conceptual content vivid (spaṣṭa) in such a way that they lose their conceptual character, coming to affect the practitioner's mind as if they were external to it. In this way, precisely in virtue of beginning the process of cultivation with certain expectations about the Buddha's teaching, Buddhist yogins come to have a direct and non-conceptual experience of the breakdown of their own mental streams and the dissolution of the sense of self.
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