1,020 research outputs found
Conditions for the onset of the current filamentation instability in the laboratory
Current Filamentation Instability (CFI) is capable of generating strong
magnetic fields relevant to explain radiation processes in astrophysical
objects and lead to the onset of particle acceleration in collisionless shocks.
Probing such extreme scenarios in the laboratory is still an open challenge. In
this work, we investigate the possibility of using neutral
beams to explore the CFI with realistic parameters, by performing 2D
particle-in-cell simulations. We show that CFI can occur unless the rate at
which the beam expands due to finite beam emittance is larger than the CFI
growth rate and as long as the role of competing electrostatic two-stream
instability (TSI) is negligible. We also show that the longitudinal energy
spread, typical of plasma based accelerated electron-positron fireball beams,
plays a minor role in the growth of CFI in these scenarios
The role of prestimulus activity in visual extinction.
Patients with visual extinction following right-hemisphere damage sometimes see and sometimes miss stimuli in the left visual field, particularly when stimuli are presented simultaneously to both visual fields. Awareness of left visual field stimuli is associated with increased activity in bilateral parietal and frontal cortex. However, it is unknown why patients see or miss these stimuli. Previous neuroimaging studies in healthy adults show that prestimulus activity biases perceptual decisions, and biases in visual perception can be attributed to fluctuations in prestimulus activity in task relevant brain regions. Here, we used functional MRI to investigate whether prestimulus activity affected perception in the context of visual extinction following stroke. We measured prestimulus activity in stimulus-responsive cortical areas during an extinction paradigm in a patient with unilateral right parietal damage and visual extinction. This allowed us to compare prestimulus activity on physically identical bilateral trials that either did or did not lead to visual extinction. We found significantly increased activity prior to stimulus presentation in two areas that were also activated by visual stimulation: the left calcarine sulcus and right occipital inferior cortex. Using dynamic causal modelling (DCM) we found that both these differences in prestimulus activity and stimulus evoked responses could be explained by enhanced effective connectivity within and between visual areas, prior to stimulus presentation. Thus, we provide evidence for the idea that differences in ongoing neural activity in visually responsive areas prior to stimulus onset affect awareness in visual extinction, and that these differences are mediated by fluctuations in extrinsic and intrinsic connectivity
Ultra-high brilliance multi-MeV -ray beam from non-linear Thomson scattering
We report on the generation of a narrow divergence (
mrad), multi-MeV ( MeV) and ultra-high brilliance ( photons s mm mrad 0.1\% BW) -ray
beam from the scattering of an ultra-relativistic laser-wakefield accelerated
electron beam in the field of a relativistically intense laser (dimensionless
amplitude ). The spectrum of the generated -ray beam is
measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear
evidence of the onset of non-linear Thomson scattering. The photon source has
the highest brilliance in the multi-MeV regime ever reported in the literature
An intervention framework for collaboration
This paper provides an intervention framework for collaboration to improve services. When collaboration is an intervention, its development and effectiveness depend on intervention logic. Intervention logic requires a precise conceptualization of collaboration. This conceptualization emphasizes its vital and unique components. It includes a developmental progression in which collaboration is contrasted with companion concepts. It also includes progress benchmarks, outcome measures, and logic models. These models depict relations among the benchmarks and outcomes, and they identify the mediating and moderating variables that account for collaboration's development and effectiveness. These models are designed to improve planning, evaluation, and their relations. This intervention framework for collaboration contrasts sharply with other conceptualizations and strategies. Although its aim is to unify and improve collaboration policy and practice, its inherent selectivity is an obvious limitation. [PUBLICATION ABSTRACT
Laser-driven electron source suitable for single-shot Gy-scale irradiation of biological cells at dose-rates exceeding Gy/s
We report on the first systematic characterisation of a tuneable laser-driven
electron source capable of delivering Gy-scale doses in a duration of 10 - 20
ps, thus reaching unprecedented dose rates in the range of
Gy/s. Detailed characterisation of the source indicates, in agreement with
Monte-Carlo simulations, single-shot delivery of multi-Gy doses per pulse over
cm-scale areas, with a high degree of spatial uniformity. The results reported
here confirm that a laser-driven source of this kind can be used for systematic
studies of the response of biological cells to picosecond-scale radiation at
ultra-high dose rates.Comment: submitted for publicatio
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