916 research outputs found
Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear
We investigate magnetic reconnection in systems simultaneously containing
asymmetric (anti-parallel) magnetic fields, asymmetric plasma densities and
temperatures, and arbitrary in-plane bulk flow of plasma in the upstream
regions. Such configurations are common in the high-latitudes of Earth's
magnetopause and in tokamaks. We investigate the convection speed of the
X-line, the scaling of the reconnection rate, and the condition for which the
flow suppresses reconnection as a function of upstream flow speeds. We use
two-dimensional particle-in-cell simulations to capture the mixing of plasma in
the outflow regions better than is possible in fluid modeling. We perform
simulations with asymmetric magnetic fields, simulations with asymmetric
densities, and simulations with magnetopause-like parameters where both are
asymmetric. For flow speeds below the predicted cutoff velocity, we find good
scaling agreement with the theory presented in Doss et al., J.~Geophys.~Res.,
120, 7748 (2015). Applications to planetary magnetospheres, tokamaks, and the
solar wind are discussed.Comment: 17 pages, 4 figures, submitted to Physics of Plasma
Asymmetric magnetic reconnection with a flow shear and applications to the magnetopause
We perform a theoretical and numerical study of anti-parallel 2D magnetic
reconnection with asymmetries in the density and reconnecting magnetic field
strength in addition to a bulk flow shear across the reconnection site in the
plane of the reconnecting fields, which commonly occurs at planetary
magnetospheres. We predict the speed at which an isolated X-line is convected
by the flow, the reconnection rate, and the critical flow speed at which
reconnection no longer takes place for arbitrary reconnecting magnetic field
strengths, densities, and upstream flow speeds, and confirm the results with
two-fluid numerical simulations. The predictions and simulation results counter
the prevailing model of reconnection at Earth's dayside magnetopause which says
reconnection occurs with a stationary X-line for sub-Alfvenic magnetosheath
flow, reconnection occurs but the X-line convects for magnetosheath flows
between the Alfven speed and double the Alfven speed, and reconnection does not
occur for magnetosheath flows greater than double the Alfven speed. We find
that X-line motion is governed by momentum conservation from the upstream
flows, which are weighted differently in asymmetric systems, so the X-line
convects for generic conditions including sub-Alfvenic upstream speeds. For the
reconnection rate, while the cutoff condition for symmetric reconnection is
that the difference in flows on the two sides of the reconnection site is twice
the Alfven speed, we find asymmetries cause the cutoff speed for asymmetric
reconnection to be higher than twice the asymmetric form of the Alfven speed.
The results compare favorably with an observation of reconnection at Earth's
polar cusps during a period of northward interplanetary magnetic field, where
reconnection occurs despite the magnetosheath flow speed being more than twice
the magnetosheath Alfven speed, the previously proposed suppression condition.Comment: 46 pages, 7 figures, abstract abridged here, accepted to Journal of
Geophysical Research - Space Physic
Chromatic Dynamics of an Electron Beam in a Plasma Based Accelerator
We present a theoretical investigation of the chromatic dynamics of the
witness beam within a plasma based accelerator. We derive the single particle
motion of an electron in an ion column within a nonlinear, blowout wake
including adiabatic dampening and adiabatic variations in plasma density. Using
this, we calculate the evolution of the beam moments and emittance for an
electron beam. Our model can handle near arbitrary longitudinal phase space
distributions. We include the effects of energy change in the beam, imperfect
wake loading, initial transverse offsets of the beam, and mismatch between the
beam and plasma. We use our model to derive analytic saturation lengths for the
projected, longitudinal slice, and energy slice emittance under different beam
loading conditions. Further, we show that the centroid oscillations and spot
sizes vary between the slices and the variation depends strongly on the beam
loading. Next, we show how a beam evolves in a full plasma source with density
ramps and show that the integral of the plasma density along the ramp
determines the impact on the beam. Finally, we derive several simple scaling
laws that show how to design a plasma based injector to produce a target beam
energy and energy spread.Comment: 17 pages, 10 figure
Antiquity at the National Memorial Arboretum
This is an Accepted Manuscript of an article published by Taylor & Francis in International Journal of Heritage Studies on 16/1/2013 available online: http://wwww.tandfonline.com/10.1080/13527258.2012.757556The paper explores the use of ancient and historic material cultures and architectures within the recent resurgence in public commemoration in the UK. Using the case study of the National Memorial Arboretum (Staffordshire), the study focuses on how ancient designs (including prehistoric, classical and medieval styles and forms) interleave with the arboreal, geological and celestial themes of the memorial gardens. Together these designs serve to create a multitude of temporal poises by which auras of commemorative perpetuity and regeneration are projected and sustained. The paper proposes that archaeologists can bring their expertise to bear on the investigation of the complex, varied allusions to the past within contemporary landscapes of memory.This book chapter was submitted to the RAE2014 for the University of Chester - Geography, Environmental Studies and
Archaeology
The importance of initial-final state correlations for the formation of fragments in heavy ion collisions
Using quantum molecular dynamics simulations, we investigate the formation of
fragments in symmetric reactions between beam energies of E=30AMeV and 600AMeV.
After a comparison with existing data we investigate some observables relevant
to tackle equilibration: dsigma/dErat, the double differential cross section
dsigma/pt.dpz.dpt,... Apart maybe from very energetic E>400AMeV and very
central reactions, none of our simulations gives evidence that the system
passes through a state of equilibrium. Later, we address the production
mechanisms and find that, whatever the energy, nucleons finally entrained in a
fragment exhibit strong initial-final state correlations, in coordinate as well
as in momentum space. At high energy those correlations resemble the ones
obtained in the participant-spectator model. At low energy the correlations are
equally strong, but more complicated; they are a consequence of the Pauli
blocking of the nucleon-nucleon collisions, the geometry, and the excitation
energy. Studying a second set of time-dependent variables (radii,
densities,...), we investigate in details how those correlations survive the
reaction especially in central reactions where the nucleons have to pass
through the whole system. It appears that some fragments are made of nucleons
which were initially correlated, whereas others are formed by nucleons
scattered during the reaction into the vicinity of a group of previously
correlated nucleons.Comment: 45 pages text + 20 postscript figures Accepted for publication in
Physical Review
Women’s tenure security on collective lands: A conceptual framework
Within discussions of land and resource rights, there is growing attention to women’s rights, mostly in terms of household and individual rights to private property. This leaves unanswered questions about whether and how women’s land rights can be secured under collective tenure, upon which billions of people worldwide depend. There is an important gap in conceptual tools, empirical understanding, and policy recommendations on women’s land rights within collective tenure. To address this gap and lay the foundations for a sound body of empirical studies and appropriate policies, we develop a conceptual framework to improve our understanding of women’s land rights under collective tenure. We begin by discussing what secure tenure for women on collective lands would entail. We then present the conceptual framework for what factors would affect women’s tenure security, building on a framework for land tenure security that focuses on individual and household tenure. We give attention to particularities of rangelands, forests, and other types of lands as well as commonalities across types of collective lands. A key theme that emerges is that for women to have secure tenure under collective tenure, two dimensions must be in place. First, the collective (group) itself must have tenure security. Second, women must have secure rights within this collective. The latter requires us to consider the governance structures, how men and women access and control land, and the extent to which women have voice and power within the collective. More consistent analyses of collective tenure systems using the framework presented in this paper can help to identify which action resources are important for groups to secure rights to collective lands, and for women to advocate for their rights within the group
Collective Flow from the Intranuclear Cascade Model
The phenomenon of collective flow in relativistic heavy ion collisions is
studied using the hadronic cascade model ARC. Direct comparison is made to data
gathered at the Bevalac, for Au+Au at GeV/c. In contrast to the
standard lore about the cascade model, collective flow is well described
quantitatively without the need for explicit mean field terms to simulate the
nuclear equation of state. Pion collective flow is in the opposite direction to
nucleon flow as is that of anti-nucleons and other produced particles. Pion and
nucleon flow are predicted at AGS energies also, where, in light of the higher
baryon densities achieved, we speculate that equation of state effects may be
observable.Comment: 9 pages, 2 figures include
Path to Facilitate the Prediction of Functional Amino Acid Substitutions in Red Blood Cell Disorders – A Computational Approach
A major area of effort in current genomics is to distinguish mutations that are functionally neutral from those that contribute to disease. Single Nucleotide Polymorphisms (SNPs) are amino acid substitutions that currently account for approximately half of the known gene lesions responsible for human inherited diseases. As a result, the prediction of non-synonymous SNPs (nsSNPs) that affect protein functions and relate to disease is an important task.In this study, we performed a comprehensive analysis of deleterious SNPs at both functional and structural level in the respective genes associated with red blood cell metabolism disorders using bioinformatics tools. We analyzed the variants in Glucose-6-phosphate dehydrogenase (G6PD) and isoforms of Pyruvate Kinase (PKLR & PKM2) genes responsible for major red blood cell disorders. Deleterious nsSNPs were categorized based on empirical rule and support vector machine based methods to predict the impact on protein functions. Furthermore, we modeled mutant proteins and compared them with the native protein for evaluation of protein structure stability.We argue here that bioinformatics tools can play an important role in addressing the complexity of the underlying genetic basis of Red Blood Cell disorders. Based on our investigation, we report here the potential candidate SNPs, for future studies in human Red Blood Cell disorders. Current study also demonstrates the presence of other deleterious mutations and also endorses with in vivo experimental studies. Our approach will present the application of computational tools in understanding functional variation from the perspective of structure, expression, evolution and phenotype
Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors
We present results from new experiments to study the dynamics of radiative
shocks, reverse shocks and radiative precursors. Laser ablation of a solid
piston by the Orion high-power laser at AWE Aldermaston UK was used to drive
radiative shocks into a gas cell initially pressurised between and $1.0 \
bar with different noble gases. Shocks propagated at {80 \pm 10 \ km/s} and
experienced strong radiative cooling resulting in post-shock compressions of {
\times 25 \pm 2}. A combination of X-ray backlighting, optical self-emission
streak imaging and interferometry (multi-frame and streak imaging) were used to
simultaneously study both the shock front and the radiative precursor. These
experiments present a new configuration to produce counter-propagating
radiative shocks, allowing for the study of reverse shocks and providing a
unique platform for numerical validation. In addition, the radiative shocks
were able to expand freely into a large gas volume without being confined by
the walls of the gas cell. This allows for 3-D effects of the shocks to be
studied which, in principle, could lead to a more direct comparison to
astrophysical phenomena. By maintaining a constant mass density between
different gas fills the shocks evolved with similar hydrodynamics but the
radiative precursor was found to extend significantly further in higher atomic
number gases (\sim4$ times further in xenon than neon). Finally, 1-D and 2-D
radiative-hydrodynamic simulations are presented showing good agreement with
the experimental data.Comment: HEDLA 2016 conference proceeding
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