817 research outputs found
On particle acceleration and trapping by Poynting flux dominated flows
Using particle-in-cell (PIC) simulations, we study the evolution of a
strongly magnetized plasma slab propagating into a finite density ambient
medium. Like previous work, we find that the slab breaks into discrete magnetic
pulses. The subsequent evolution is consistent with diamagnetic relativistic
pulse acceleration of \cite{liangetal2003}. Unlike previous work, we use the
actual electron to proton mass ratio and focus on understanding trapping vs.
transmission of the ambient plasma by the pulses and on the particle
acceleration spectra. We find that the accelerated electron distribution
internal to the slab develops a double-power law. We predict that emission from
reflected/trapped external electrons will peak after that of the internal
electrons. We also find that the thin discrete pulses trap ambient electrons
but allow protons to pass through, resulting in less drag on the pulse than in
the case of trapping of both species. Poynting flux dominated scenarios have
been proposed as the driver of relativistic outflows and particle acceleration
in the most powerful astrophysical jets.Comment: 25 pages, Accepted by Plasma Physics and Controlled Fusio
Development of an Interpretive Simulation Tool for the Proton Radiography Technique
Proton radiography is a useful diagnostic of high energy density (HED)
plasmas under active theoretical and experimental development. In this paper we
describe a new simulation tool that interacts realistic laser-driven point-like
proton sources with three dimensional electromagnetic fields of arbitrary
strength and structure and synthesizes the associated high resolution proton
radiograph. The present tool's numerical approach captures all relevant physics
effects, including effects related to the formation of caustics.
Electromagnetic fields can be imported from PIC or hydrodynamic codes in a
streamlined fashion, and a library of electromagnetic field `primitives' is
also provided. This latter capability allows users to add a primitive, modify
the field strength, rotate a primitive, and so on, while quickly generating a
high resolution radiograph at each step. In this way, our tool enables the user
to deconstruct features in a radiograph and interpret them in connection to
specific underlying electromagnetic field elements. We show an example
application of the tool in connection to experimental observations of the
Weibel instability in counterstreaming plasmas, using particles
generated from a realistic laser-driven point-like proton source, imaging
fields which cover volumes of mm. Insights derived from this
application show that the tool can support understanding of HED plasmas.Comment: Figures and tables related to the Appendix are included in the
published journal articl
Genes required for survival in microgravity revealed by genome-wide yeast deletion collections cultured during spaceflight
Spaceflight is a unique environment with profound effects on biological systems including tissue redistribution and musculoskeletal stresses. However, the more subtle biological effects of spaceflight on cells and organisms are difficult to measure in a systematic, unbiased manner. Here we test the utility of the molecularly barcoded yeast deletion collection to provide a quantitative assessment of the effects of microgravity on a model organism. We developed robust hardware to screen, in parallel, the complete collection of ~4800 homozygous and ~5900 heterozygous (including ~1100 single-copy deletions of essential genes) yeast deletion strains, each carrying unique DNA that acts as strain identifiers. We compared strain fitness for the homozygous and heterozygous yeast deletion collections grown in spaceflight and ground, as well as plus and minus hyperosmolar sodium chloride, providing a second additive stressor. The genome-wide sensitivity profiles obtained from these treatments were then queried for their similarity to a compendium of drugs whose effects on the yeast collection have been previously reported. We found that the effects of spaceflight have high concordance with the effects of DNA-damaging agents and changes in redox state, suggesting mechanisms by which spaceflight may negatively affect cell fitness
Microwave-induced control of Free Electron Laser radiation
The dynamical response of a relativistic bunch of electrons injected in a
planar magnetic undulator and interacting with a counterpropagating
electromagnetic wave is studied. We demonstrate a resonance condition for which
the free electron laser (FEL) dynamics is strongly influenced by the presence
of the external field. It opens up the possibility of control of short
wavelength FEL emission characteristics by changing the parameters of the
microwave field without requiring change in the undulator's geometry or
configuration. Numerical examples, assuming realistic parameter values
analogous to those of the TTF-FEL, currently under development at DESY, are
given for possible control of the amplitude or the polarization of the emitted
radiation.Comment: 14 pages, 5 figures, accepted for publication in Phys. Rev.
A comparison of weak-turbulence and PIC simulations of weak electron-beam plasma interaction
Quasilinear theory has long been used to treat the problem of a weak electron
beam interacting with plasma and generating Langmuir waves. Its extension to
weak-turbulence theory treats resonant interactions of these Langmuir waves
with other plasma wave modes, in particular ion-sound waves. These are strongly
damped in plasma of equal ion and electron temperatures, as sometimes seen in,
for example, the solar corona and wind. Weak turbulence theory is derived in
the weak damping limit, with a term describing ion-sound wave damping then
added. In this paper we use the EPOCH particle-in-cell code to numerically test
weak turbulence theory for a range of electron-ion temperature ratios. We find
that in the cold ion limit the results agree well, but increasing ion
temperature the three-wave resonance becomes broadened in proportion to the
ion-sound wave damping rate. This may be important in, for example, the theory
of solar radio bursts, where the spectrum of Langmuir waves is critical.
Additionally we establish lower limits on the number of simulation particles
needed to accurately reproduce the electron and wave distributions in their
saturated states, and to reproduce their intermediate states and time
evolution.Comment: Accepted by PO
Coherent Cherenkov-Cyclotron Radiation Excited by an Electron Beam in a Metamaterial Waveguide
An electron beam passing through a metamaterial structure is predicted to generate reversed Cherenkov radiation, an unusual and potentially very useful property. We present an experimental test of this phenomenon using an intense electron beam passing through a metamaterial loaded waveguide. Power levels of up to 5Â MW are observed in backward wave modes at a frequency of 2.40Â GHz using a one microsecond pulsed electron beam of 490Â keV, 84Â A in a 400Â G magnetic field. Contrary to expectations, the output power is not generated in the Cherenkov mode. Instead, the presence of the magnetic field, which is required to transport the electron beam, induces a Cherenkov-cyclotron (or anomalous Doppler) instability at a frequency equal to the Cherenkov frequency minus the cyclotron frequency. Nonlinear simulations indicate that the Cherenkov-cyclotron mode should dominate over the Cherenkov instability at a lower magnetic field where the highest output power is obtained.United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative (Grant FA9550-12-1-0489
Quasi-stationary States of Two-Dimensional Electron Plasma Trapped in Magnetic Field
We have performed numerical simulations on a pure electron plasma system
under a strong magnetic field, in order to examine quasi-stationary states that
the system eventually evolves into. We use ring states as the initial states,
changing the width, and find that the system evolves into a vortex crystal
state from a thinner-ring state while a state with a single-peaked density
distribution is obtained from a thicker-ring initial state. For those
quasi-stationary states, density distribution and macroscopic observables are
defined on the basis of a coarse-grained density field. We compare our results
with experiments and some statistical theories, which include the
Gibbs-Boltzmann statistics, Tsallis statistics, the fluid entropy theory, and
the minimum enstrophy state. From some of those initial states, we obtain the
quasi-stationary states which are close to the minimum enstrophy state, but we
also find that the quasi-stationary states depend upon initial states, even if
the initial states have the same energy and angular momentum, which means the
ergodicity does not hold.Comment: 9 pages, 7 figure
Spinodal Decomposition in Binary Gases
We carried out three-dimensional simulations, with about 1.4 million
particles, of phase segregation in a low density binary fluid mixture,
described mesoscopically by energy and momentum conserving Boltzmann-Vlasov
equations. Using a combination of Direct Simulation Monte Carlo(DSMC) for the
short range collisions and a version of Particle-In-Cell(PIC) evolution for the
smooth long range interaction, we found dynamical scaling after the ratio of
the interface thickness(whose shape is described approximately by a hyperbolic
tangent profile) to the domain size is less than ~0.1. The scaling length R(t)
grows at late times like t^alpha, with alpha=1 for critical quenches and
alpha=1/3 for off-critical ones. We also measured the variation of temperature,
total particle density and hydrodynamic velocity during the segregation
process.Comment: 11 pages, Revtex, 4 Postscript figures, submitted to PR
A Monte Carlo simulation of ion transport at finite temperatures
We have developed a Monte Carlo simulation for ion transport in hot
background gases, which is an alternative way of solving the corresponding
Boltzmann equation that determines the distribution function of ions. We
consider the limit of low ion densities when the distribution function of the
background gas remains unchanged due to collision with ions. A special
attention has been paid to properly treat the thermal motion of the host gas
particles and their influence on ions, which is very important at low electric
fields, when the mean ion energy is comparable to the thermal energy of the
host gas. We found the conditional probability distribution of gas velocities
that correspond to an ion of specific velocity which collides with a gas
particle. Also, we have derived exact analytical formulas for piecewise
calculation of the collision frequency integrals. We address the cases when the
background gas is monocomponent and when it is a mixture of different gases.
The developed techniques described here are required for Monte Carlo
simulations of ion transport and for hybrid models of non-equilibrium plasmas.
The range of energies where it is necessary to apply the technique has been
defined. The results we obtained are in excellent agreement with the existing
ones obtained by complementary methods. Having verified our algorithm, we were
able to produce calculations for Ar ions in Ar and propose them as a new
benchmark for thermal effects. The developed method is widely applicable for
solving the Boltzmann equation that appears in many different contexts in
physics.Comment: 14 page
Assessing the socioeconomic and infrastructure development disparity–a case study of city districts of Punjab, Pakistan
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