1,171 research outputs found
Limiting stable currents in bounded electron and ion streams
The classical static analysis of the infinite planar diode has been extended to include the effects of finite transverse beam size. Simple expressions have been found for the increase in maximum stable current density over that of an infinite stream for finite cylindrical and strip streams flowing between plates of infinite diodes. The results are also given in terms of stream perveance. The effect of a nonuniform distribution of current across the stream is shown to be relatively small. Experimental values of maximum stable current agree with those obtained from the analysis. A further extension of the static analysis has been made to include the effects of additional conducting plane boundaries parallel to the stream motion. For length-to-width ratios L/D less than 0.25 the tube is adequately described by the results for the infinite planar diode and for L/D greater than 4, the infinitely-long drift tube theory suffices. At intermediate values of L/D, the maximum amount of current that can be stably passed through the tube is greater than that predicted by either asymptotic theory
Effects of high energy photon emissions in laser generated ultra-relativistic plasmas: real-time synchrotron simulations
We model the emission of high energy photons due to relativistic charged
particle motion in intense laser-plasma interactions. This is done within a
particle-in-cell code, for which high frequency radiation normally cannot be
resolved due to finite time steps and grid size. A simple expression for the
synchrotron radiation spectra is used together with a Monte-Carlo method for
the emittance. We extend previous work by allowing for arbitrary fields,
considering the particles to be in instantaneous circular motion due to an
effective magnetic field. Furthermore we implement noise reduction techniques
and present validity estimates of the method. Finally, we perform a rigorous
comparison to the mechanism of radiation reaction, and find the emitted energy
to be in excellent agreement with the losses calculated using radiation
reaction
Inductive and Electrostatic Acceleration in Relativistic Jet-Plasma Interactions
We report on the observation of rapid particle acceleration in numerical
simulations of relativistic jet-plasma interactions and discuss the underlying
mechanisms. The dynamics of a charge-neutral, narrow, electron-positron jet
propagating through an unmagnetized electron-ion plasma was investigated using
a three-dimensional, electromagnetic, particle-in-cell computer code. The
interaction excited magnetic filamentation as well as electrostatic plasma
instabilities. In some cases, the longitudinal electric fields generated
inductively and electrostatically reached the cold plasma wave-breaking limit,
and the longitudinal momentum of about half the positrons increased by 50% with
a maximum gain exceeding a factor of 2 during the simulation period. Particle
acceleration via these mechanisms occurred when the criteria for Weibel
instability were satisfied.Comment: Revised for Phys. Rev. Lett. Please see publised version for best
graphic
X-band microwave generation caused by plasma-sheath instability
It is well known that oscillations at the electron plasma frequency may
appear due to instability of the plasma sheath near a positively biased
electrode immersed in plasma. This instability is caused by transit-time
effects when electrons, collected by this electrode, pass through the sheath.
Such oscillations appear as low-power short spikes due to additional ionization
of a neutral gas in the electrode vicinity. Herein we present first results
obtained when the additional ionization was eliminated. We succeeded to prolong
the oscillations during the whole time a positive bias was applied to the
electrode. These oscillations could be obtained at much higher frequency than
previously reported (tens of GHz compared to few hundreds of MHz) and power of
tens of mW. These results in combination with presented theoretical estimations
may be useful, e.g., for plasma diagnostics.Comment: 12 pages, 7 figure
Kinetic Vlasov Simulations of collisionless magnetic Reconnection
A fully kinetic Vlasov simulation of the Geospace Environment Modeling (GEM)
Magnetic Reconnection Challenge is presented. Good agreement is found with
previous kinetic simulations using particle in cell (PIC) codes, confirming
both the PIC and the Vlasov code. In the latter the complete distribution
functions () are discretised on a numerical grid in phase space.
In contrast to PIC simulations, the Vlasov code does not suffer from numerical
noise and allows a more detailed investigation of the distribution functions.
The role of the different contributions of Ohm's law are compared by
calculating each of the terms from the moments of the . The important role
of the off--diagonal elements of the electron pressure tensor could be
confirmed. The inductive electric field at the X--Line is found to be dominated
by the non--gyrotropic electron pressure, while the bulk electron inertia is of
minor importance. Detailed analysis of the electron distribution function
within the diffusion region reveals the kinetic origin of the non--gyrotropic
terms
Comparison between hybrid and fully kinetic models of asymmetric magnetic reconnection: coplanar and guide field configurations
Magnetic reconnection occurring in collisionless environments is a
multi-scale process involving both ion and electron kinetic processes. Because
of their small mass, the electron scales are difficult to resolve in numerical
and satellite data, it is therefore critical to know whether the overall
evolution of the reconnection process is influenced by the kinetic nature of
the electrons, or is unchanged when assuming a simpler, fluid, electron model.
This paper investigate this issue in the general context of an asymmetric
current sheet, where both the magnetic field amplitude and the density vary
through the discontinuity. A comparison is made between fully kinetic and
hybrid kinetic simulations of magnetic reconnection in coplanar and guide field
systems. The models share the initial condition but differ in their electron
modeling. It is found that the overall evolution of the system, including the
reconnection rate, is very similar between both models. The best agreement is
found in the guide field system, which confines particle better than the
coplanar one, where the locality of the moments is violated by the electron
bounce motion. It is also shown that, contrary to the common understanding,
reconnection is much faster in the guide field system than in the coplanar one.
Both models show this tendency, indicating that the phenomenon is driven by ion
kinetic effects and not electron ones.Comment: 11 pages, 8 figures, accepted in Physics of Plasma
Laser-plasma interactions with a Fourier-Bessel Particle-in-Cell method
A new spectral particle-in-cell (PIC) method for plasma modeling is presented
and discussed. In the proposed scheme, the Fourier-Bessel transform is used to
translate the Maxwell equations to the quasi-cylindrical spectral domain. In
this domain, the equations are solved analytically in time, and the spatial
derivatives are approximated with high accuracy. In contrast to the
finite-difference time domain (FDTD) methods that are commonly used in PIC, the
developed method does not produce numerical dispersion, and does not involve
grid staggering for the electric and magnetic fields. These features are
especially valuable in modeling the wakefield acceleration of particles in
plasmas. The proposed algorithm is implemented in the code PLARES-PIC, and the
test simulations of laser plasma interactions are compared to the ones done
with the quasi-cylindrical FDTD PIC code CALDER-CIRC.Comment: submitted to Phys. Plasma
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