1,594 research outputs found
Laser acceleration of monoenergetic protons via a double layer emerging from an ultra-thin foil
We present theoretical and numerical studies of the acceleration of monoenergetic protons in a double layer formed by the laser irradiation of an ultra-thin film. The ponderomotive force of the laser light pushes the electrons forward, and the induced space charge electric field pulls the ions and makes the thin foil accelerate as a whole. The ions trapped by the combined electric field and inertial force in the accelerated frame, together with the electrons trapped in the well of the ponderomotive and ion electric field, form a stable double layer. The trapped ions are accelerated to monoenergetic energies up to 100 MeV and beyond, making them suitable for cancer treatment. We present an analytic theory for the laser-accelerated ion energy and for the amount of trapped ions as functions of the laser intensity, foil thickness and the plasma number density. We also discuss the underlying physics of the trapped and untrapped ions in a double layer. The analytical results are compared with those obtained from direct Vlasov simulations of the fully nonlinear electron and ion dynamics that is controlled by the laser light
Vlasov simulation of laser-driven shock acceleration and ion turbulence
We present a Vlasov, i.e. a kinetic Eulerian simulation study of nonlinear
collisionless ion-acoustic shocks and solitons excited by an intense laser
interacting with an overdense plasma. The use of the Vlasov code avoids
problems with low particle statistics and allows a validation of
particle-in-cell results. A simple original correction to the splitting method
for the numerical integration of the Vlasov equation has been implemented in
order to ensure the charge conservation in the relativistic regime. We show
that the ion distribution is affected by the development of a turbulence driven
by the relativistic "fast" electron bunches generated at the laser-plasma
interaction surface. This leads to the onset of ion reflection at the shock
front in an initially cold plasma where only soliton solutions without ion
reflection are expected to propagate. We give a simple analytic model to
describe the onset of the turbulence as a nonlinear coupling of the ion density
with the fast electron currents, taking the pulsed nature of the relativistic
electron bunches into account
Stabilisation of BGK modes by relativistic effects
Context. We examine plasma thermalisation processes in the foreshock region of astrophysical shocks within a fully kinetic and self-consistent treatment. We concentrate on proton beam driven electrostatic processes, which are thought to play a key role in the beam relaxation and the particle acceleration. Our results have implications for the effectiveness of electron surfing acceleration and
the creation of the required energetic seed population for first order Fermi acceleration at the shock front.
Aims. We investigate the acceleration of electrons via their interaction with electrostatic waves, driven by the relativistic Buneman instability, in a system dominated by counter-propagating proton beams.
Methods. We adopt a kinetic Vlasov-Poisson description of the plasma on a fixed Eulerian grid and observe the growth and saturation of electrostatic waves for a range of proton beam velocities, from 0.15c to 0.9c.
Results. We can report a reduced stability of the electrostatic wave (ESW) with increasing non-relativistic beam velocities and an improved wave stability for increasing relativistic beam velocities, both in accordance with previous findings. At the highest beam speeds, we find the system to be stable again for a period of ≈160 plasma periods. Furthermore, the high phase space resolution
of the Eulerian Vlasov approach reveals processes that could not be seen previously with PIC simulations. We observe a, to our knowledge, previously unreported secondary electron acceleration mechanism at low beam speeds. We believe that it is the result of parametric couplings to produce high phase velocity ESW’s which then trap electrons, accelerating them to higher energies. This
allows electrons in our simulation study to achieve the injection energy required for Fermi acceleration, for beam speeds as low as 0.15c in unmagnetised plasma
Vlasov simulation in multiple spatial dimensions
A long-standing challenge encountered in modeling plasma dynamics is
achieving practical Vlasov equation simulation in multiple spatial dimensions
over large length and time scales. While direct multi-dimension Vlasov
simulation methods using adaptive mesh methods [J. W. Banks et al., Physics of
Plasmas 18, no. 5 (2011): 052102; B. I. Cohen et al., November 10, 2010,
http://meetings.aps.org/link/BAPS.2010.DPP.NP9.142] have recently shown
promising results, in this paper we present an alternative, the Vlasov Multi
Dimensional (VMD) model, that is specifically designed to take advantage of
solution properties in regimes when plasma waves are confined to a narrow cone,
as may be the case for stimulated Raman scatter in large optic f# laser beams.
Perpendicular grid spacing large compared to a Debye length is then possible
without instability, enabling an order 10 decrease in required computational
resources compared to standard particle in cell (PIC) methods in 2D, with
another reduction of that order in 3D. Further advantage compared to PIC
methods accrues in regimes where particle noise is an issue. VMD and PIC
results in a 2D model of localized Langmuir waves are in qualitative agreement
Vlasov-Maxwell, self-consistent electromagnetic wave emission simulations in the solar corona
1.5D Vlasov-Maxwell simulations are employed to model electromagnetic
emission generation in a fully self-consistent plasma kinetic model for the
first time in the solar physics context. The simulations mimic the plasma
emission mechanism and Larmor drift instability in a plasma thread that
connects the Sun to Earth with the spatial scales compressed appropriately. The
effects of spatial density gradients on the generation of electromagnetic
radiation are investigated. It is shown that 1.5D inhomogeneous plasma with a
uniform background magnetic field directed transverse to the density gradient
is aperiodically unstable to Larmor-drift instability. The latter results in a
novel effect of generation of electromagnetic emission at plasma frequency.
When density gradient is removed (i.e. when plasma becomes stable to
Larmor-drift instability) and a density, super-thermal, hot beam is
injected along the domain, in the direction perpendicular to the magnetic
field, plasma emission mechanism generates non-escaping Langmuir type
oscillations which in turn generate escaping electromagnetic radiation. It is
found that in the spatial location where the beam is injected, the standing
waves, oscillating at the plasma frequency, are excited. These can be used to
interpret the horizontal strips observed in some dynamical spectra. Quasilinear
theory predictions: (i) the electron free streaming and (ii) the beam long
relaxation time, in accord with the analytic expressions, are corroborated via
direct, fully-kinetic simulation. Finally, the interplay of Larmor-drift
instability and plasma emission mechanism is studied by considering
electron beam in the Larmor-drift unstable (inhomogeneous) plasma.
http://www.maths.qmul.ac.uk/~tsiklauri/movie1.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie2.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie3.mpgComment: Solar Physics (in press, the final, accepted version
On the formation and decay of a molecular ultracold plasma
Double-resonant photoexcitation of nitric oxide in a molecular beam creates a
dense ensemble of Rydberg states, which evolves to form a plasma of
free electrons trapped in the potential well of an NO spacecharge. The
plasma travels at the velocity of the molecular beam, and, on passing through a
grounded grid, yields an electron time-of-flight signal that gauges the plasma
size and quantity of trapped electrons. This plasma expands at a rate that fits
with an electron temperature as low as 5 K, colder that typically observed for
atomic ultracold plasmas. The recombination of molecular NO cations with
electrons forms neutral molecules excited by more than twice the energy of the
NO chemical bond, and the question arises whether neutral fragmentation plays a
role in shaping the redistribution of energy and particle density that directs
the short-time evolution from Rydberg gas to plasma. To explore this question,
we adapt a coupled rate-equations model established for atomic ultracold
plasmas to describe the energy-grained avalanche of electron-Rydberg and
electron-ion collisions in our system. Adding channels of Rydberg
predissociation and two-body, electron- cation dissociative recombination to
the atomic formalism, we investigate the kinetics by which this relaxation
distributes particle density and energy over Rydberg states, free electrons and
neutral fragments. The results of this investigation suggest some mechanisms by
which molecular fragmentation channels can affect the state of the plasma
Apar-T: code, validation, and physical interpretation of particle-in-cell results
We present the parallel particle-in-cell (PIC) code Apar-T and, more
importantly, address the fundamental question of the relations between the PIC
model, the Vlasov-Maxwell theory, and real plasmas.
First, we present four validation tests: spectra from simulations of thermal
plasmas, linear growth rates of the relativistic tearing instability and of the
filamentation instability, and non-linear filamentation merging phase. For the
filamentation instability we show that the effective growth rates measured on
the total energy can differ by more than 50% from the linear cold predictions
and from the fastest modes of the simulation.
Second, we detail a new method for initial loading of Maxwell-J\"uttner
particle distributions with relativistic bulk velocity and relativistic
temperature, and explain why the traditional method with individual particle
boosting fails.
Third, we scrutinize the question of what description of physical plasmas is
obtained by PIC models. These models rely on two building blocks:
coarse-graining, i.e., grouping of the order of p~10^10 real particles into a
single computer superparticle, and field storage on a grid with its subsequent
finite superparticle size. We introduce the notion of coarse-graining dependent
quantities, i.e., quantities depending on p. They derive from the PIC plasma
parameter Lambda^{PIC}, which we show to scale as 1/p. We explore two
implications. One is that PIC collision- and fluctuation-induced thermalization
times are expected to scale with the number of superparticles per grid cell,
and thus to be a factor p~10^10 smaller than in real plasmas. The other is that
the level of electric field fluctuations scales as 1/Lambda^{PIC} ~ p. We
provide a corresponding exact expression.
Fourth, we compare the Vlasov-Maxwell theory, which describes a phase-space
fluid with infinite Lambda, to the PIC model and its relatively small Lambda.Comment: 24 pages, 14 figures, accepted in Astronomy & Astrophysic
The filamentation instability driven by warm electron beams: Statistics and electric field generation
The filamentation instability of counterpropagating symmetric beams of
electrons is examined with 1D and 2D particle-in-cell (PIC) simulations, which
are oriented orthogonally to the beam velocity vector. The beams are uniform,
warm and their relative speed is mildly relativistic. The dynamics of the
filaments is examined in 2D and it is confirmed that their characteristic size
increases linearly in time. Currents orthogonal to the beam velocity vector are
driven through the magnetic and electric fields in the simulation plane. The
fields are tied to the filament boundaries and the scale size of the
flow-aligned and the perpendicular currents are thus equal. It is confirmed
that the electrostatic and the magnetic forces are equally important, when the
filamentation instability saturates in 1D. Their balance is apparently the
saturation mechanism of the filamentation instability for our initial
conditions. The electric force is relatively weaker but not negligible in the
2D simulation, where the electron temperature is set higher to reduce the
computational cost. The magnetic pressure gradient is the principal source of
the electrostatic field, when and after the instability saturates in the 1D
simulation and in the 2D simulation.Comment: 10 pages, 6 figures, accepted by the Plasma Physics and Controlled
Fusion (Special Issue EPS 2009
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