54 research outputs found
Ion-acoustic shocks with reflected ions: modeling and PIC simulations
Non-relativistic collisionless shock waves are widespread in space and
astrophysical plasmas and are known as efficient particle accelerators.
However, our understanding of collisionless shocks, including their structure
and the mechanisms whereby they accelerate particles remains incomplete. We
present here the results of numerical modeling of an ion-acoustic collisionless
shock based on one-dimensional (1D) kinetic approximation both for electrons
and ions with a real mass ratio. Special emphasis is made on the
shock-reflected ions as the main driver of shock dissipation. The reflection
efficiency, velocity distribution of reflected particles and the shock
electrostatic structure are studied in terms of the shock parameters.
Applications to particle acceleration in geophysical and astrophysical shocks
are discussed.Comment: 6 pages, 7 figures, International Workshop "Complex Plasma Phenomena
in the Laboratory and in the Universe", January 19-20, 2015, Rome, Ital
Nonlinear Mirror and Weibel modes: peculiarities of quasi-linear dynamics
A theory for nonlinear evolution of the mirror modes near
the instability threshold is developed. It is shown that during initial
stage the major instability saturation is provided by the flattening of the
velocity distribution function in the vicinity of small parallel ion
velocities. The relaxation scenario in this case is accompanied by rapid
attenuation of resonant particle interaction which is replaced by a weaker
adiabatic interaction with mirror modes. The saturated plasma state can be
considered as a magnetic counterpart to electrostatic BGK modes. After
quasi-linear saturation a further nonlinear scenario is controlled by the
mode coupling effects and nonlinear variation of the ion Larmor radius. Our
analytical model is verified by relevant numerical simulations. Test
particle and PIC simulations indeed show that it is a modification of
distribution function at small parallel velocities that results in fading
away of free energy driving the mirror mode. The similarity with resonant
Weibel instability is discussed
Monoenergetic proton beams accelerated by a radiation pressure driven shock
High energy ion beams (> MeV) generated by intense laser pulses promise to be
viable alternatives to conventional ion beam sources due to their unique
properties such as high charge, low emittance, compactness and ease of beam
delivery. Typically the acceleration is due to the rapid expansion of a laser
heated solid foil, but this usually leads to ion beams with large energy
spread. Until now, control of the energy spread has only been achieved at the
expense of reduced charge and increased complexity. Radiation pressure
acceleration (RPA) provides an alternative route to producing laser-driven
monoenergetic ion beams. In this paper, we show the interaction of an intense
infrared laser with a gaseous hydrogen target can produce proton spectra of
small energy spread (~ 4%), and low background. The scaling of proton energy
with the ratio of intensity over density (I/n) indicates that the acceleration
is due to the shock generated by radiation-pressure driven hole-boring of the
critical surface. These are the first high contrast mononenergetic beams that
have been theorised from RPA, and makes them highly desirable for numerous ion
beam applications
Energy transport in stochastically perturbed lattice dynamics
We consider lattice dynamics with a small stochastic perturbation of order
ε and prove that for a space-time scale of order \varepsilon\^-1 the local
spectral density (Wigner function) evolves according to a linear transport
equation describing inelastic collisions. For an energy and momentum conserving
chain the transport equation predicts a slow decay, as 1/\sqrt{t}, for the
energy current correlation in equilibrium. This is in agreement with previous
studies using a different method.Comment: Changed title and introductio
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Charged particle acceleration in nonuniform plasmas
The high-gradient electron acceleration schemes that have been demonstrated using LWFA appear promising for the development of plasma-based laser accelerators into practical devices. However, a question still exists: how to avoid the wake field deterioration and the loss of the phase synchronism between the plasma wave and the electrons that prevent them from being accelerated up to the theoretical limit. In order to obtain the highest possible values of the wake electric field one must use as intense laser pulses as possible i.e., pulses with dimensionless amplitudes a {much_gt} 1. Pulses that have a dimensionless amplitude larger than one tend to be subject to a host of instabilities, such as relativistic self-focusing, self modulation and stimulated Raman scattering, that affect their propagation in the plasma. Such processes could be beneficial, in so far as they increase the pulse energy density, enhance the wake field generation, and provide the mechanism for transporting the laser radiation over several Rayleigh lengths without diffraction spreading. However, it is still far from certain that these processes can be exploited in a controlled form and can lead to regular, stationary wake fields. It is known that, in order to create good quality wake fields, it would be preferable to use laser pulses with steep fronts of order {lambda}{sub p}. The present paper aims at analyzing the influence of the laser pulse shape and of the plasma nonuniformity on the charged particle acceleration. This study is based on the results obtained with one dimensional PIC simulations
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A high current, high gradient, laser excited, pulsed electron gun
This paper describes a pulsed electron gun that can be used as an FEL, as an injector for electron linear accelerators or for rf power generation. It comprises a 1 to 5 MeV, 1 to 2 ns pulsed power supply feeding a single diode, photoexcited acceleration gap. Beam quality of a {approximately}1nC charge in {approximately}1 GV/m field was studied. Computations of the beam parameters as a function of electrode configuration and peak electron current are presented together with descriptions of the power supply, laser and beam diagnostics systems
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