Numerical simulations of energy transfer in two collisionless interpenetrating plasmas (original) (raw)
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Ion Dynamics and Acceleration in Relativistic Shocks
The Astrophysical Journal, 2009
Ab-initio numerical study of collisionless shocks in electron-ion unmagnetized plasmas is performed with fully relativistic particle in cell simulations. The main properties of the shock are shown, focusing on the implications for particle acceleration. Results from previous works with a distinct numerical framework are recovered, including the shock structure and the overall acceleration features. Particle tracking is then used to analyze in detail the particle dynamics and the acceleration process. We observe an energy growth in time that can be reproduced by a Fermi-like mechanism with a reduced number of scatterings, in which the time between collisions increases as the particle gains energy, and the average acceleration efficiency is not ideal. The in depth analysis of the underlying physics is relevant to understand the generation of high energy cosmic rays, the impact on the astrophysical shock dynamics, and the consequent emission of radiation.
Theoretical studies of collisionless shocks for laser-acceleration of ions
Proceedings of SPIE - The International Society for Optical Engineering, 2013
Recently, strong effort has been done in exploring shock acceleration for the generation of highly energetic ion beams, with applications e.g. for medical purposes. The heating of a near-critical density plasma target with a laser, increases the electron temperature and excites ion acoustic waves, which can lead to electrostatic shock formation due to non-linear wave breaking. The higher inertia background ions are reflected and accelerated at the shock potential, showing a quasi-monoenergetic profile. For the first time, its feasibility has been demonstrated experimentally, gaining 20 MeV protons with a very narrow energy spread 1 and a predicted scaling up to 200 MeV for lasers with a 0 = 10. 2 In the quest for high proton energies, optimal conditions for shock formation have to be found. We developed a relativistic model that connects the initial parameters with the steady state shock Mach number, which is based on the Sagdeev approach, 3, 4 showing an increase of the ion energy for high upstream electron temperatures and low downstream to upstream density ratios 5 and high temperature ratios, which has been confirmed by particle-in-cell simulations. In the context of producing a quasi-monoenergetic beam profile, we studied the enhancement of the Weibel instability in an electrostatic shock setup. Governing parameter regimes for the transition to an electromagnetic shock, which is associated with a broadening of the ion spectrum, were determined analytically and confirmed with simulations.
Plasma Physics and Controlled Fusion, 2012
The growth and saturation of Buneman-type instabilities is examined with a particle-in-cell (PIC) simulation for parameters that are representative for the foreshock region of fast supernova remnant (SNR) shocks. A dense ion beam and the electrons correspond to the upstream plasma and a fast ion beam to the shock-reflected ions. The purpose of the 2D simulation is to identify the nonlinear saturation mechanisms, the electron heating and potential secondary instabilities that arise from anisotropic electron heating and result in the growth of magnetic fields. We confirm that the instabilities between both ion beams and the electrons saturate by the formation of phase space holes by the beam-aligned modes. The slower oblique modes accelerate some electrons, but they can not heat up the electrons significantly before they are trapped by the faster beam-aligned modes. Two circular electron velocity distributions develop, which are centred around the velocity of each ion beam. They develop due to the scattering of the electrons by the electrostatic wave potentials. The growth of magnetic fields is observed, but their amplitude remains low.
Collisionless Plasma Shocks. Field Generation and Particle Acceleration
2005
Gamma ray bursts are among the most energetic events in the known universe. A highly relativistic fireball is ejected. In most cases the burst itself is followed by an afterglow, emitted under deceleration as the fireball plunges through the circum-stellar media. To interpret the observations of the afterglow emission, two physical aspects need to be understood: 1) The origin and nature of the magnetic field in the fireball and 2) the particle velocity distribution function behind the shock. Both are necessary in existing afterglow models to account for what is believed to be synchrotron radiation. To answer these questions, we need to understand the microphysics at play in collisionless shocks. Using 3D particle-in-cell simulations we can gain insight in the microphysical processes that take place in such shocks. We discuss the results of such computer experiments. It is shown how a Weibel-like two-stream plasma instability is able to create a strong transverse intermittent magnetic field and how this points to a connected mechanism for in situ particle acceleration in the shock region.
Ion Acceleration by Collisionless Shocks in High-Intensity-Laser–Underdense-Plasma Interaction
Physical Review Letters, 2004
Ion acceleration by the interaction of an ultraintense short-pulse laser with an underdense-plasma has been studied at intensities up to 3 10 20 W=cm 2 . Helium ions having a maximum energy of 13:2 1:0 MeV were measured at an angle of 100 from the laser propagation direction. The maximum ion energy scaled with plasma density as n 0:700:05 e . Two-dimensional particle-in-cell simulations suggest that multiple collisionless shocks are formed at high density. The interaction of shocks is responsible for the observed plateau structure in the ion spectrum and leads to an enhanced ion acceleration beyond that possible by the ponderomotive potential of the laser alone.
Proton Shock Acceleration in Laser-Plasma Interactions
Physical Review Letters, 2004
The formation of strong, high Mach number (2-3), electrostatic shocks by laser pulses incident on overdense plasma slabs is observed in one-and two-dimensional particle-in-cell simulations, for a wide range of intensities, pulse durations, target thicknesses, and densities. The shocks propagate undisturbed across the plasma, accelerating the ions (protons). For a dimensionless field strength parameter a 0 16 (I 2 3 10 20 W cm ÿ2 m 2 , where I is the intensity and the wavelength), and target thicknesses of a few microns, the shock is responsible for the highest energy protons. A plateau in the ion spectrum provides a direct signature for shock acceleration.
Radiation-pressure-driven ion Weibel instability and collisionless shocks
Physical Review E
The Weibel instability from counterstreaming plasma flows is a basic process highly relevant for collisionless shock formation in astrophysics. In this paper we investigate, via two-and three-dimensional simulations, suitable configurations for laboratory investigations of the ion Weibel instability (IWI) driven by a fast quasineutral plasma flow launched into the target via the radiation pressure of an ultra-high-intensity laser pulse ("hole-boring" process). The use of S-polarized light at oblique incidence is found to be an optimal configuration for driving IWI, as it prevents the development of surface rippling observed at normal incidence that would lead to strong electron heating and would favor competing instabilities. Conditions for the evolution of IWI into a collisionless shock are also investigated.