Three-dimensional Particle Simulation Research Articles

Effects of sidelobes of focused flat-topped laser beams on vacuum electron acceleration

Using three-dimensional test particle simulations, we investigated electrons accelerated by a focused flat-top laser beam at different intensities and flatness levels of the beam profile before focusing in vacuum. The results show that the presence of sidelobes around the main focal spot of the focused flat-top laser beam influences the optimum (as far as electron acceleration is concerned) initial momentum (and incident angle) of electrons for acceleration. The difference of initial conditions between laser beams with and without sidelobes becomes evident when the laser field is strong enough (a0>10, corresponding to intensities I>1×1020 W/cm2 for the laser wavelength λ=1 μm, where a0 is a dimensionless parameter measuring laser intensity). The difference becomes more pronounced at increasing a0. Because of the presence of sidelobes, there exist three typical CAS (capture and acceleration scenario) channels when a0≥30 (corresponding to I>1×1021 W/cm2 for λ=1 μm). The energy spread of the outgoing electrons is also discussed in detail.

Field structure and electron acceleration in a laser beam of a high-order Hermite-Gaussian mode

We analyze the axial electric field intensity distribution and the phase velocity distribution of high-order Hermite-Gaussian (HG) mode laser beams. Using a three-dimensional test particle simulation, the numerical results of electrons accelerated by Hermite-Gaussian (0,0), and (3,0) mode laser beams are presented. It is established that electrons can be more favorably captured and accelerated in an odd high-order Hermite-Gaussian mode laser beam.

A particle simulation for the global pulsar magnetosphere: the pulsar wind linked to the outer gaps

Soon after the discovery of radio pulsars in 1967, the pulsars are identified as strongly magnetic (typically 10 12 G) rapidly rotating (∼10 2 - 0.1 Hz) neutron stars. However, the mechanism of particle acceleration in the pulsar magnetosphere has been a longstanding problem. The central problem is why the rotation power manifests itself in both gamma-ray beams and a highly relativistic wind of electron-positron plasmas, which excites surrounding nebulae observed in X-ray. Here we show with a three-dimensional particle simulation for the global axisymmetric magnetosphere that a steady outflow of electron-positron pairs is formed with associated pair sources, which are the gamma-ray emitting regions within the light cylinder. The magnetic field is assumed to be a dipole, and to be consistent, the pair creation rate is taken to be small, so that the model might be applicable to old pulsars such as Geminga. The pair sources are charge-deficient regions around the null surface, and we identify them as the outer gap. The wind mechanism is the electromagnetic induction which brings about fast azimuthal motion and eventually trans-field drift by radiation drag in the close vicinity of the light cylinder and beyond. The wind causes loss of particles from the system. This maintains charge deficiency in the outer gap and pair creation. The model is thus in a steady state, balancing loss and supply of particles. Our simulation implies how the wind coexists with the gamma-ray emitting regions in the pulsar magnetosphere.

Production of proton beams with narrow-band energy spectra from laser-irradiated ultrathin foils

Three-dimensional gridless particle simulations of proton acceleration via irradiation of a very thin foil by a short-pulse, high-intensity laser have been performed to evaluate recently proposed microstructured target configurations. It is found that a pure proton microdot target does not by itself result in a quasimonoenergetic proton beam. Such a beam can only be produced with a very lightly doped target, in qualitative agreement with one-dimensional theory. The simulations suggest that beam quality in current experiments could be dramatically improved by choosing microdot compositions with a 5-10 times lower proton fraction.

Generation of High-Density Atto-Second Electron Bunch by Intense Short Pulse Laser

An attosecond high-density electron bunch is produced by an intense short-pulse TEM10+TEM01-mode laser. The transverse ponderomotive force of the TEM10+TEM01-mode laser confines pre-accelerated electrons in the transverse direction, and at the same time the laser longitudinal electric field accelerates the electrons. Our three-dimensional particle simulations show that the pre-accelerated electrons are accelerated to a few hundreds MeV and are compressed to ∼499 attoseconds with a low energy spread (∼±3.5 %). The electron density of the attosecond bunch reaches ∼43 times as high as the initial number density. Such the attosecond electron bunch is now available to attophysics.

Three-dimensional particle simulation on structure formation and plasma instabilities in collisionless driven reconnection

Structure formation and plasma instabilities in collisionless driven reconnection are investigated by three-dimensional electromagnetic (EM) particle simulation in an open system. In the early period, the lower-hybrid drift instability is excited in tlie periphery of the current layer. In the intermediate period, magnetic islands are created in the downstream due to magnetic reconnection. and they become unstable against a kink instability. In the late period, after the magnetic islands go out through the boundary, a wide, thin current sheet is generated and a low-frequency EM mode is excited in the central region. This mode has a frequency comparable to the ion gyration frequency, and thus it is considered to be the drift kink instability.

Low-frequency instabilities locally enhanced by parallel flow velocity shears in magnetized plasmas

Three-dimensional electrostatic particle simulations are performed in order to investigate the effects of ion flow parallel to magnetic-field lines and their velocity shears on low-frequency plasma instabilities in detail. In the case where the ion drift speed is so small that the low-frequency instabilities cannot take place. the ion-acoustic wave is destabilized by introducing the ion-flow velocity shear. The ion-acoustic wave is locally destabilized in the large velocity shear region. On the other hand, the ion-cyclotron instability is also destabilized when the ion drift speed exceeds a certain threshold. The flow velocity shear enhances not, only the fundamental mode but also tlie high harmonic modes. As a result. the spiky fluctuations in the time domain are locally observed in the velocity shear region. which are caused by the simultaneous existence of several coherent ion-evclotron harmonics.

Magnetotail field topology in a three-dimensional global particle simulation

We present the temporal evolution of magnetic field topology (FT) and its ‘bifurcation’, i.e. ‘magnetic reconnection’ taking place in the magnetotail with a southward interplanetary magnetic field. According to the FT theory, the magnetic FT is uniquely determined by the signs of real part of Jacobian matrix eigenvalues of the critical points (CPs) i.e. magnetic null points, if their Jacobian matrix of the CPs is not degenerated. This is because FT can change only at CP locations. The signs of the real part of eigenvalues characterize the magnetic field patterns around CPs. At the CP locations, the magnetic field becomes zero. The CPs can be classified into different types of ‘saddles points’. In the saddles points, the eigenvectors span characteristic both one- and two-dimensional manifolds, according to their signs. These manifolds and CPs form FT ‘skeletons’ and determine essentially the global FT. These FT ‘skeletons’ that include the CPs, characteristic curves and surfaces which are spanned by the eigenvectors of their CPs provide a clear view of the three-dimensional FT. The change in the skeleton, i.e. the change in the FT, is the so-called ‘bifurcation’ and allows us to show the occurrence of the magnetic reconnections.

Shear viscosity of bulk suspensions at low Reynolds number with the three-dimensional lattice Boltzmann method

We report three-dimensional parallel Lagrangian particle simulations using the lattice Boltzmann method, conducted at a low Reynolds number. Using modified Lees-Edwards boundary conditions and directly calculated viscous dissipation, we show that it is possible to recover excellent agreement with the Einstein viscosity formula in the low concentration limit and to predict viscosity corrections for larger concentrations.

Energetic electron bunch generation by an intense laser pulse

In this paper, three-dimensional particle simulation results show a generation of high-energy micro-electron bunch by using an intense short-pulse laser composed of a linear combination of a TEM 10 mode and a TEM 01 mode in vacuum. The longitudinal electric field of this laser accelerates electrons in the longitudinal direction, and the electron beam size becomes a few micrometers. In this paper we found that this electron bunch has several energy spectrum peaks and the energy spread is smaller than 10% in each spectrum peak. If the laser intensity is enough high, the electron energy is proportional to a 0 and the electron energy is not influenced much by the variation of the initial electron energy. The simulation results are compared with the scaling law, and the scaling results coincide well with the simulation results.

Energetic ion generation by Coulomb explosion in cluster gas and a low-density plastic foam with an intense femtosecond laser

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1-keV protons under laser irradiation of intensity 6 × 1016W/cm2. The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of 1016–1017 W/cm2 and also when extrapolated with the results of three-dimensional particle simulations at 1020–1021 W/cm2. Energetic proton generation in low-density plastic (C5H10) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm3. The foam target was irradiated by 100-fs pulses of a laser with intensity 1 × 1018 W/cm2. A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape explained well by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.

Impact of the laser wavelength on the dynamics of Xe cluster plasma produced by an intense ultrashort laser pulse

The dynamics of Xe clusters irradiated by a high intensity subpicosecond laser pulse is investigated through a relativistic time-dependent three-dimensional particle simulation model. In order to explore the effect of transition from underdense to overdense plasma, we performed calculations for laser wavelengths between 100 and 800 nm. The ionization of clusters and charge accumulation was found to be independent of the laser wavelength, while the removal rate of electrons from the cluster into the intercluster space, mean electron and ion energies, and energy absorption increase with the wavelength.

Three dimensional pondermotive scattering of ultrashort electron beam in the field of focused ultraintense laser pulse

Using a three-dimensional test particle simulation code, we study the pondermotive scattering of an ultrashort electron beam subject to a focused ultraintense femtosecond laser pulse. The dependence of the spatial distribution of electron beam and electron energy spectrum on the time delay between laser pulse and electron pulse is discussed in detail. Through the measurement of the spatial distribution of electron beam or electron spectrum, we present a method to measure the duration of an ultrashort electron pulse.

A second-order method for three-dimensional particle simulation

This paper describes a numerical method for the direct numerical simulation of Navier–Stokes flows with one or more solid spheres. The particles may be fixed or mobile, and they may have different radii. The basic idea of the method stems from the observation that, due to the no-slip condition, in the reference frame of each particle, the velocity near the particle boundary is very small so that the Stokes equations constitute an excellent approximation to the full Navier–Stokes problem. The general analytic solution of the Stokes equations can then be used to “transfer” the no-slip condition from the particle surface to the adjacent grid nodes. In this way the geometric complexity arising from the irregular relation between the particle boundary and the underlying mesh is avoided and fast solvers can be used. The method is validated by a detailed comparison with spectral solutions for the flow past a sphere at Reynolds numbers of 50 and 100. The existence in these situations of a Stokes region near the particle is explicitly demonstrated. Other numerical experiments to show the performance of the code are also described. To illustrate the power and efficiency of the method, a simulation of decaying homogeneous turbulence in a cell containing 100 moveable spheres is described. As implemented here, the method can only be applied to simple body shapes such as spheres and cylinders. Extensions to more general situations are mentioned in the last section.

Dynamics of a Xe cluster plasma produced by an intense ultrashort pulse KrF laser

The dynamics of Xe clusters with initial radius between 10 and 100 Å irradiated by an IR subpicosecond laser pulse is investigated. The evolution of the cluster is modeled with a relativistic time-dependent three-dimensional particle simulation model. The focus of this investigation is to understand the energy absorption of clusters and how the absorbed energy is distributed among the various degrees of freedom. The consequence of the initial cluster radius on the absorbed energy, average charge per atom, mean electron and ion energies, ionization, removal of electrons from the cluster, and cluster expansion was studied. The absorbed energy per cluster scales as N5∕3, and the mean electron and ion energies scale as N1∕3 and N2∕3, respectively (N is the number of atoms per cluster). A significant fraction of the absorbed energy (∼90%) is converted into kinetic energy with comparable contribution to electrons and ions. The energy balance suggests that smaller clusters are more efficient as radiators, while larger clusters are more conducive to particle acceleration. The radiation yield of clusters with initial radius 20–50 Å irradiated by a laser with peak intensity 1016W∕cm2 is determined to be 1%–2%.

Quasistatic magnetic and electric fields generated in intense laser plasma interaction

A self-consistent kinetic model based on relativistic Vlasov–Maxwell equations is presented for the generation of quasistatic spontaneous fields, i.e., both the quasistatic magnetic (QSM) field and the quasistatic electric (QSE) field, in intense laser plasma interaction. For the circularly polarized laser, QSM field includes two parts, the axial part Bz as well as the azimuthal Bθ; the QSE field Es, corresponding to the space-charge potential, forms a plasma density channel. For the linearly polarized laser, Bz is absent. Equations for Bz, Bθ, and Es are uniformly derived from one self-consistent model under the static-state approximation, which satisfies the conservation law of charge. The profile of the plasma density channel and the dependence of the peak QSM fields on the laser intensity are discussed. The experiment and simulation results are explained by the model. The predicted QSM and QSE fields are also observed in the three-dimensional particle simulation.

Modeling of clusters in a strong248‐nmlaser field by a three-dimensional relativistic molecular dynamic model

A relativistic time-dependent three-dimensional particle simulation model has been developed to study the interaction of intense ultrashort KrF (248 nm) laser pulses with small Xe clusters. The trajectories of the electrons and ions are treated classically according to the relativistic equation of motion. The model has been applied to a different regime of ultrahigh intensities extending to 10(21) W/ cm(2). In particular, the behavior of the interaction with the clusters from intensities of approximately 10(15) W/cm(2) to intensities sufficient for a transition to the so-called "collective oscillation model" has been explored. At peak intensities below 10(20) W/cm(2), all electrons are removed from the cluster and form a plasma. It is found that the "collective oscillation model" commences at intensities in excess of 10(20) W/cm(2), the range that can be reached in stable relativistic channels. At these high intensities, the magnetic field has a profound effect on the shape and trajectory of the electron cloud. Specifically, the electrons are accelerated to relativistic velocities with energies exceeding 1 MeV in the direction of laser propagation and the magnetic field distorts the shape of the electron cloud to give the form of a pancake.

Magnetic Field Generation in Collisionless Shocks: Pattern Growth and Transport

We present results from three-dimensional particle simulations of collisionless shock formation, with relativistic counterstreaming ion-electron plasmas. Particles are followed over many skin depths downstream of the shock. Open boundaries allow the experiments to be continued for several particle crossing times. The experiments confirm the generation of strong magnetic and electric fields by a Weibel-like kinetic streaming instability and demonstrate that the electromagnetic fields propagate far downstream of the shock. The magnetic fields are predominantly transversal and are associated with merging ion current channels. The total magnetic energy grows as the ion channels merge and as the magnetic field patterns propagate downstream. The electron populations are quickly thermalized, while the ion populations retain distinct bulk speeds in shielded ion channels and thermalize much more slowly. The results help reveal processes of importance in collisionless shocks and may help to explain the origin of the magnetic fields responsible for afterglow synchrotron/jitter radiation from gamma-ray bursts.

Generation of high-energy protons from the Coulomb explosion of hydrogen clusters by intense femtosecond laser pulses

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1 keV protons under laser irradiation of intensity $6\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb-explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of ${10}^{16}--{10}^{17}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ and also when extrapolated with the results of three-dimensional particle simulations at ${10}^{20}--{10}^{21}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$

Characteristics of multimode combined intense laser-induced electron acceleration and violent bunch compression

The interaction of low energy electrons with multimode combined intense laser beams in vacuum was studied by using three-dimensional test particle simulation. The process of interaction and the ponderomotive potential structures of some multimode combined intense laser beams are analyzed in detail. This article presents the detailed characteristics of the multimode combined intense laser-induced electron acceleration and violent bunch compression [Wang et al., Appl. Phys. Lett. 82, 2752 (2003)]. The properties of the output electron bunch are also investigated.