Ultra-short Ultra-intense Laser Research Articles

Simulation study of indirect positron generation by an ultra-short laser

Positron generation by an ultra-short ultra-intense laser in an indirect manner has been studied with two-dimensional particle-in-cell (PIC) and Monte Carlo (MC) simulations. In this generation scheme, positrons are produced with energetic electrons accelerated by an ultra-shot laser pulse propagating through an underdense plasma. The dependence of the positron beam properties on the plasma length and secondary target (converter) thickness was investigated in detail. The simulation results reveal that the positron yield is strongly correlated with the total energy of laser-accelerated electrons; both the temperature and divergence of the positron beam are sensitive to the plasma length; and the positron beam has a pulse duration comparable to the incident electron beam. In addition, it is indicated that even with the optimal converter thickness, only a small fraction (11.4%) of positrons can escape out and most of the detected positrons originate from the back edge of the converter.

Ultra-short ultra-intense laser guiding and its influence on electron acceleration

Simulations and analyses of ultra-short ultra-intense laser propagting in plasmas with uniform and parabolic density profiles, as well as the electron injection into the wake field and the electron spectra in the stable transmission state are performed by using a particle-in-cell code. Fixing the incident laser focal spot size but changing the plasma density in a range of (0.42)1019/cm3, comparative analyses are carried out of the evolutions of the laser beam spot during the propagation of the laser pulse in the plasmas with aforementioned two density profiles, with the normalized laser intensity ranging from 1 to 6. The results show that a plasma channel with a parabolic density profile can realize a good guiding of an ultra-short ultra-intense pulse, which is beneficial for high energy electron acceleration. However, at higher densities, self-guiding can be realized by relativistic self-focusing in uniform plasma, which is conducive to simplifying the experiment and to producing more accelerated electrons.

超短超强激光产生正电子的蒙特卡罗模拟

超短超强激光产生正电子的蒙特卡罗模拟

Thomson Scattering Imaging From Ultrashort Ultraintense Laser Interaction With Gas

Laser-plasma acceleration can provide acceleration gradients which are thousands of times stronger than conventional electron accelerators. The laser propagation length is a crucial parameter that must be extended to achieve high-energy electrons. Here, we show that color images of the laser-plasma interaction region taken from the direction perpendicular to the polarization plane are a powerful tool to discriminate between Thomson scattering and plasma self-emission, leading to a precise measurement of the propagation length.

超短超强激光与薄膜靶作用的光学光谱

超短超强激光与薄膜靶作用的光学光谱

Target normal sheath acceleration: theory, comparison with experiments and future perspectives

Ions can be effectively accelerated during the interaction of an ultra-intense ultra-short laser pulse irradiating a thin solid target via the so-called target normal sheath acceleration (TNSA) mechanism. One of the pivotal questions at this stage of the research is how to predict the properties of the accelerated ions, both from a fundamental point of view and in the light of foreseen applications. In this context, it is desirable to have a simple but reliable description to be used to extrapolate current results to future regimes, which will be made available in the near future, thanks to developments in laser technology. In this paper, the possible approaches for an analytical description of TNSA are discussed, and a theoretical TNSA model is developed. This model is then used to investigate the maximum ion energy as a function of laser parameters. Detailed comparisons with available experimental data and scaling laws are presented. In particular, the relative role played by both the laser pulse energy and irradiance in determining the ion features is investigated.

超短超强激光在稀薄等离子体中的自聚焦传输

超短超强激光在稀薄等离子体中的自聚焦传输

Efficient acceleration of a small dense plasma pellet by consecutive action of multiple short intense laser pulses

AbstractThe acceleration of a micrometer-sized plasma pellet at 100 critical densities (1023 cm−3) by consecutive application of ultra-short ultra-intense laser pulses is studied using two-dimensional particle-in-cell simulation. It is shown that due to the repeated actions of the laser ponderomotive force, a small dense plasma pellet can be efficiently accelerated, with a considerable fraction of the plasma ions accelerated to high speeds. The proposed scheme can provide a high-density flux of energetic ions, which should be valuable in many practical applications.

Radiography with Low Energy Protons Generated from Ultraintense Laser-plasma Interactions

In order to obtain high quality images of thin objects, we performed an experiment of proton radiography by using low energy protons generated from the interaction of an ultrashort ultraintense laser with solid targets. The protons were produced from a thin polyimide target irradiated by the laser pulse, and their maximum energy was estimated at up to 1.8 MeV. A CR-39 nuclear track detector was used as a proton radiography screen. The proton images were obtained by using an optical microscope and the spatial resolution was evaluated by a Modulation Transfer Function (MTF). We have achieved about <TEX>$10\;{\mu}m$</TEX> spatial resolution of images. The obtained spatial resolution shows about <TEX>$4{\sim}5$</TEX> times better value than the conventional X-ray radiography for inspection or non-destructive test (NDT) purpose.

Observation of macroscopic postsolitons in ultrashort ultraintense laser-plasma interaction

Experiments were performed using ultrashort (30fs) ultraintense (up to 1018 W/cm2) laser pulse focused into underdense deuterium plasmas (ne＜1020/cm3). Macroscopic bubblelike structures were observed using monochromatic plasma irradiance imaging method. These structures are interpreted as the overdense shells of postsolitons generated in the plasma by the laser pulse, which is found to have important influence on the formation of plasma channel.

Acceleration of Initially Moving Electrons by a Copropagation Intense Laser Pulse

Acceleration of an initially moving electron by a copropagation ultra-short ultra-intense laser pulse in vacuum is studied. It is shown that when appropriate laser pulse parameters and focusing conditions are imposed, the acceleration of electron by ascending front of laser pulse can be much stronger compared to the deceleration by descending part. Consequently, the electron can obtain significantly high net energy gain. We also report the results of the new scheme that enables a second-step acceleration of electron using laser pulses of peak intensity in the range of 1019 − 1020 Wμm2/cm2. In the first step the electron acceleration from rest is limited to energies of a few MeV, while in the second step the electron acceleration can be considerably enhanced to about 100 MeV energy.

Intense local plasma heating by stopping of ultrashort ultraintense laser pulse in dense plasma

AbstractSelf-trapping, stopping, and absorption of an ultrashort ultraintense linearly polarized laser pulse in a finite plasma slab of near-critical density is investigated by particle-in-cell simulation. As in the underdense plasma, an electron cavity is created by the pressure of the transmitted part of the light pulse and it traps the latter. Since the background plasma is at near-critical density, no wake plasma oscillation is created. The propagating self-trapped light rapidly comes to a stop inside the slab. Subsequent ion Coulomb explosion of the stopped cavity leads to explosive expulsion of its ions and formation of an extended channel having extremely low plasma density. The energetic Coulomb-exploded ions form shock layers of high density and temperature at the channel boundary. In contrast to a propagating pulse in a lower density plasma, here the energy of the trapped light is deposited onto a stationary and highly localized region of the plasma. This highly localized energy-deposition process can be relevant to the fast ignition scheme of inertial fusion.

Measurements of angular distribution and energy spectrum of hot electrons

The angular distribution and energy spectrum of hot electrons produced in the interaction of ultra-short ultra-intense laser pulses with solid foil targets are investigated. Results show that bath the angular distribution and the energy spectrum reveal anisotropic behavior. This anisotropy is believed to be related to the electron generation mechanism. The existence of energy anisotropy explains why different experimental groups obtained quite different elctron temperatures under the same laser intensity.

Experimental study of optical emission from the rear surface in ultrashort ultra-intense laser interaction with solid targets

Optical emission generated by hot electrons passing through solid targets is measured from the rear surface of a metallic foil. The integrated image taken by CC D camera shows that the optical emission forms a ring, on the edge of the ring t here is a bright localized signal, which is the optical transition radiation (OT R). The optical emission spectrum shows a series of sharp nonperiodic peaks bet ween 300—500nm,obviously near 400nm(2ω). The sharp peaks are ascribed to the c oherent transition radiation (CTR) of hot electron-beam microbunching generated by v×B acceleration mechanism. The OTR intensity decreases with the increase of the target thickness.

Electron capture and violent acceleration by a tightly focused ultra-short ultra-intense laser pulse in vacuum

A low-energy relativistic free electron capture and violently acceleration by a tightly focused ultra-short ultra-intense laser pulse in vacuum is presented in this paper. The results show that the electron is mainly accelerated by the long itudinal ponderomotive force and the energy gain is sensitively dependent on the retarded time, initial energy and the incident angle of electron. On the other hand, the energy gain is not very sensitive to the pulse width when it is larger than 10λ.

Possibility of transmutation of Cs135 by ultraintense laser

The possibility of photo-transmutation of long-lived nuclide Cs135 by ultrashort ultraintense laser was analytically evaluated. The yield of Cs135(γ,n) Cs134 was strongly dependent on the laser intensity at around 1020W∕cm2. If Cs135 were irradiated by such a laser with the intensity of 1021W∕cm2 and 10 Hz for 30 min, characteristic γ-ray counting rate was estimated to be 3 Bq.

Effects of Pre-plasma on Energetic Electrons Generation in Ultra-short Ultra-intense Laser–Solid Interaction

The interactions between an ultra-short ultra-intense laser pulse and a solid target with and without a low density pre-plasma are examined. The electrons in the pre-plasma can be trapped and accelerated by the ponderomotive force of a propagating laser pulse, and escape from the pulse as the latter reflects from the target surface, preserving high energy. While in laser interaction with a target without pre-plasma, the maximum energy achieved is much lower. In this sense, the pre-plasma is advantageous for energetic electron generation in the ultra-short ultra-intense laser–solid interaction.

Collisional electron heating by an ultraintense ultrashort laser pulse focused in a gas

Collisional heating of plasma electrons in the field of an ultraintense ultrashort laser pulse is studied. The numerical results obtained by the method of molecular dynamics are compared with the well-known results from kinetic simulations. A model is proposed that provides a good agreement with the results of calculations for both linearly and circularly polarized high-intensity laser pulses.

One-dimensionalparticlesimulations of fast electron transportin solid targets

Fast electron transport in a dense plasma is investigated with one-dimensional particle-in-cell simulations. We show the importance of collisions in the interaction of an ultra-intense ultrashort laser pulse with a solid target. We demonstrate that the fast electron penetration depth and distribution depend on target resistivity. Collisions reinforce the fast electron confinement near the irradiated surface.

Nonlinear filamentation instability driven by an inhomogeneous current in a collisionless plasma

Beams of fast electrons in a cold electron background play a key role in the generation of a magnetic field in the wake of an ultrashort ultraintense laser pulse propagating in an underdense plasma. Here we study the linear and nonlinear evolution of the electromagnetic beam-plasma instability in a collisionless inhomogeneous plasma by using a set of two-fluid electron equations in the nonrelativistic and relativistic regimes. We show the characteristic spatial structures in the current and magnetic field distributions that are generated by this instability. These structures can be used as a signature of the physical mechanism at play in the analysis of the numerical and experimental results of the laser-plasma interaction.