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Intense attosecond light sources are required for investigating ultrafast electron dynamics, such as that in molecules and atoms, etc. However, so far the achievable photon energy remains at the keV level, thereby limiting their range of applications. For instance, probing intranuclear dynamics requires photon energies in the gamma-ray regime. We propose here a scheme for generating intense attosecond gamma-ray pulses by irradiating a thin solid-density foil with intense vortex laser. In the interaction, the laser driven-foil electrons are differentially accelerated within a quarter-wave field and become highly bunched. Three-dimensional particle-in-cell simulations show that they can efficiently (with ∼ 30 % energy conversion efficiency) radiate ultrashort ( ∼ 400 as), high-flux ( > 10 10 photons per pulse), and ultrabrilliant (up to 1027 photons s−1 mm−2 mrad−2 per 0.1% BW at 1 MeV photon energy) gamma rays. With an x-ray driving laser, the scheme can even produce isolated bright sub-attosecond gamma-ray pulses that are useful for time-resolved nuclear spectroscopy and many other areas. [ABSTRACT FROM AUTHOR]

Particle collisions can have significant effects on plasma instabilities, especially in dense and/or low temperature plasmas. To understand the influence of collisional effects on the plasma waves, the Vlasov–Poisson system with Krook collisions is applied to study the long-term evolution of the two-stream (TS) and bump-on-tail (BOT) instabilities. The system is solved numerically with the fourth-order Runge–Kutta scheme and the Thomas algorithm. It is found that collisions can enhance the wave damping and mitigate the energy of the characteristic slow evolving nonlinear Landau damping oscillations associated with the wave-trapped electrons, especially if the collision rate ν is higher than 0.01 ω p , where ωp is the plasma frequency of the background plasma. Collisions can also decrease the growth rate and saturation level of the TS and BOT unstable waves and tend to shrink the phase space vortex and narrow the phase-mixed region of the trapped electrons. However, our simulations show that collisions cannot readily prevent the nonlinear Landau damping oscillations. In fact, only with ν > 0.001 ω p for the TS instability and ν > 0.01 ω p for the BOT instability, as well as evolution times greater than several hundred ω p − 1 , the vortex structure of the wave-trapped electrons can be undetectable. The corresponding growth rates also drop dramatically, and the maximum wave energy can be one or two orders lower than that of the collisionless limits. [ABSTRACT FROM AUTHOR]

Generation of a thin plasma jet with embedded gigagauss axial magnetic fields from the frontal impact of a short linearly polarized laser pulse on an overdense microtube target is considered. It is a new scheme of axial magnetic field generation without initial laser angular momentum. Three-dimensional particle-in-cell simulations show that the space-charge field of the laser expelled tube-front electrons will pull out carbon ions to form at the tube entrance a long-living low-density plasma bunch with gigagauss magnetic fields. The front center of the plasma bunch then stretches forward to form a thin gigagauss-magnetized plasma jet, which survives for sub-picosecond after the core of the laser has passed through the tube. [ABSTRACT FROM AUTHOR]

Background: Because of the close relationship between seed coat color and forage seed quality, it is of great significance to reveal the effect of seed coat color on seed germination and seedling growth under drought stress. Methods: We explore the effect of different seed coat colors (yellow, dark brown) on the seed germination and seedling growth of alfalfa under simulated drought stress (polyethylene glycol (PEG-6000) with eight osmotic potentials: 0 MPa, -0.1 MPa, -0.2 MPa, -0.3 MPa, -0.4 MPa, -0.6 MPa, -0.8 MPa, -1.0 MPa). Result: Under control conditions, the germination rate and germination energy of yellow seeds (91.6% and 91.2%, respectively) were higher than dark brown seeds (46.2% and 42.0%). The maximum seed germination rate and germination energy were observed in the -0.1MPa treatment for both yellow and dark brown seeds and were higher than control by 1.53% and 0.66% (yellow) and 9.96%, 14.76% (dark brown), respectively. The root length and seedling growth of two kinds of seeds were significantly inhibited when drought intensity increased. In summary, the germination and seedling growth traits of yellow seeds were much better than dark brown ones. Agronomically, selecting seed with high proportions of yellow coat color is more beneficial for seed storage and establishment of new alfalfa grasslands. [ABSTRACT FROM AUTHOR]

Precision spectroscopy of molecular hydrogen can be used to test the quantum electrodynamics theory and determine the proton-to-electron mass ratio in a four-body system. Cavity-enhanced double-resonance spectroscopy is proposed to determine highly-excited rotation-vibration energies of HD with high precision. As a demonstration, the V-type comb-locked cavity-assisted double resonance (COCA-DR) spectroscopy of HD is measured by pumping the P(1) (2-0) line and probing the R(1) line in the same overtone band. The sum value of the two transition frequencies is determined to be 426 889 423 917 (26) kHz, representing a new test of theoretical calculations. [ABSTRACT FROM AUTHOR]

Branched flow is an interesting phenomenon that can occur in diverse systems. It is usually linear in the sense that the flow does not alter the properties of the medium. Branched flow of light on thin films has recently been discovered. It is therefore of interest to know whether nonlinear light branching can also occur. Here, using particle-in-cell simulations, we find that in the case of an intense laser propagating through a randomly uneven medium, cascading local photoionization by the incident laser, together with the response of freed electrons in the strong laser fields, triggers space–time-dependent optical unevenness. The resulting branching pattern depends dramatically on the laser intensity. That is, the branching here is distinct from the existing linear ones. The observed branching properties agree well with theoretical analyses based on the Helmholtz equation. Nonlinear branched propagation of intense lasers potentially opens up a new area for laser–matter interaction and may be relevant to other branching phenomena of a nonlinear nature. [ABSTRACT FROM AUTHOR]

In experiments on the effect of magnetic field on electron transportation in laser–plasma interaction, the magnetic field is often produced by two coils and is mirror-like. In this paper, the transport and the reflection of fast electron beam generated in laser–plasma interaction in solid-density plasma immersed in a mirror magnetic field are studied using particle-in-cell simulation. The helicoidal motion of fast electrons in the field and the convergence of magnetic induction lines leads to the collimated transport and focusing of the fast electrons. The reflection of the fast electrons can lead to the decrease in the transmission ratio, and this reflection increases with the magnetic mirror ratio, but saturates at a certain level. [ABSTRACT FROM AUTHOR]

We use particle-in-cell simulation to demonstrate that the use of two split intense femtosecond laser pulses can significantly improve the proton energy in the target normal sheath acceleration scheme. By splitting a weak laser pulse from the main laser pulse and irradiating it on the target rear side, the hot electrons are greatly enhanced on both the front and back sides of the solid target, forming a low-density electron layer of a few microns thickness, which in turn further improves the absorption rate of the laser and leads to enhanced acceleration of protons. It is shown that for a wide range of laser intensities, the energy conversion efficiency to protons, the number of high-energy protons, and the cutoff proton energy are almost doubled compared to the single-laser case with the same total laser energy. This simple scheme potentially opens a degree of freedom for manipulating the proton acceleration process and should arouse broad interest due to its wide applications. [ABSTRACT FROM AUTHOR]

The generation and transport of fast electrons play an important role in intense laser interaction with dense plasma. Here, we experimentally study fast electron generation using a planar aluminum target obliquely irradiated at a large incident angle by an ultrashort high-temporal-contrast relativistic laser pulse. Simulations show that for near-relativistic laser intensities, s-polarized lasers above a certain energy threshold can generate energetic electrons () with a high yield, which is different from the case of low laser contrast. It is also shown that the most energetic electrons are emitted along the target's surface and accelerated to high energies. The key features of the acceleration regimes of lasers with different polarizations are also given. [ABSTRACT FROM AUTHOR]

Collision of intermediate-energy heavy ions is an important way to investigate the state of high-density nuclear matter and formation of quark gluon plasmas. In this paper, we demonstrate from two-dimensional particle-in-cell simulations that hundreds-GeV Au79+ ions can be generated by ultrashort ∼1022–24 W cm−2 lasers interacting with high-Z grain doped gas. Under extremely intense laser field, strong γ-ray radiation has resulted in massive laser energy loss, and the invoked radiation reaction trapping of electrons weakens the heavy ion acceleration. However, due to hole-boring acceleration and multi-body expansion acceleration of high-Z grains, the energy conversion efficiency from laser to heavy ions is still up to 32%. Our results might provide an effective routine to carry out the experimental explorations of heavy-ion collisions by using hundreds-petawatt laser facilities. [ABSTRACT FROM AUTHOR]

Terahertz (THz) radiation from a plasma cylinder with embedded radial electric and axial magnetic fields is investigated. The plasma density and the electric and magnetic fields are such that the electron plasma frequency is near the electron cyclotron frequency and in the THz regime. Two-dimensional particle-in-cell simulations show that the plasma electrons oscillate not only in the azimuthal direction but also in the radial direction. Spectral analysis shows that the resulting oscillating current pattern has a clearly defined characteristic frequency near the electron cyclotron frequency, suggesting resonance between the cyclotron and plasma oscillations. The resulting far-field THz radiation in the axial direction is also discussed. [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulations show that when an intense picosecond laser pulse irradiates a target with steep but smooth density profile, the target protons can be accelerated to high energies with small divergence by a combination of target normal sheath acceleration and radiation pressure acceleration. The effects of plasma density profile on proton acceleration and collimation are investigated. In general, smaller(larger) density gradients lead to larger(smaller) self-generated azimuthal magnetic fields and smaller(larger) target-back electrostatic sheath fields, and thus proton beams with smaller(larger) divergence angle as well as cutoff energy. Accordingly, within limits, proton beams with desired peaked spectrum, energy, and divergence angle can be obtained by tailoring the target density profiles. It is also demonstrated that target tailoring can be achieved by having two suitable nanosecond lasers separately irradiating the front and back sides of a uniform plane slab. [ABSTRACT FROM AUTHOR]

The interaction of two parallel relativistic laser beams in underdense plasmas is investigated by considering the evolution of their wave envelopes. The energy transfer between the two lasers is given by an expression based on the evolution of the total laser power in a regime without beam mixing. It is shown that how the energy is transferred depends nonlinearly on the initial phase difference of the lasers, and the result of the interaction depends on the laser intensity, spot radius, and their separation distance. The results are verified by direct numerical solution of the relativistic nonlinear Schrödinger equations for the laser envelopes as well as particle-in-cell simulation. The study and results should be helpful for understanding the energy transfer behavior of multiple co-propagating laser beams in underdense plasmas. [ABSTRACT FROM AUTHOR]

Target-normal sheath acceleration (TNSA) of ions by >100-fs relativistic laser pulses irradiating a multichannel target consisting of a row of parallel long wires and a plane back foil is studied. Two-dimensional particle-in-cell simulations show that the laser light pulls out from the wires a large number of dense hot attosecond electron bunches, which are synergetically accelerated forward by the relativistic ponderomotive force of the laser as well as the longitudinal electric field of a transverse magnetic mode that is excited in the vacuum channels between the wires. These electrons are characterized by a distinct two-temperature energy spectrum, with the temperature of the more energetic electrons close to twice the ponderomotive potential energy. After penetrating through the foil, they induce behind its rear surface a sheath electric field that is both stronger and frontally more extended than that without the channels. As a result, the TNSA ions have much higher maximum energy and the laser-to-ion energy conversion efficiency is also much higher. It is found that a laser of intensity 1.37 × 1020 W/cm2, duration 165 fs, and energy 25.6 J can produce 85 MeV protons and 31 MeV/u carbon ions, at 30% laser-to-ion energy conversion efficiency. The effects of the channel size and laser polarization on the TNSA ions are also investigated. [ABSTRACT FROM AUTHOR]

The growth of single-walled carbon nanotubes (SWCNTs) in plasma-enhanced chemical vapor deposition (PECVD) is studied using a surface diffusion model. It is shown that at low substrate temperatures (<=1000 K), the atomic hydrogen and ion fluxes from the plasma can strongly affect nanotube growth. The ion-induced hydrocarbon dissociation can be the main process that supplies carbon atoms for SWCNT growth and is responsible for the frequently reported higher (compared to thermal chemical vapor deposition) nanotube growth rates in plasma-based processes. On the other hand, excessive deposition of plasma ions and atomic hydrogen can reduce the diffusion length of the carbon-bearing species and their residence time on the nanotube lateral surfaces. This reduction can adversely affect the nanotube growth rates. The results here are in good agreement with the available experimental data and can be used for optimizing SWCNT growth in PECVD. [ABSTRACT FROM AUTHOR]

Oscillations of charged dust grains in the sheath region of a radio-frequency plasma containing negative ions are studied using a one-dimensional model for collisional sheaths. The forces on a dust grain in the sheath region are first analyzed and the force-balance conditions found. Oscillations of the levitated dusts are then investigated. Linear lattice waves, including one-dimensional longitudinal and two-dimensional hexagonal transverse lattice waves, as well as the effects of the negative ions and external magnetic field on the waves, are discussed. [ABSTRACT FROM AUTHOR]

The acceleration of protons in dense plastic foils irradiated by ultrahigh intensity laser pulses is simulated using a two-dimensional hybrid particle-in-cell scheme. For the chosen parameters of the overdense foils of densities ρ=0.2, 1, and 3 g/cm3 and of an ultrahigh intensity (2×1020 W/cm2) laser pulse, our simulations illustrate that a high-density target is favorable to high collimation of the target-normal-sheath acceleration protons but less energy for a short acceleration time (<100 fs). In particular, the difference of strong local heating of the carbon ion for different plasma densities is clearly observed at both the front and rear surfaces of thin solid targets, suggesting that the effect of the density and composition of the targets are also important for correctly simulating energetic ion generation in ultraintense laser-solid interactions. [ABSTRACT FROM AUTHOR]

Plasma transport in a hybrid dc vacuum arc plasma source for ion deposition and plasma immersion treatment is considered. It is found that external crossed electric and magnetic fields near the substrate can significantly reduce the relative amplitude of ion current fluctuations Īf at the substrate surface. In particular, Īf decreases with the applied magnetic field when the bias voltage exceeds 300 V, thus allowing one to reduce the deviations from the rated process parameters. This phenomenon can be attributed to an interaction between the metal-plasma jet from the arc source and the discharge plasma in the crossed fields. [ABSTRACT FROM AUTHOR]

Investigates the operation and mode jumps in low-frequency radio-frequency inductively coupled plasmas. Excitation of the electrostatic and electromagnetic discharge modes by a flat inductive coil; Examination of the power transfer efficiency and mode transition behavior; Increase of the discharge efficiency after the transition to the H discharge regime.

Presents information on a study which investigated the effect of charged particulates or dust particles on surface wave produced microwave discharges. Formulation; Calculation of frequencies of standing electromagnetic eigenmodes of a large-area of flat plasmas; Discussion.

An all-optical method for generating ultra-intense high-order-mode light pulse is investigated with three-dimensional particle-in-cell simulation. We find that the conversion from a short intense circularly polarized incident Gaussian laser pulse into a transverse magnetic (TM) mode occurs as it propagates into a micro plasma waveguide. The strength of the longitudinal electric field of the excited TM modes can be almost two orders of magnitude higher than that of the original laser. The simulation results show that, for the lower-order modes, the trapped electrons lead to their revolving transverse structures. A linear plasma waveguide model is presented to predict the mode pattern and intensity of the longitudinal electric fields, which are in excellent agreement with those in the simulations. Relativistic-intense high-order-mode light can be useful for many applications, including accelerating charged particles to high energies. [ABSTRACT FROM AUTHOR]

The acceleration of protons from the interaction of two intersecting laser pulses in a double-layer target, consisting of a carbon slab with a thin hydrogen layer behind it, is investigated. It is found that a bunch of well-collimated, up to 270 MeV, quasi-monoenergetic protons can be generated. The affected target electrons are heated and accelerated forward by the laser pulses so that the target gradually becomes transparent as the lasers bore into it. Eventually, the thin target becomes fully transparent. The converging laser pulses can then pass through it and the hot backside electron cloud, together with the electrons trapped in them. As a result, behind the target, an inhomogeneous sheath electric field is formed, which not only accelerates the protons but also merges them into a quasi-monoenergetic bunch with low divergence. The energy and spatial spread of the bunch, as well as the peak energy, can be controlled by manipulating the locations of the two lasers. [ABSTRACT FROM AUTHOR]

When an intense right-hand circularly polarized laser pulse propagates through a highly magnetized (in the direction of laser propagation) foil into the vacuum region between this foil and an unmagnetized foil, it can be multiply reflected and thus quasitrapped between the two foils until its energy is depleted. During the multiple reflections at the magnetized foil, foil electrons are accelerated by the laser light. Electrons that have gained sufficient energy can escape and appear as periodic energetic electron bunches at a time interval determined precisely by the trapping time or the distance between the two foils. [ABSTRACT FROM AUTHOR]

Target-normal sheath acceleration (TNSA) of protons from a solid-density plasma target consisting of a thin foil, with a thin hydrogen layer behind it and a plasma-filled tube with a parabolic density profile at its front, is investigated using two-dimensional particle-in-cell simulation. It is found that the targetback sheath field induced by the laser driven hot electrons is double peaked, so that the protons are additionally accelerated. The hot sheath electrons, and thus the TNSA protons, depend strongly on the tube plasma, which unlike the preplasma caused by the laser prepulse can be easily controlled. It is also found that the most energetic and best collimated TNSA protons are produced when the tube plasma is of near-critical density. [ABSTRACT FROM AUTHOR]

A normalized weighting function model for the average period of particles trapped in the potential wells of waves undergoing nonlinear Landau damping is considered. The model preserves the conservation properties of the trapped particles and relates them to the evolution of the wave envelope. It is valid when there are more shallowly trapped particles than deeply trapped particles, such as in long-time nonlinear Landau damping, as well as stabilities due to trapped particles at long times. The model is also useful for estimating the validity of the often invoked assumption that most trapped particles in nonlinear waves are deeply trapped. [ABSTRACT FROM AUTHOR]

Long distance propagation of an energetic laser pulse with intensity slightly below that for multi-photon ionization in air is considered analytically, by noting that in the process, it is mainly the peak region of the pulse that interacts with the air molecules. Similar to that of much shorter femtosecond laser pulses of similar intensity, the affected air becomes slightly ionized and self-consistently forms a co-propagating thin and low-density plasma filament along the axis. It is found that a hundred-Joule-level laser pulse with a relatively large spot radius and pulse duration can propagate (also in the form of a self-consistent filament) tens of kilometers through the atmosphere. Such laser propagation properties should have applications in many areas. [ABSTRACT FROM AUTHOR]

The effect of perpendicular (to the direction of shock propagation) magnetic fields on electron shock-surfing acceleration (ESSA) is investigated. Two-dimensional particle-in-cell simulations show that the strength of background magnetic fields can significantly affect the shock formation process and the electrostatic field structure near the leading edge of the shock. It is found that ESSA is most efficient when the ratio of electron cyclotron frequency to plasma frequency is about 0.1. In this case, there are two types of electrostatic field structures near the shock edge, namely, small-scale filamentary and large-scale dipole electric fields, induced by the Buneman instability and fluid compression, respectively. In such a mixed field structure, the affected electrons can undergo multiple accelerations and gain much higher energies than that under weaker or stronger background magnetic fields. [ABSTRACT FROM AUTHOR]

A practical approach to achieve strong coherent synchrotron emissions (CSE) in relativistic laser-plasma interaction is proposed, where a plane target with its electron density satisfying the self-similar parameter S≃ne0/a0nc=1 is obliquely irradiated by a P-polarized laser pulse. In this case, electrons at the target surface are periodically dragged out into the vacuum by the laser field component perpendicular to the target surface, resulting in the formation of a series of dense electron bunches propagating along the target surface. Intense CSE is generated by these electron bunches under acceleration by the laser field component parallel to the target surface. Two-dimensional particle-in-cell simulations show that an intense attosecond light pulse at intensity 9.1 × 1020 W/cm2 (electric field strength ~41% as that of the drive laser) can be obtained through such CSE. In the high-order harmonics with 15ω0<ωn500ω0 (ω0 is the laser frequency), the power spectrum of the emission scales as I(n)~n-1.8 and the conversion efficiency from laser to emission reaches ~10-2. [ABSTRACT FROM AUTHOR]

In magnetized toroidal plasmas, neoclassical effects and turbulent drift waves can induce the geodesic acoustic mode (GAM). We simulate the GAM using the gyro-kinetic code GTS for typical tokamak parameters and investigate its properties, especially its frequency continuum, evolution of its radial wave number, and propagation characteristics. The simulation results are compared with those of the relevant theory and experiment. It is found that the radial phase velocity of the GAM is roughly proportional to the ion thermal speed. [ABSTRACT FROM AUTHOR]

Instability of a force-free current sheet with a localized nonuniform guide field in a Vlasov-Maxwell plasma is investigated. Two oppositely propagating linearly unstable modes with quite different mode structures are found. The growth rate of one mode varies nonmonotonically with the magnitudes of the guide field and the main-field aligned component of its wave vector. The unstable mode is weakened as the guide-field nonuniformity is reduced, and it vanishes when the guide field becomes uniform. The growth rate of the other mode is almost unaffected by the guide field nonuniformity. However, it decreases monotonously with the magnitudes of the guide field and the wavevector component in the direction of the main field. [ABSTRACT FROM AUTHOR]

Ion acceleration from the interaction of two intersecting intense laser pulses with an overdense plasma is investigated using a three-dimensional particle-in-cell simulation. It is found that, comparing with the single-pulse case, the charge of the resulting energetic ion bunch can be increased by more than an order of magnitude without much loss of quality. Dependence of the ion charge on the interaction parameters, including separation distance and incidence angles of the lasers, is considered. It is shown that the charge of the accelerated ion bunch can be optimized by controlling the degree of laser overlapping. The improved performance can be attributed to the enhanced laser intensity as well as stochastic heating of the accelerated electrons. Since at present the intensity of readily available lasers is limited, the two pulse scheme should be useful for realizing higher laser intensity in order to achieve higher-energy target normal sheath acceleration ions. [ABSTRACT FROM AUTHOR]

The evolution of a small spatially periodic perturbation in the electron velocity distribution function in collisionless plasma is reconsidered by numerically solving the Vlasov and Poisson equations. The short as well as long time behaviors of the excited oscillations and damping/ modulation are followed. In the small but finite-amplitude excited plasma wave, resonant electrons become trapped in the wave potential wells and their motion affects the low-velocity electrons participating in the plasma oscillations, leading to modulation of the latter at an effective trapping frequency. It is found that the phase space of the resonant and low-velocity electrons becomes chaotic, but then self-organization takes place but remains fine-scale chaotic. It is also found that as long as particles are trapped, there is only modulation and no monotonic damping of the excited plasma wave. The modulation period/amplitude increases/decreases as the magnitude of the initial disturbance is reduced. For the initial and boundary conditions used here, linear Landau damping corresponds to the asymptotic limit of the modulation period becoming infinite, or no trapping of the resonant electrons. [ABSTRACT FROM AUTHOR]

Propagation of intense circularly polarized laser pulses in strongly magnetized inhomogeneous plasmas is investigated. It is shown that a left-hand circularly polarized laser pulse propagating up the density gradient of the plasma along the magnetic field is reflected at the left-cutoff density. However, a right-hand circularly polarized laser can penetrate up the density gradient deep into the plasma without cutoff or resonance and turbulently heat the electrons trapped in its wake. Results from particle-incell simulations are in good agreement with that from the theory. [ABSTRACT FROM AUTHOR]

Influence of the system temperature on the micro-structures and dynamics of dust clusters in dusty plasmas is investigated through laboratory experiment and molecular dynamics simulation. The micro-structures, defect numbers, and pair correlation function of the dust clusters are studied for different system temperatures. The dust grains' trajectories, the mean square displacement, and the corresponding self-diffusion coefficient of the clusters are calculated for different temperatures for illustrating the phase properties of the dust clusters. The simulation results confirm that with the increase in system temperature, the micro-structures and dynamics of dust clusters are gradually changed, which qualitatively agree with experimental results. [ABSTRACT FROM AUTHOR]

Effects of the system temperature on dust aggregation in plasmas are investigated using two-dimensional molecular dynamics simulations. It is shown that as the system temperature increases, the boundary of the clusters becomes gradually irregular (i.e., deviating from sphere-like), and the cluster system gradually changes from solid to liquid and finally to gas state. The mean square displacement, mean nearest-neighbor distance in the clusters, cluster size and coupling parameter of the system are obtained and the properties of the system structure and dynamics are investigated. The time τ needed for reaching equilibrium for different temperatures is obtained. It is shown that τ firstly decreases and then increases with the temperature, indicating that there is an optimum temperature allowing a dust aggregation to reach an equilibrium state most rapidly. The simulation results agree qualitatively with the experimental observations. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

In a long subcritical density plasma, an ultrashort ultraintense laser pulse can self-organize into a fast but sub-relativistic propagating structure consisting of the modulated laser light and a large number of trapped electrons from the plasma. Upon impact of the structure with a solid foil target placed in the latter, the remaining laser light is reflected, but the dense and hot trapped electrons pass through the foil, together with the impact-generated target-frontsurface electrons to form a dense hot electron cloud at the back of the target suitable for enhancing target normal sheath acceleration of the target-backsurface ions. The accelerated ions are well collimated and of high charge and energy densities, with peak energies a full order of magnitude higher than that from target normal sheath acceleration without the subcritical density plasma. In the latter case, the space-charge field accelerating the ions is limited since they are formed only by the target-frontsurface electrons during the very short instant of laser reflection. [ABSTRACT FROM AUTHOR]

Backward Raman amplification (BRA) in plasmas has been promoted as a means for generating ultrapowerful laser pulses. For the purpose of achieving the maximum intensities over the shortest distances, an envelope matching between the seed pulse and the amplification gain is required, i.e., the seed pulse propagates at the same velocity with the gain such that the peak of the seed pulse can always enjoy the maximum gain. However, such an envelope matching is absent in traditional BRA because in the latter the amplification gain propagates at superluminous velocity while the seed pulse propagates at the group velocity, which is less than the speed of light. It is shown here that, by using self-ionizing plasmas, the speed of the amplification gain can be well reduced to reach the envelope matching regime. This results in a favorable BRA process, in which higher saturated intensity, shorter interaction length and higher energy-transfer efficiency are achieved. [ABSTRACT FROM AUTHOR]

Expansion of the ion and electron fronts of a cold non-neutral plasma slab with a quasi-neutral core bounded by layers containing only ions is investigated analytically and exact solutions are obtained. It is found that on average, the plasma expansion time scales linearly with the initial inverse ion plasma frequency as well as the degree of charge imbalance, and no expansion occurs if the cold plasma slab is stationary and overall neutral. However, in both cases, there can exist prominent oscillations on the electron front. [ABSTRACT FROM AUTHOR]

Excitation of streamers by modulationally unstable small-scale drift-Alfven wave (SSDAW) is investigated. It is found that the excitation depends strongly on the propagation direction of the SSDAW, and the ion and electron diamagnetic drift waves are both unstable due to the generation of streamers. It is also shown that zonal flows can be effectively excited by the SSDAW with the propagation direction different from that for streamer excitation. [ABSTRACT FROM AUTHOR]

Electron injection into the wakefield of an intense short laser pulse by a weaker laser pulse propagating in the opposite direction is reconsidered using two-dimensional (2D) particle-in-cell simulations as well as analytical modeling. It is found that for linearly polarized lasers the injection efficiency and the quality of the wakefield accelerated electrons increase with the intensity of the injection laser only up to a certain level, and then decreases. Theory and simulation tracking test electrons originally in the beat region of the two laser pulses show that the reduction of the injection efficiency at high injection-laser intensities is caused by stochastic overheating of the affected electrons. [ABSTRACT FROM AUTHOR]

Superelasticity and mechanical damping are important functional properties of nickel-titanium (NiTi) shape memory alloys (SMAs). Owing to the generation and accumulation of dislocation during loading, the recovery strain of commercial NiTi SMAs is usually smaller than 10%, which limits their ability to dissipate energy. In this paper, the superelasticity and mechanical damping of a nanocrystalline NiTi SMA was studied. The results show that the nanocrystalline NiTi SMA alloy possessed a large recovery strain of about 14%, greater than that of the commercial NiTi SMAs, and a high level of absorbed energy, or toughness, of 111 MJ m−3, which is higher than the highest value (about 81 MJ m−3) of all SMAs reported so far. The transmission electron microscopy (TEM) studies suggest that few full dislocations were generated in the nanocrystalline NiTi alloy during loading. Instead, the dominant deformation modes after stress induced martensitic transformation were elastic deformation and detwinning. The detwinning process decreased the twin boundary energy, which stabilised the martensitic phase. [ABSTRACT FROM AUTHOR]

Three-dimensional particle-in-cell simulation is used to investigate radiation-pressure driven acceleration and compression of small solid-density plasma by intense laser pulses. It is found that multiple impacts by presently available short-pulse lasers on a small hemispheric shell target can create a long-living tiny quasineutral monoenergetic plasma bunch of very high energy density. [ABSTRACT FROM AUTHOR]

It is shown that the intense quasistatic electric and magnetic fields self-generated near the axis of the laser-driven channel in an appropriately profiled preplasma during ultraintense laser interaction with a thin target can create dense relativistic electron bunches. The latter easily penetrate through the target and can greatly enhance the sheath field at the rear, resulting in significant increase in the laser-to-ion energy conversion efficiency and the maximum energy of the target normal sheath accelerated ions. Particle-in-cell simulations show that with a hydrogen targets a proton beam of peak energy ∼38 MeV and energy conversion efficiency ≥6.5% can be produced by a linearly polarized 5 × 1019 W/cm2 laser. An analytical model is also proposed and its results agree well with those of the simulations. [ABSTRACT FROM AUTHOR]

Electron acceleration by transversely echelon phase-modulated (EPM) circularly polarized (CP) intense laser pulse is investigated. Solution of the relativistic electron equations of motion shows that the CP EPM light wave structure can disrupt the harmonic response of a trapped electron not only in the transverse direction but also in the direction of laser propagation. In each laser cycle, there can be a net gain in the electron's transverse momentum, which is promptly converted into the forward direction by the Lorentz force. As a result, the electron can be trapped and accelerated in the favorable phase of the laser for a rather long time. Its momentum gain then accumulates and can eventually reach high levels. It is also found that with the CP EPM laser, the net acceleration of the electron is not sensitive to its initial position and velocity relative to the phase of the laser fields, so that such a laser can also be useful for accelerating thermal electron bunches to high energies. [ABSTRACT FROM AUTHOR]