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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 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]

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]

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]

A two-phase proton acceleration scheme using an ultra-intense laser pulse irradiating a proton foil with a tenuous heavier-ion plasma behind it is presented. The foil electrons are compressed and pushed out as a thin dense layer by the radiation pressure and propagate in the plasma behind at near the light speed. The protons are in turn accelerated by the resulting space-charge field and also enter the backside plasma, but without the formation of a quasistationary double layer. The electron layer is rapidly weakened by the space-charge field. However, the laser pulse originally behind it now snowplows the backside-plasma electrons and creates an intense electrostatic wakefield. The latter can stably trap and accelerate the pre-accelerated proton layer there for a very long distance and thus to very high energies. The two-phase scheme is verified by particle-in-cell simulations and analytical modeling, which also suggests that a 0.54 TeV proton beam can be obtained with a 1023 W/cm2 laser pulse. [ABSTRACT FROM AUTHOR]

A cone-nanolayer target that combines the advantages of the conical and layered geometries for electron acceleration in laser-target interaction is proposed. Two-dimensional particle-in-cell (PIC) simulations show that the cone-nanolayer target can enhance laser absorption and electron yield. With suitable choice of the laser and target parameters, the cone-nanolayer target can be a controllable source of hot electrons at desired energy ranges. [ABSTRACT FROM AUTHOR]

A simple but comprehensive two-dimensional analytical model for the interaction of a normally incident short intense laser pulse with a solid-density plasma is proposed. Electron cavitation near the target surface by the laser ponderomotive force induces a strong local electrostatic charge-separation field. The cavitation makes possible mode conversion of the laser light into longitudinal electron oscillation at laser frequency, even for initial normal incidence of laser pulse. The intense charge-separation field in the cavity can significantly enhance the laser induced u×B electron oscillation at twice laser frequency to density levels even higher than that of the initial target. [ABSTRACT FROM AUTHOR]

Generation and propagation of attosecond electron bunches along a cone-and-channel target are investigated by particle-in-cell simulation. The target electrons are pulled out by the oscillating electric field of an intense laser pulse irradiating a cone target and accelerated forward along the cone walls. It is shown that the energetic electrons can be further guided and confined by a channel attached to the cone tip. The propagation of these electrons along the channel induces a strong quasistatic magnetic field as well as a sheath electric field since a part of the energetic electrons expands into the surrounding vacuum. The electromagnetic field in turn confines the surface currents. With the cone-and-channel target the energetic electrons can be much better collimated and propagate much farther than that from the classical cone target. [ABSTRACT FROM AUTHOR]

Hot electrons generated by short-pulse-laser interaction with nanolayered target (NT) are investigated using two-dimensional particle-in-cell simulation. Compared to the planar target, the NT leads to more efficient conversion of laser energy to the kinetic energy of the accelerated electrons. However, the energy absorption by the NT decreases at both too-low and too-high laser intensities. At lower laser intensities it is because of the weaker electric and magnetic fields generated by the hot-electron jets and smaller relativistic skin depth. At higher laser intensities it is because of the damage or destruction of the layered structure by the laser field. On the other hand, the dependence of the conversion efficiency and hot-electron number on the duration of the (short) laser pulse and the nanolayer length is weak. Control of the hot-electron characteristics by tailoring the parameters of the laser and the NT is discussed. [ABSTRACT FROM AUTHOR]

Nonlinear coupling of the motion in the three spatial degrees of freedom of a cold fluid electron-positron plasma is investigated. Exact solutions describing expanding flows with oscillations are obtained. It is found that the energy in the irrotational flow component is in general transferred to the rotational components, but not in the reversed direction. Furthermore, since the density evolution need not be related to all the three flow components, oscillations in one or two of the flow fields can be purely electromagnetic and are not accompanied by density oscillations. [ABSTRACT FROM AUTHOR]

Fully nonlinear electrostatic waves in a plasma containing electrons, positrons, and ions are investigated by solving the governing equations exactly. It is found that both smooth and spiky quasistationary waves exist, and large-amplitude waves necessarily have large-phase velocities, but small-amplitude waves can be both fast and slow. [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulation is used to investigate the acceleration of foil ions by the radiation pressure of an ultraintense short laser pulse in the presence of a background plasma of proper density and length behind the foil. It is shown that under appropriate conditions a central foil segment with a transverse dimension ∼σL/2, where σL is the laser spot size, can be stably accelerated. In this plasma backed acceleration scheme, foil electron heating and ion expansion are greatly suppressed by the cool backside electrons that replace the expelled fast electrons in the target, so that a monoenergetic collimated GeV ion sheet is produced. The simulation results agree with that from a physical model for the stably accelerated foil segment. [ABSTRACT FROM AUTHOR]

Electron acceleration in the low-density preplasma of a thin solid target by a short intense laser pulse is investigated by particle-in-cell simulation. Electrons in the preplasma are trapped and accelerated by the laser ponderomotive force as well as the wake bubble field. When the laser pulse is stopped and reflected by the target, the trapped electrons continue to move forward inertially, passing through the target with small energy spread and emittance. The use of the present scheme in practical applications is discussed. [ABSTRACT FROM AUTHOR]

The trajectories of electrons originating from different initial locations in the unperturbed plasma during the interaction of an ultraintense laser with underdense plasma in the bubble regime are followed by particle-in-cell simulation. It is found that plasma electrons initially aligned with the rim of the laser focal spot contribute most to the bow wave in front of the bubble and those aligned with the lateral bubble sheath edge contribute most to the self-injection at the back of the bubble. A scaling law between the transverse electric and magnetic fields for the case where there are many electrons in the bubble is given and discussed in terms of betatron oscillations of the injected electrons. [ABSTRACT FROM AUTHOR]

The formation of collisionless electrostatic shock (CES) and ion acceleration in thin foils irradiated by intense laser pulse is investigated using particle-in-cell simulation. The CES can appear in the expanding plasma behind the foil when self-induced transparency occurs. The transmitting laser pulse can expel target-interior electrons, in addition to the electrons from the front target surface. The additional hot electrons lead to an enhanced and spatially-extended sheath field behind the foil. As the CES propagates in the plasma, it also continuously forward-reflects many of the upstream ions to higher energies. The latter ions are further accelerated by the enhanced sheath field and can overtake and shield the target-normal sheath accelerated ions. The energy gain of the CES accelerated ions can thus be considerably higher than that of the latter. [ABSTRACT FROM AUTHOR]

The lower-hybrid drift instability (LHDI) in a thin current sheet in the intermediate-wavelength ([formula], where ky, ρe, and ρi are the wave vector and the electron and ion gyroradii, respectively) regime for particles with κ velocity distribution is studied. The latter is more suitable for describing nonthermal distributions with an enhanced high-energy tail and includes the Maxwellian as a limiting case. It is shown that linear electromagnetic LHDI can be excited near the center of the current sheet. The growth rate decreases, but the electromagnetic component of the LHD mode increases with increase in hot particles. [ABSTRACT FROM AUTHOR]

Energetic proton generation from thin foil targets by ultraintense laser pulse is investigated using two-dimensional particle-in-cell simulation. Foil targets with concave, flat, and convex front surfaces are considered. The maximum energy of the accelerated protons depends on the front-surface curvature, and the highest-energy protons are from the concave foil. The result can be attributed to an enhancement of the generation as well as concentration of the laser-driven hot electrons by the concave surface. [ABSTRACT FROM AUTHOR]

Resonance absorption in relativistic laser-plasma interaction is studied via two-dimensional particle-in-cell simulation. As the laser intensity increases from the linear regime, the absorption rate first decreases due to relativistic modulation of the electron plasma oscillations excited at the mode conversion layer. However, the trend reverses after a critical intensity. The reversal can be attributed to the fact that the relativistic critical layer depends on the local intensity of the laser pulse, so that instead of occurring in a thin layer, resonance absorption occurs in a plasma bulk region, leading absorption rate increase. The reflected-light spectrum also shows broadening and splitting of the harmonics at high laser intensities, which can be attributed to critical-surface oscillations driven by the laser ponderomotive force. [ABSTRACT FROM AUTHOR]

Ion acceleration by ultrashort circularly polarized laser pulse in a solid-density target is investigated using two-dimensional particle-in-cell simulation. The ions are accelerated and compressed by the continuously extending space-charge field created by the evacuation and compression of the target electrons by the laser light pressure. For a sufficiently thin target, the accelerated and compressed ions can reach and exit from the rear surface as a high-density high-energy ion bunch. The peak ion energy depends on the target thickness and reaches maximum when the compressed ion layer can just reach the rear target surface. The compressed ion layer exhibits lateral striation which can be suppressed by using a sharp-rising laser pulse. [ABSTRACT FROM AUTHOR]

The interaction of an ultraintense laser pulse with parabolic concave target is studied using two-dimensional particle-in-cell simulation. It is found that ions at the back of the concave target can be accelerated more effectively than that with a plane target because of a large electrostatic field on the propagation axis induced by focusing of the laser expelled electrons. A highly collimated ion bunch with high particle and current densities appears. The dependence of the quality of ion acceleration and bunching on the width of the concave cavity is examined. It is found that the maximum ion energy is greatly enhanced when the width of the cavity attains a threshold, and an optimum width for generating a high-quality ion bunch exists. The results should be useful in the design of targets used in producing highly collimated energetic ions in high-energy-density physics. [ABSTRACT FROM AUTHOR]

Nonlinear characteristics including spatial chaos and patterns associated with relativistically intense laser-plasma interaction are studied theoretically and numerically using a model relativistic nonlinear Schrödinger equation. It is shown that in the phase space irregular homoclinic orbit crossings exist. The latter are verified and investigated numerically. The spatial chaos and complex patterns of the laser wave field can be attributed to the relativistic electron mass variation as well as the ponderomotive-force driven electron-density modulation. The formation of complex patterns results from stochastic partition of energy in the Fourier modes. [ABSTRACT FROM AUTHOR]

The electric potential and the electron- and ion-density profiles near a spherical probe in electronegative plasmas containing negative as well as positive ions are studied. It is shown that for relatively large probe radius and neutral-gas pressure, and/or small electron-to-ion temperature ratio, the density profile of the negative ions can differ from that of Boltzmann if their flux near the probe is outwards. The existence of stationary states is considered and the corresponding maximum negative-ion flux is obtained. [ABSTRACT FROM AUTHOR]

An electron with an appropriate initial velocity injected into an oncoming, ultraintense circularly polarized laser pulse can execute a circular relativistic motion at the peak of the laser pulse. The circulating electron then radiates in the same manner as that in the storage ring of a conventional synchrotron source. Owing to the extremely small orbit radius, the laser-field synchrotron radiation thus generated can be a compact source of radiation pulses at short wavelength and short duration. [ABSTRACT FROM AUTHOR]

The ambipolar diffusion of plasma particles in a column of inhomogeneous plasma containing dust grains is investigated analytically as well as numerically. The grains act as charge sinks and behave like an immobile but charge-varying negative background. They can lead to significant depletion of the electron density and decrease the diffusion scale length. The theoretically obtained election density distribution agrees well with that from experiment. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The transport and heating of laser produced relativistic electron beam propagating through coaxial two-layer solid-density plasma are studied using two-dimensional hybrid fluid-particle-in-cell simulation. When the energetic electrons enter the plastic and copper target, stronger heating is observed in the plastic plasma. The heating of the plasma electrons and the formation of tens of million Gauss interface magnetic fields results from a large difference of the velocities of the returning electrons at the two sides of the interface. Moreover, the electron beam can have a completely different emittance behavior when the target consists of different-material layers. [ABSTRACT FROM AUTHOR]

An equation describing the propagation and instability behavior of magnetic electron drift vortex modes in a cylindrically symmetric rotating plasma is derived without using the local approximation. The effects of equilibrium density, velocity, and temperature, as well as magnetic field gradients, are included. An approximate analytical solution for modes near the center of the plasma is obtained. [ABSTRACT FROM AUTHOR]

It is shown that magnetic electron drift vortex waves can cause electron energy transport in a nonuniform plasma. Using a quasilinear approach, an expression for the corresponding anomalous thermal conductivity is derived. The result may be relevant to the study of energy transport in laser-produced as well as other plasmas. [ABSTRACT FROM AUTHOR]

The interaction of two energetic electron bunches generated in the wakefields of two intense intersecting laser pulses in rarefied plasmas is investigated using particle-in-cell simulations. It is found that, with suitable intersection angle between the two laser pulses, the initially independent wakefield accelerated electron bunches can merged into a single one with high charge, energy, and narrow energy spread. The dynamics of the laser-pulse intersection and wake-bubble merging process is also investigated, and the crucial roles of the intersection angle are pointed out and analyzed. [ABSTRACT FROM AUTHOR]

The penetration of an ultrashort laser pulse into an overdense plasma in the relativistic transparency regime is reexamined. The interaction is governed by the Maxwell and relativistic hydrodynamic equations and investigated using a fully implicit energy-conserving numerical scheme. It is found that for a circularly polarized laser, the penetrated pulse has the expected soliton structure. However, for a linearly polarized laser, the penetrated light exhibits a breather structure, and energy exchange between it and the plasma is at twice the laser frequency. [ABSTRACT FROM AUTHOR]

It is demonstrated experimentally and by numerical simulations that the presence of a long-pulse-laser created back plasma on the target backside can focus the relativistic electrons produced by short-pulse laser interaction with the front of a solid target. Comparing this to that without the back plasma, the number density of the fast electrons is increased by one order of magnitude, and their divergence angle is reduced fivefold. The effect is attributed to the absence of the backside sheath electric field and the collimation effect of the megagauss self-generated baroclinic magnetic field there. Such an acceleration scheme can be useful to applications requiring high-energy and charge-density electron bunches, such as fast ignition in inertial fusion. [ABSTRACT FROM AUTHOR]

Reconnection of the self-generated magnetic fields in laser-plasma interaction was first investigated experimentally by Nilson et al. [Phys. Rev. Lett. 97, 255001 (2006)] by shining two laser pulses a distance apart on a solid target layer. An elongated current sheet (CS) was observed in the plasma between the two laser spots. In order to more closely model magnetotail reconnection, here two side-by-side thin target layers, instead of a single one, are used. It is found that at one end of the elongated CS a fanlike electron outflow region including three well-collimated electron jets appears. The (> 1 MeV) tail of the jet energy distribution exhibits a power-law scaling. The enhanced electron acceleration is attributed to the intense inductive electric field in the narrow electron dominated reconnection region, as well as additional acceleration as they are trapped inside the rapidly moving plasmoid formed in and ejected from the CS. The ejection also induces a secondary CS. [ABSTRACT FROM AUTHOR]

An electron injection regime in laser wake-field acceleration, namely electron bow-wave injection, is investigated by two- and three-dimensional particle-in-cell simulation as well as analytical model. In this regime electrons in the intense electron bow wave behind the first bubble catch up with the bubble tail and are trapped by the bubble finally, resulting in considerable enhancement of the total trapped electron number. For example, with the increase of the laser intensity from 2 × 1019 to 1 × 1020 W/cm2, the electron trapping changes from normal self-injection to bow-wave injection and the trapped electron number is enhanced by two orders of magnitude. An analytical model is proposed to explain the numerical observation. [ABSTRACT FROM AUTHOR]

Transport of high-current relativistic electron beams in dense plasmas is of interest in many areas of research. However, so far the mechanism of such beam-plasma interaction is still not well understood due to the appearance of small time- and space-scale effects. Here we identify a new regime of electron beam transport in solid-density plasma, where kinetic effects that develop on small time and space scales play a dominant role. Our three-dimensional particle-in-cell simulations show that in this regime the electron beam can evolve into layered short microelectron bunches when collisions are relatively weak. The phenomenon is attributed to a secondary instability, on the space- and timescales of the electron skin depth (tens of nanometers) and few femtoseconds of strong electrostatic modulation of the microelectron current filaments formed by Weibel-like instability of the original electron beam. Analytical analysis on the amplitude, scale length, and excitation condition of the self-generated electrostatic fields is clearly validated by the simulations. [ABSTRACT FROM AUTHOR]

The dynamics of electron injection in the direct laser acceleration (DLA) regime is investigated using three-dimensional particle-in-cell simulations and theoretical analyses. It is shown that, as an ultraintense laser pulse propagates into a near-critical density or relativistically transparent plasma, the longitudinal charge-separation electric field excites ion density spikes, which modulate the local electric field. The corresponding electric field acts as a series of potential wells to guide the electrons on the edge of the plasma channel into its center where the DLA can take place. On the other hand, an azimuthal magnetic field is self-generated, and it can deflect the injected electrons from the intense laser-field region. Understanding these physical processes paves the way for further optimizing the properties of direct-laser accelerated electron beams and the associated x- and γ-ray sources. [ABSTRACT FROM AUTHOR]

Generation of intense coherent THz radiation by obliquely incidenting an intense laser pulse on a wire target is studied using particle-in-cell simulation. The laser-accelerated fast electrons are confined and guided along the surface of the wire, which then acts like a current-carrying line antenna and under appropriate conditions can emit electromagnetic radiation in the THz regime. For a driving laser intensity ~3×1018W/cm2 and pulse duration ~10 fs, a transient current above 10 KA is produced on the wire surface. The emission-cone angle of the resulting ~0.15 mJ (~58 GV/m peak electric field) THz radiation is ~30°. The conversion efficiency of laser-to-THz energy is ~0.75%. A simple analytical model that well reproduces the simulated result is presented. [ABSTRACT FROM AUTHOR]