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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.

The authors propose a scheme utilizing an underdense plasma of spiral thickness in generating relativistic vortex laser light with axial orbital angular momentum. The laser ponderomotive and the plasma charge-separation forces exert a torque on the plasma ions, resulting in creation of oppositely directed forces in the plasma ions and the laser light.

We propose to use intense optical vortex to control laser-based ion acceleration for obtaining high-quality ion beams. An acceleration field favorable for generating well-collimated energetic proton beams results from the interaction of a tailored vortex laser pulse with thin solid-density foil in a blowout regime. Three-dimensional particle-in-cell simulations show that the foil protons can be efficiently accelerated to the GeV level in the form of a beam with small radius (μm), narrow divergence (π mm mrad). The proton beam is of high energy density (>1018 J m−3) and high brightness (>1022 A m−2 rad−2), exceeding that of the Gaussian laser case by four orders of magnitude, and the energy conversion efficiency is about 12 times that under the same laser intensity. The scheme can also be used to accelerate heavier, such as carbon, ions. The resulting ion beams should be useful as compact neutron source, for creation of warm dense matters, as well as ion-beam direct and indirect drive inertial confinement fusion, ultrafast diagnostics of the implosion dynamics in the latter, etc.

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 $\ensuremath{\gamma}$-ray sources.

Target-normal sheath acceleration (TNSA) of monoenergetic proton bunches using a triple-layer target consisting of a long slightly-above-critical-density carbon preplasma, a thin solid-density carbon main target layer, and a thin hydrogen layer attached to its back, is considered. The target-back space-charge field the consists of mainly the preplasma electrons that have been accelerated and heated by the relativistic laser pulse. The backside electron cloud is thus denser and more favorably profiled for TNSA of the protons from the hydrogen layer. It is found that a proton beam with a $\ensuremath{\sim}460\text{ }\text{ }\mathrm{MeV}$ monoenergetic peak energy can be generated. The corresponding energy and angular spread are $\ensuremath{\sim}33\text{ }\text{ }\mathrm{MeV}$ and $\ensuremath{\sim}3\ifmmode^\circ\else\textdegree\fi{}$, respectively.

It is shown by particle-in-cell simulation that intense circularly polarized (CP) laser light can be contained in the cavity of a solid-density circular Al-plasma shell for hundreds of light-wave periods before it is dissipated by laser-plasma interaction. A right-hand CP laser pulse can propagate with almost no reflection and attenuation into the cavity through a highly magnetized overdense H-plasma slab filling the entrance hole. The entrapped laser light is then multiply reflected at the inner surfaces of the slab and shell plasmas, slowly losing energy to the latter. Compared to that of the incident laser, the frequency is only slightly broadened and the wave vector slightly modified by the appearance of weak nearly isotropic and homogeneous fluctuations.

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.

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.

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.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.

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.

Radially polarized intense terahertz (THz) radiation behind a thin foil irradiated by ultrahigh-contrast ultrashort relativistic laser pulse is recorded by a single-shot THz time-domain spectroscopy system. As the thickness of the target is reduced from 30 to 2 \textmu{}m, the duration of the THz emission increases from 5 to over 20 ps and the radiation energy increases dramatically, reaching $\ensuremath{\sim}10.5\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}$ per pulse, corresponding to a laser-to-THz radiation energy conversion efficiency of $1.7%$. The efficient THz emission can be attributed to reflection (deceleration and acceleration) of the laser-driven hot electrons by the target-rear sheath electric field. The experimental results are consistent with that of a simple model as well as particle-in-cell simulation.

Radially polarized intense terahertz (THz) radiation is measured experimentally behind a thin foil irradiated by ultrashort relativistic intense laser pulse of ultrahigh contrast. As the target thickness is reduced from 30 to 2 μm, the energy of the THz emission increases dramatically, which even reaches up to 10.5 mJ per pulse, corresponding to energy conversion efficiency of 1.7%. This efficient THz emission is attributed to bremsstrahlung radiation of the energetic electrons rapidly refluxing between the target front and back electrostatic sheaths fields created during the laser-target interaction. Analytical modeling and particle-in-cell simulation confirms this interpretation.

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 ∼3×10^{18}W/cm^{2} 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.

The effect of a transient magnetic field on right-hand circularly polarized (RHCP) laser light propagation in overcritical-density plasma is investigated. When the electron gyrofrequency is larger than the wave frequency, RHCP light can propagate along the external magnetic field in an overcritical density plasma without resonance or cutoff. However, when the magnetic field falls to below the cyclotron resonance point, the propagating laser pulse will be truncated and the local plasma electrons resonantly heated. Particle-in-cell simulation shows that when applied to a thin slab, the process can produce intense two-cycle light pulses as well as long-lasting self-magnetic fields.

We have investigated the polarization of attosecond light pulses generated from relativistic few-cycle laser pulse interaction with the surface of overdense plasmas using particle-in-cell simulation. Under suitable conditions, a desired polarization state of the generated attosecond pulse can be achieved by controlling the polarization of the incident laser. In particular, an elliptically polarized laser pulse of suitable ellipticity can generate an almost circularly polarized attosecond pulse without compromising the harmonic generation efficiency. The process is thus applicable as a new tabletop circularly-polarized XUV radiation source for probing attosecond phenomena with high temporal resolution.

It is shown by particle-in-cell simulations that a narrow electron beam with high energy and charge density can be generated in a subcritical-density plasma by two consecutive laser pulses. Although the first laser pulse dissipates rapidly, the second pulse can propagate for a long distance in the thin wake channel created by the first pulse and can further accelerate the preaccelerated electrons therein. Given that the second pulse also self-focuses, the resulting electron beam has a narrow waist and high charge and energy densities. Such beams are useful for enhancing the target-back space-charge field in target normal sheath acceleration of ions and bremsstrahlung sources, among others.

Intense Compton-scattering γ-ray radiation driven by laser wakefield acceleration (LWFA) and generation of ultrashort positron beams are investigated by Monte Carlo simulation. Using an LWFA driven GeV electron bunch and a 45 femtosecond, 90 mJ/pulse, and 10 Hz Ti:Sapphire laser for driving the Compton scattering, fs γ-ray pulses were generated. The latter have a flux of ≥108/s, peak brightness of ≥1020 photons/(s mm2 mrad2 0.1% bandwidth), and photon energy of 5.9 to 23.2 MeV. The γ-ray pulses then impinge on a thin high-Z target. More than 107 positrons/s in the form of sub-100 fs pulses at several MeV can be produced. Such ultrashort positron pulses can be useful as the pump-probe type positron annihilation spectroscopy as well as in other applications.

An advanced cone-nanolayer target with nanolayers on both inside and outside of the hollow-cone tip is proposed. Two-dimensional particle-in-cell simulations show that laser interaction with such cone-nanolayer targets can efficiently produce fast electron beams with manageable spotsize, and the beams can propagate for a relatively long distance in the vacuum beyond the cone tip.

A two-dimensional theoretical model for the evolution of solid-density plasma irradiated by short, intense laser pulse is introduced. The electrons near the target surface are pushed inward by the radiation pressure, leading to a receding electron density jump where the laser is reflected. The electrostatic field of the resulting charge separation eventually balances the radiation pressure at the laser peak. After that the charge separation field becomes dominant. It accelerates and compresses the ions that are left behind until they merge with the compressed electrons, resulting in a high-density plasma peak. The laser pulse reflected from the receding electron density jump loses energy in plasma and suffers Doppler frequency red-shift, which can provide valuable information on the laser absorption rate and the speed of the receding electrons. Electron oscillations, including the u × B oscillations across the density jump at twice the laser frequency during the laser action, as well as the low-frequency oscillations appearing after laser action, are identified.

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.

Collimated proton beams from laser interaction with a slab having a hole on its backside are investigated using particle-in-cell simulation. The hot target electrons driven by the laser expand rapidly into the hole. However, at the hole's corners the electrons are strongly compressed and an intense electron jet is emitted from each corner, tightly followed by the ions. The plasma jets focus and collimate along the axis of the hole and can propagate without divergence within the hole. The effect of the hole diameter on the collimated proton beam is considered.

It is shown that an intense laser pulse can be focused by a conical channel. This anomalous light focusing can be attributedto a hitherto ignored effect in nonlinear optics, namely that the boundary response depends on the light intensity: the innercone surface is ionized and the laser pulse is in turn modified by the resulting boundary plasma. The interaction creates anew self-consistently evolving light-plasma boundary, which greatly reduces reflection and enhances forward propagationof the light pulse. The hollow cone can thus be used for attaining extremely high light intensities for applications in strong-field and high energy-density physics and other areas.Keywords: Laser-cone interaction; Laser focusing; Particle-in-cell simulation Strong-field or high-energy-density physics relies much on thevery high concentration of laser energy in an extremely smalltime/space interval. Chirped pulse amplification technologyhas opened a new era in strong-field physics. By compressingthe laser pulse to the femtosecond regime, terawatt, or evenpetawatt lasers have become available (Perry & Mourou,1994; Perry et al., 1999; Mourou et al., 1998). Furthermore,considerable effort has been made in concentrating laserenergy to a small or tightly focused spot. Recently, powerfullasers have been focused to the 10-wavelength level (Fritzleretal., 2003; Cowan etal.,2004),withpeakintensities reaching10

Dimensional effects in particle-in-cell (PIC) simulation of target normal sheath acceleration (TNSA) of protons are considered. As the spatial divergence of the laser-accelerated hot sheath electrons and the resulting space-charge electric field on the target backside depend on the spatial dimension, the maximum energy of the accelerated protons obtained from three-dimensional (3D) simulations is usually much less that from two-dimensional (2D) simulations. By closely examining the TNSA of protons in 2D and 3D PIC simulations, we deduce an empirical ratio between the maximum proton energies obtained from the 2D and 3D simulations. This ratio may be useful for estimating the maximum proton energy in realistic (3D) TNSA from the results of the corresponding 2D simulation. It is also shown that the scaling law also applies to TNSA from structured targets., Comment: 20 pages, 7 figures, 3 tables, 45 references

The 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.

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.

Channeling by a train of laser pulses into homogeneous and inhomogeneous plasmas is studied using particle-in-cell simulation. When the pulse duration and the interval between the successive pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single long-pulse laser of similar peak intensity and total energy. The increased penetration distance can be attributed to the repeated actions of the ponderomotive force, the continuous between-pulse channel lengthening by the inertially evacuating ions, and the suppression of laser-driven plasma instabilities by the intermittent laser-energy cut-offs.

The acceleration of electrons by the self-generated quasi-static magnetic field in the interaction of n laser pulses with a plasma is investigated. The relativistic dynamics of a single electron is followed numerically. It is found that the electron can be much more effectively accelerated by n laser pulses, each with amplitude a0 and having a fixed frequency difference from the preceding one, than by a single pulse with amplitude na0. The large enhancement of acceleration can be attributed to stochastic-like resonance between the electron motion at the beat-frequencies of lasers and at the cyclotron motion frequency.

Ultra-intense radiation in the terahertz (THz) regime emitted from the rear side of laser-irradiated solid target is experimentally investigated using a 20 TW fs laser system. The THz radiation emitted in a conical angle around 45 degrees is of radially polarized and has its duration of tens of ps. The waveform, angular distribution, polarization and laser intensity dependence of the observed radiation are in good agreement with those predicted for the radiation from deceleration of >sub-MeV electrons passing through the sheath electric field behind the target.

Introduce the measuring instrument of plumb coaxial error for long-distance orifices which is according to the measuring requests of Flange Place of experiment fast neutron reactor in nuclear power equipment and designed by combining the laser collimation technique and CCD imaging technique. The measuring instrument constructs the plumb line with utilizing the characteristic of laser and making the CDD as imaging screen, and the line is regarded as the datum line in measurement and used for measuring coaxial error of the large orifices' manufacture and assemblage under plumb state. Angle resolving power is: 0.3; displacement resolving power is: 0.02 mm; respective degree of uncertainty of measurement results are: 0.1; 0.01 mm. The paper detailed introduces the idiographic design principle and measure method of the measuring instrument, and analyzes the measure error. It is applied to measure the precision of manufacture and the coaxial error of assemblage of the large or heavy pipe casting equipment.

Long-distance propagation of intense laser pulse in air is reconsidered analytically by generalizing the analogy between the laser spotsize and the orbit of a classical particle. It is shown that multiphoton ionization introduces unique features to the laser-air interaction, thereby enabling the long-distance behavior. Several interesting characteristics of the latter are pointed out.

The optical behavior of a four-level system in a ring cavity driven by two coherent laser fields is studied. One laser field is treated as the incident field and the other the control field, respectively. It is found that there is optical transparency when the difference between the two frequency detunings of the incident and control fields from the corresponding atomic transition frequencies is zero. Optical bistability can be produced and controlled by increasing the magnitude of the frequency difference. The bistable hysteresis becomes larger when the frequency difference is increased. Further increase of the latter can lead to onset of multistability.

A classical model for high-harmonic generation from the interaction of a laser pulse and an atomic gas is introduced. The effects of finite pulse width and laser magnetic field are included. The optimum intensity-pulse width relation for high-harmonic emission and the corresponding power spectrum are presented. The existing high-harmonic cutoff law is found to remain valid.

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 ≃ n e 0 / a 0 n c = 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 15 ω 0 < ω n < 500 ω 0 (ω0 is the laser frequency), the power spectrum of the emission scale...

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.

The collision of wake bubbles behind two counterpropagating laser pulses in rarefied plasma is investigated using particle-in-cell simulation. Special attention is paid to the highly nonlinear dynamics of the electrons in the interaction region. It is found that, as the two bubbles approach each other and collide, the electrons in the interaction region first oscillate in a periodic fashion, forming a quasistationary dense electron density ripple with fairly regular spatial structure. At longer times, the electron motion becomes chaotic, and the density grating is gradually smeared. The electrons escape in the transverse direction, and eventually the two bubbles merge to form a single one. The transition of the electron motion from regular to chaotic is confirmed by analytical modeling using test electrons moving in counterpropagating planar electromagnetic waves. The findings shed light on the dynamics of wake-bubble collisions and the complex behavior induced by multiple laser pulses in plasmas.

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.

It is shown that the transverse Rayleigh-Taylor-like (RT) instability in the hole boring radiation pressure acceleration can be suppressed by using elliptically polarized (EP) laser. A moderate ${J}\times{B}$ heating of the EP laser will thermalize the local electrons, which leads to the transverse diffusion of ions, suppressing the short wavelength perturbations of RT instability. A proper condition of polarization ratio is obtained analytically for the given laser intensity and plasma density. The idea is confirmed by two dimensional Particle-in-Cell simulations, showing that the ion beam driven by the EP laser is more concentrated and intense compared with that of the circularly polarized laser., 5 pages, 4 figures. arXiv admin note: substantial text overlap with arXiv:1212.2435

A novel two-step catalytic reaction is developed to synthesize gallium nitride nanowires encapsulated inside carbon nanotubes (GaN@CNT). The nanowires are prepared from the reaction of gallium metal and ammonium using metals or metal alloys as a catalyst. After the formation of the nanowires, carbon nanotubes are subsequently grown along the nanowires by chemical vapor deposition of methane. The structural and optical properties of pure GaN nanowires and GaN@CNT are characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results show that GaN nanowires are indeed encapsulated inside carbon nanotubes. The field emission studies show that the turn-on field of GaN@CNT is higher than that of carbon nanotubes, but substantially lower than that of pure GaN nanowires. This work provides a wide route toward the preparation and applications of new one-dimensional semiconductor nanostructures.

Ponderomotive-force driven acceleration of an electron at the focus of a high-intensity short-pulse laser is considered using a model that accounts for its averaged drift motion but neglects its fast-varying quiver motion. It is shown that at relativistic laser intensities the ponderomotive acceleration mechanism can be significant even for an electron initially at rest. The latter can easily be driven out of the interaction region by the radial component of the ponderomotive force. For intensities above ${10}^{19}$ W $\ensuremath{\mu}{\mathrm{m}}^{2}/{\mathrm{cm}}^{2},$ energy gains in the range of MeV can be realized.

A model for the penetration of ultrahigh-intensity ultrashort-pulse lasers through a highly overdense foil target is proposed. The model involves only electron motion. It is shown that the ponderomotive force of the light waves can push the electrons towards the back surface of the foil, such that the effective thickness of the latter is reduced to the order of the skin depth, thus allowing wave penetration. The results agree well with that from a particle-in-cell simulation.

Speciation of zinc in phosphor wastes during electrokinetic treatments has been studied by in situ X-ray absorption near edge structure (XANES) spectroscopy in the present work. The least-square fits of the in situ XANES spectra show that the major zinc species in the phosphor waste are ZnS (77%), ZnO (10%), and Zn(OH)2 (13%). During the electrokinetic treatment for 90 min, 25% of ZnS and 4% of ZnO are dissolved. About 42% of zinc is enriched on the cathode under the electric field (5 V/cm). Prolonging the electrokinetic treatment time to 4 h under the electric field of 5 V/cm, at least 80% of zinc in the phosphor waste can be recovered. © 2007 Elsevier B.V. All rights reserved.

A simple surface current model is proposed for studying harmonic generation by a short-pulse high-intensity circularly polarized laser irradiating a solid target normally. Exact relativistic electron dynamics is taken into account. It is found that high harmonics peaked at directions normal to that of incidence are efficiently generated.