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As a typical plasma-based optical element that can sustain ultra-high light intensity, plasma density gratings driven by intense laser pulses have been extensively studied for wide applications. Here, we show that the plasma density grating driven by two intersecting driver laser pulses is not only nonuniform in space but also varies over time. Consequently, the probe laser pulse that passes through such a dynamic plasma density grating will be depolarized, that is, its polarization becomes spatially and temporally variable. More importantly, the laser depolarization may spontaneously take place for crossed laser beams if their polarization angles are arranged properly. The laser depolarization by a dynamic plasma density grating may find application in mitigating parametric instabilities in laser-driven inertial confinement fusion.

Bright, energetic, and directional electron bunches are generated through efficient energy transfer of relativistic intense (~ 1019 W/cm2), 30 femtosecond, 800 nm high contrast laser pulses to grating targets (500 lines/mm and 1000 lines/mm), under surface plasmon resonance (SPR) conditions. Bi-directional relativistic electron bunches (at 40° and 150°) are observed exiting from the 500 lines/mm grating target at the SPR conditions. The surface plasmon excited grating target enhances the electron flux and temperature by factor of 6.0 and 3.6, respectively, compared to that of the plane substrate. Particle-in-Cell simulations indicate that fast electrons are emitted in different directions at different stages of the laser interaction, which are related to the resultant surface magnetic field evolution. This study suggests that the SPR mechanism can be used to generate multiple, bright, ultrafast relativistic electron bunches for a variety of applications.

The efficiency of high-order harmonic generation from a relativistic laser interacting with solid targets depends greatly on surface plasma distribution. The usual method of enhancing efficiency involves tuning the plasma scale length carefully by improving the laser contrast. Here, we experimentally demonstrate that efficient harmonics can be achieved directly by compressing large-scale surface plasma via the radiation pressure of a circularly polarized normally incident prepulse. The harmonic generation efficiency obtained by this method is comparable to that obtained with optimized plasma scale length by high-contrast lasers. Our scheme does not rely on high-contrast lasers and is robust and easy to implement. Thus, it may pave a way for the development of intense extreme ultraviolet sources and future applications with high repetition rates.

Electron beam polarization in the bubble regime of the interaction between a high-intensity laser and a longitudinally pre-polarized plasma is investigated by means of the Thomas-Bargmann-Michel-Telegdi equation. Using a test-particle model, the dependence of the accelerated electron polarization on the bubble geometry is analyzed in detail. Tracking the polarization dynamics of individual electrons reveals that although the spin direction changes during both the self-injection process and acceleration phase, the former has the biggest impact. For nearly spherical bubbles, the polarization of electron beam persists after capture and acceleration in the bubble. By contrast, for aspherical bubble shapes, the electron beam becomes rapidly depolarized, and the net polarization direction can even reverse in the case of a oblate spheroidal bubble. These findings are confirmed via particle-in-cell simulations., 8pages, 4 figures

Electromagnetic emission via linear mode conversion from electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) of an incident laser pulse in inhomogeneous plasma is investigated theoretically and numerically. It is found that the mode conversion can occur naturally in underdense plasma region below the quarter critical density provided that EPWs are generated due to the development of backward SRS when the laser pulse is incident at certain angle with the plasma density gradient. The produced radiation may cover a broad frequency range up to half of the incident laser frequency. The dependence of the radiation conversion efficiency on the laser intensity, incident angle, laser pulse duration, plasma density scale length, and initial electron temperature is analyzed based on one-dimensional particle-in-cell simulation. In two-dimensional geometry, due to the development of sideward SRS, it is found that the mode conversion to occur even at normal incidence of the laser pulse. The radiation frequency, bandwidth, duration, and amplitude can be well controlled by the laser and plasma parameters, suggesting that it may provide a new source of tunable broadband radiation as well as a diagnosis of the development of SRS.

In laser wakefield accelerators, guiding of drive laser pulses in preformed plasma channels plays a key role to overcome laser diffraction for effective acceleration. Different from guiding schemes studied previously, where a Gaussian laser pulse and a parabolic plasma channel were investigated, here we investigate the guiding of Laguerre–Gaussian (LG) pulses in plasma channels. Analytical studies and three-dimensional particle-in-cell simulations show that the matched conditions still exist for high order laser pulses and high order plasma channels. For usual Gaussian and high order LG pulses, the second order parabolic channel gives the best guiding. Although the laser pulse can also be guided in even higher order channels, its envelope deforms during propagation. For laser pulses with combined multi-LG modes, determined by their azimuthal orbit angular momenta, there is axisymmetric or non-axisymmetric evolution for the transverse laser intensity profile. The preformed plasma channel can guide the combined pulses but the transverse intensity profile of the laser pulses always evolves.

Nonlinear structures such as shock waves and vortices widely exist in nature and the Universe. They have also been separately observed in laser–plasma interactions. We show for the first time that two perpendicularly propagated collisionless electrostatic shock waves (CESs) can be excited by a moving electron vortex (EV). The latter is driven by an ultrashort intense laser pulse propagating through a sandwich nonuniform underdense plasma slab and is found to move perpendicularly to the density gradient. Two CESs are observed on both sides of the passing route of the EVs. The left-side CES is induced by a high-density electron layer, which originates from the vortex front and is compressed and accelerated during the EV motion. The right-side CES is induced by supersonic ions accelerated by the EVs directly. Ion acceleration by such CESs along the directions perpendicular to the vortex propagation is also observed. This study reveals the transformation of nonlinear structures and provides new routes for laser energy dissipation in plasmas.

Moving electron vortices have been observed in laser interaction with non-uniform near-critical-density plasma by multi-dimensional Particle-in-Cell simulations. In two dimensional geometry, there are two vortices with opposite magnetic polarity, moving perpendicularly to the plasma density gradient direction. The field distribution and particle motion composing such a moving structure have been clearly observed in simulations, which explains the vortex motion. Two components of loop currents are formed around each electron vortex, which dominate the vortex motion. The moving velocity can be as large as a 0.2c level, forming relativistic vortices inside the plasma. Laser plasma conditions such as intensity, polarization, density profile, and external magnetic field effects on the vortex motion and evolution are also studied. In three dimensions, the structure appears as an expanding magnetic ring with an internal magnetic field up to 1000 Tesla. Such vortex structures suggest an interesting way of energy (with more than 5% of the laser energy) transportation to ambient plasmas as far as 50 μm away from the laser-plasma interaction region, which may have applications in laser plasma-based inertial confinement fusion and laboratory astrophysics.

we propose an all-optical scheme for ultra-bright γ-ray emission and dense positron production with lasers at intensity of 1022-23 Wcm-2. By irradiating two colliding elliptically-polarized lasers onto two diamondlike carbon foils, electrons in the focal region of one foil are rapidly accelerated by the laser radiation pressure and interact with the other intense laser pulse which penetrates through the second foil due to relativistically induced foil transparency. This symmetric configuration enables efficient γ-photon emission with unprecedented brightness of 1025 photons/s/mm2/mrad2/0.1%BW at 15 MeV and intensity of 5×1023 Wcm-2. A GeV positron beam with density of 2.5×1022 cm-3 and flux of 1.6×1010/shot is achieved.

Although the texture of Ti alloys is currently evaluated by the pole figure, it can only provide incomplete and qualitative information. In this investigation, the textures of Ti-3Al-2.5V tubing were studied to seek for an appropriate method for evaluating Ti tubing texture. The texture measurement was performed by thinning the tubing chemically into foils and flattening them, followed by x-ray diffraction analysis. A Bruker D8 Advance texture goniometer with Cu Kα radiation was employed for measuring the {0002}, {10-10}, {10-12}, {11-20} and {10-13} incomplete pole figures using the Schultz reflection technique. Based on the pole data, the crystallite orientation distribution function (ODF) was synthesized using TexEval V2.5 software, Bruker AXS GmbH. The texture strengths of the three specimens were compared using pole figures and ODF. It is found that it gives rise to a large error in the texture strength using the pole figure to evaluate texture. The orientation densities of the maxima in ODF and the tilt angle of their basal poles from the normal toward the tangential direction can describe the tubing texture accurately, which can evaluate the tubing mechanical properties. Subsequently, ODF can be used to predict the mechanical properties of batches of tubing.

Self-injection in a laser-plasma wakefield accelerator is usually achieved by increasing the laser intensity until the threshold for injection is exceeded. Alternatively, the velocity of the bubble accelerating structure can be controlled using plasma density ramps, reducing the electron velocity required for injection. We present a model describing self-injection in the short-bunch regime for arbitrary changes in the plasma density. We derive the threshold condition for injection due to a plasma density gradient, which is confirmed using particle-in-cell simulations that demonstrate injection of subfemtosecond bunches. It is shown that the bunch charge, bunch length, and separation of bunches in a bunch train can be controlled by tailoring the plasma density profile.

Broadband terahertz (THz) radiation with extremely high peak power, generated by the interaction of a femtosecond laser with a thin solid target, has been investigated via particle-in-cell simulations. The spatial (angular) and temporal profiles of the THz radiation reveal that it is caused by the coherent transition radiation emitted when laser-produced hot electrons pass through the front or rear surface of the target. Dependence of the THz radiation on laser and target parameters is studied; it is shown to have a strong correlation with hot electron production. The THz radiation conversion efficiency can be as high as a few times 10^{-3}. This radiation is not only a potentially high power THz source, but may also be used as a unique diagnostic of hot electron generation and transport in relativistic laser-solid interactions.

We investigated powerful terahertz (THz) radiation from laser-driven gas plasma by theory and particle-in-cell (PIC) simulation. We proposed a model to describe THz radiation emission from a net current in plasma. We found that the well-known two-color scheme can be extended to the second laser at any even harmonic of the main laser. To strengthen the THz radiation, we proposed mid-infrared laser scheme, picosecond laser scheme, and asymmetric laser scheme. Recently, we proposed an approach to generate tunable, circularly-polarized, narrow-band THz radiation.

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.

Hot electron emission is studied using 798 nm, 150 fs, p-polarized laser pulses at 1 × 1016 W cm−2, irradiating on plastic (CH), aluminium (Al), titanium (Ti) and copper (Cu) planar targets. The dependence of hot electron generation on atomic number Z is observed experimentally.

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.

The magnetic field generation through inverse Faraday effect and its effects on the propagation of a circularly polarized light wave are studied in a self-consistent way for relativistic intensities. The following results are presented. (i) The magnetic field is produced by two sources, the circular motion of single electrons which produces plasma magnetization, and the inhomogeneity of both the electron density and light intensity which produces nonzero currents in the azimuthal direction. The magnetic field is calculated for various profiles of electron density and light intensity. (ii) For the case of a plane wave in a homogeneous plasma, the cutoff frequency is calculated as a function of light intensity, which is different from that without consideration of magnetic field generation. An ultra-intense magnetic field as large as hundreds of MG is obtainable in an overdense plasma where the wave can propagate owing to the induced transparency. (iii) The evolution equations for a laser beam of finite width are derived. Due to magnetic field generation, the critical power for self-focusing of the laser beam is reduced by a factor of (1+${\mathrm{\ensuremath{\omega}}}_{\mathit{p}}^{2}$/${\mathrm{\ensuremath{\omega}}}^{2}$${)}^{\mathrm{\ensuremath{-}}1}$; the magnetic field tends to reduce the effect of the electron cavitation resulting from the transverse ponderomotive force. \textcopyright{} 1996 The American Physical Society.

A total of 80 primary human breast carcinoma DNAs were analysed for loss of heterozygosity (LOH) on the long arm of chromosome 6, using microsatellite markers whose location has been defined physically and by linkage analysis. Loss of heterozygosity was observed in 38 of 80 (48%) tumours that were informative for at least one locus. The analysis revealed partial or interstitial deletions of chromosome 6q. Detailed mapping of chromosome 6q in these tumour DNAs identified two and perhaps three commonly deleted regions. One of these is located between markers D6S251 and D6S252 (6q14-q16.2), another between D6S268 and D6S261 (6q16.3-q23) and a third between D6S287 and D6S270 (6q22.3-q23.1). Images Figure 1

Effect of lower‐hybrid current drive on tearing mode instability is investigated in a drift kinetic formalism. In collisional regime the co‐driven current plays a stabilizing role to tearing mode but a destabilizing role to drift tearing mode. A competitive effect among the gradients of density, temperature and driven current at the rational surface may exist. In semicollisional regime, lower hybrid current drive increases the growth rate of tearing mode and reduces the growth rate of drift tearing mode.

Hard Kα x‐ray source profile and intense Terahertz radiation from femtosecond laser‐plasma interactions are studied. When a preplasma is presented, a weak halo surrounding a bright spot is observed in the x‐ray source. The x‐ray photons in the halo are mainly excited by the fast electrons that flow laterally near the target surface. Intense Terahertz radiation has been observed in the laser specular direction. The pulse energy of the radiation reaches tens of μJ/sr.

With many high power laser systems ranging from a few TW to multi‐100 TW installed in some laboratories in Asia, significant progress on laser‐driven wakefield acceleration of electrons has been achieved. Generation of quasi‐monoenergetic electron beams from tens of MeV to nearly GeV has been demonstrated. Several programs for ion/proton acceleration aiming at potential medical applications are running or planned based upon their significant theoretical and numerical findings. There are quite a few collaborations existing among Asian research groups.

We report on a self-organizing, quasistable regime of laser proton acceleration, producing 1 GeV nanocoulomb proton bunches from laser foil interaction at an intensity of 7 x 10;{21} W/cm;{2}. The results are obtained from 2D particle-in-cell simulations, using a circular polarized laser pulse with Gaussian transverse profile, normally incident on a planar, 500 nm thick hydrogen foil. While foil plasma driven in the wings of the driving pulse is dispersed, a stable central clump with 1-2lambda diameter is forming on the axis. The stabilization is related to laser light having passed the transparent parts of the foil in the wing region and enfolding the central clump that is still opaque. Varying laser parameters, it is shown that the results are stable within certain margins and can be obtained both for protons and heavier ions such as He;{2+}.

Plasma Bragg grating is composed of periodic variations of plasma and dielectric or vacuum. The characteristic of the bandgap of the plasma Bragg grating without or with defect is studied by theory and particle‐in‐cell (PIC) simulation. A one‐dimensional (1D) defective plasma Bragg grating is proposed to increase the conversion efficiency of the third harmonic of the incident laser pulse as compared with a simple plane target. In such a structure, the high laser field intensity and high plasma density are produced by laser localization in defect states, from which the third harmonic is enhanced by one order of magnitude.

The c-Ha-ras gene was analysed by Southern blot hybridisation in 67 specimens of lymph node metastases and in 25 specimens of primary tumours obtained from 85 untreated patients with head and neck squamous cell carcinoma. The loss of one c-Ha-ras allele was observed in 10/46 (22%) tumours from heterozygous patients for this locus. Different genes, located as the c-Ha-ras gene on the short arm of chromosome 11, were also found to be deleted suggesting that the deletion of other genes could play a role in aggressiveness of head and neck carcinomas. Using polymerase chain reaction, mutation at codon 12 was detected in only 2/54 (3.8%) tumours but no mutation involving codon 61 was found. Neither gene amplification nor gene rearrangement could be observed. Total RNA was prepared from 79 of these tumour specimens and analysed by Northern and slot blot hybridisation. A 1.2 kb c-Ha-ras transcript band was detected in all the RNA preparations. Relatively high c-Ha-ras transcript levels were found in 18% of lymph node metastases and in 21% of primary tumours, indicating no significant differences between these cancers. Moreover, the c-Ha-ras mRNA levels were not significantly greater in the primary tumours than in the normal mucosae in 10/12 cases for which both tissues were analysed. These data indicate that c-Ha-ras gene does not seem to be strongly involved in head and neck carcinomas at that advanced stage of the disease, as this was previously reported for earlier clinical stages. Images Figure 1 Figure 3 Figure 4 Figure 6 Figure 7 Figure 8

Plasma as a nonlinear optical medium has the potential to produce extremely powerful terahertz (THz) emission when irradiated by ultrashort high-intensity laser pulses. Different schemes to produce THz emission associated with laser wakefield excitation will be discussed, which include THz emission from inhomogeneous plasma via linear mode conversion, emission from magnetized plasma, as well as from a plasma channel and a homogeneous plasma slab. Potential applications include diagnostic of laser wakefields, particle acceleration, and nonlinear THz spectroscopy, etc.

Recent experimental and theoretical studies on surface electron emission will be presented. A collimated fast electron beam was observed along the target surface irradiated by intense laser pulses up to 20TW when the laser is incident with large angles such as over 45 degree. Numerical simulations suggest that such an electron beam is formed due to the confinement of the surface quasistatic electric and magnetic fields. Meanwhile, an acceleration process similar to the inverse‐free‐electron‐laser is found to occur and is responsible for the generation of the most energetic electrons. A general formula for electron angular distributions accounting for the quasistatic electric and magnetic fields is given. In certain conditions, quasi‐monoenergetic electron beams are also produced. These results are of interest for potential applications of laser‐produced electron beams and helpful to the undersanding of the cone‐target physics in the fast ignition related experiments.

A unified description of wave breaking for relativistic plasma waves having arbitrary phase velocity is obtained on the basis of warm relativistic electron fluid theory [Phys. Fluids 25, 846 (1982)]. Limiting cases found in the literature are reproduced.

The emission direction of outward-ejecting fast electrons generated in laser-solid interactions by 30 fs laser pulses is measured for laser intensities varying from the nonrelativistic to the relativistic. For an s-polarized incident laser beam at nonrelativistic intensities, the ejected electrons are close to the polarization direction of the laser beam. With the increase of the laser intensity, the ejected electrons are still mainly within the polarization plane, but turn away from the laser polarization direction towards the opposite direction of the incident laser beam. At relativistic intensities, electrons eject towards the direction of the reflected laser beam. The increasing ponderomotive force acceleration with the laser intensities might be responsible for the observed changes.

Different-sized CuO nanocrystals were prepared by precipitation-pyrolysis method, with sizes of 7 nm, 10 nm, and 30 nm. The absorption spectra of CuO nanocrystals showed clear bandshift as an evidence of the quantum size effect. On a measurement device with a diamond anvil cell, the pressure-resistance relations of different-sized CuO nanocrystals under pressure up to 20 GPa at room temperature were obtained. The experimental results showed varying pressure-dependent resistance of CuO nanocrystals of different sizes. There was an interesting phenomenon occurring for the smallest CuO nanocrystal (7 nm), that is, a significant resistance fluctuation of large amplitude under pressure appears with a frequency-field dependence. This fluctuation clearly originates from surface charge oscillations on the surface of many nanocrystals with the incident field due to the size effects.

A different parameter regime of laser wakefield acceleration driven by sub-petawatt femtosecond lasers is proposed, which enables the generation of relativistic electron mirrors further accelerated by the plasma wave. Integrated particle-in-cell simulation, including both the mirror formation and Thomson scattering, demonstrates that efficient coherent backscattering up to keV photon energy can be obtained with moderate driving laser intensities and high density gas targets.

By using three-dimensional particle-in-cell simulations with synchrotron radiation damping incorporated, dynamics of ultra-intense laser driven mass-limited foils is presented. When a circularly polarized laser pulse with a peak intensity of ∼1022 W/cm2 irradiates a mass-limited nanofoil, electrons are pushed forward collectively and a strong charge separation field forms which acts as a “light sail” and accelerates the protons. When the laser wing parts overtake the foil from the foil boundaries, electrons do a betatron-like oscillation around the center proton bunch. Under some conditions, betatron-like resonance takes place, resulting in energetic circulating electrons. Finally, bright femto-second x rays are emitted in a small cone. It is also shown that the radiation damping does not alter the foil dynamics radically at considered laser intensities. The effects of the transverse foil size and laser polarization on x-ray emission and foil dynamics are also discussed.

It is proposed that laser hole-boring at a steady speed in inhomogeneous overdense plasma can be realized by the use of temporally tailored intense laser pulses, producing high-fluence quasi-monoenergetic ion beams. A general temporal profile of such laser pulses is formulated for arbitrary plasma density distribution. As an example, for a precompressed deuterium-tritium fusion target with an exponentially increasing density profile, its matched laser profile for steady hole-boring is given theoretically and verified numerically by particle-in-cell simulations. Furthermore, we propose to achieve fast ignition by the in-situ hole-boring accelerated ions using a tailored laser pulse. Simulations show that the effective energy fluence, conversion efficiency, energy spread, and collimation of the resulting ion beam can be significantly improved as compared to those found with un-tailored laser profiles. For the fusion fuel with an areal density of 1.5 g cm–2, simulation indicates that it is promising to realize fast ion ignition by using a tailored driver pulse with energy about 65 kJ.