Ultrabright γ-ray emission from the interaction of an intense laser pulse with a near-critical-density plasma

An efficient scheme for generating ultrabright γ-rays from the interaction of an intense laser pulse with a near-critical-density plasma is studied by using the two-dimensional particle-in-cell simulation including quantum electrodynamic effects. We investigate the effects of target shape on γ-ray generation efficiency using three configurations of the solid foils attached behind the near-critical-density plasma: a flat foil without a channel (target 1), a flat foil with a channel (target 2), and a convex foil with a channel (target 3). When an intense laser propagates in a near-critical-density plasma, a large number of electrons are trapped and accelerated to GeV energy, and emit γ-rays via nonlinear betatron oscillation in the first stage. In the second stage, the accelerated electrons collide with the laser pulse reflected from the foil and emit high-energy, high-density γ-rays via nonlinear Compton scattering. The simulation results show that compared with the other two targets, target 3 affords better focusing of the laser field and electrons, which decreases the divergence angle of γ-photons. Consequently, denser and brighter γ-rays are emitted when target 3 is used. Specifically, a dense γ-ray pulse with a peak brightness of 4.6×10²⁶ photons/s/mm²/mrad²/0.1%BW (at 100 MeV) and 1.8×10²³ photons/s/mm²/mrad²/0.1%BW (at 2 GeV) are obtained at a laser intensity of 8.5×10²² W/cm² when the plasma density is equal to the critical plasma density n_c. In addition, for target 3, the effects of plasma channel length, foil curvature radius, laser polarization, and laser intensity on the γ-ray emission are discussed, and optimal values based on a series of simulations are proposed.     

High intensity laser–matter interaction and electron plasma acceleration

The basic principles of the electron acceleration in laser produced plasmas and the related secondary sources of energetic radiation with a particular attention to betatron radiation are presented.     

Studies on laser–plasma interaction physics for shock ignition

We realized a series of experiments to study the physics of laser–plasma interaction in an intensity regime of interest for the novel “Shock Ignition” approach to Inertial Fusion. Experiments were performed at the Prague Asterix Laser System laser in Prague using two laser beams: an “auxiliary” beam, for pre-plasma creation, with intensity around 7?×?10¹³?W/cm² (250?ps, 1ω, λ?=?1315?nm) and the “main” beam, up to 10¹⁶?W/cm (250?ps, 3ω, λ?=?438?nm), to launch a shock. The main goal of these experiments is to study the process of the formation of a very strong shock and the influence of hot electrons in the generation of very high pressures. The shock produced by the ablation of the plastic layer is studied by shock breakout chronometry. The generation of hot electrons is analyzed by imaging Kα emission.     

Femtosecond X-ray generation through relativistic electron beam–laser interaction

Methods for generating ultra-short X-rays using the interaction of intense laser pulses with relativistic electron beams, and their application to measuring ultra-fast phenomena in solid state materials, are reviewed. Two different methods that use a long electron bunch and short laser pulse are discussed: Thomson scattering and optical slicing which have been implemented on linac and storage ring beams, respectively. The possibility of generating ultrashort electrons bunches from laser-plasma injectors is discussed.     

Particle-in-cell modeling of relativistic laser–plasma interaction with the adjustable-damping,direct implicit method

Implicit particle-in-cell codes offer advantages over their explicit counterparts in that they suffer weaker stability constraints on the need to resolve the higher frequency modes of the system. This feature may prove particularly valuable for modeling the interaction of high-intensity laser pulses with overcritical plasmas, in the case where the electrostatic modes in the denser regions are of negligible influence on the physical processes under study. To this goal, we have developed the new two-dimensional electromagnetic code ELIXIRS (standing for ELectromagnetic Implicit X-dimensional Iterative Relativistic Solver) based on the relativistic extension of the so-called Direct Implicit Method [D. Hewett, A.B. Langdon, Electromagnetic direct implicit plasma simulation, J. Comput. Phys. 72 (1987) 121–155]. Dissipation-free propagation of light waves into vacuum is achieved by an adjustable-damping electromagnetic solver. In the high-density case where the Debye length is not resolved, satisfactory energy conservation is ensured by the use of high-order weight factors. In this paper, we first derive the electromagnetic direct implicit method as a simplified Newton scheme. Its linear properties are then investigated through numerically solving the relation dispersions obtained for both light and plasma waves, accounting for finite space and time steps. Finally, our code is successfully benchmarked against explicit particle-in-cell simulations for two kinds of physical problems: plasma expansion into vacuum and relativistic laser–plasma interaction. In both cases, we will demonstrate the robustness of the implicit solver for crude discretizations, as well as the gains in efficiency which can be realized over standard explicit simulations.     

Using fence pulses to suppress stimulated Raman scattering effect in laser–plasma interaction

In inertial confinement fusion, the laser–plasma interaction(LPI) happens when the high-energy laser irradiates on the target where the scattered light share generated from the stimulated Raman scattering(SRS) effect is difficult to suppress. We propose a method using fence pulses(FPs) to suppress the backward SRS by inhibiting the growth of the intensity of electron plasma waves. Based on our simulation, the FPs can weaken SRS effect in the LPI effectively.     

Nitrogen optical emission during nanosecond laser ablation of metals: prompt electrons or photo-ionization?

Experiments on the interaction of metal targets with a Nd:YAG laser beam ( \(\lambda \)  = 1,064 nm, intensity \(10^{10}\) – \(10^{11}\,\hbox {W/cm}{^2}\) ) are carried out in a finite Nitrogen pressure environment. The observed \(\hbox {N}_2\) spectra are unambiguous evidence of the existence of an ionization and excitation source, arriving at the observation volume prior to the plume. Such a source can be either prompt electrons or VUV radiation. The analysis reveals that the prompt electron interpretation requires energies in excess of 1 keV, incompatible with any acceleration mechanisms relevant for such laser intensities. On the other hand, VUV radiation is sufficiently strong to explain the observed spectra.     

Study on the effect of laser parameters on wakefield excitation in femtosecond pulsed laser–plasma interaction using PIC method

Propagation of an intense short laser pulse through under-dense plasma can produce huge amplitude plasma wake field. A 3D particle in cell (PIC) method was used to simulate the wakefield generation for different laser parameters such as intensity, pulse duration, spot size and temporal pulse shape. Our study shows that the amplitude of wakefield is increased with laser intensity, but it is decreased with spot size. The results for pulse shape and pulse duration depend on their optimum values.     

Spin-dependent transport through a serial double-quantum-dot with Rashba spin–orbit interaction

We study the spin-dependent electron transport through a serial double-quantum-dot (DQD) by using Green’s function equation of motion technique. Special attention is paid to the functions of the Rashba spin–orbit (RSO) effect in one of the DQD and the inter-dot tunneling coupling _tc$t_c$. When the electrons transport from the left or the right lead into the middle lead, a quasi-two channel is established due to the existence of _tc$t_c$. Then, the RSO interaction will induce into the tunnel matrix element a spin-dependent extra phase factor σ_?R$\sigma {?}_R$ as the electrons flowing through different conduction channels, and thus making the current in the middle lead to be spin-polarized. Moreover, by properly adjusting the value of _tc$t_c$, the dot-lead coupling strength, dots’ levels and the external bias voltages, a net spin current without the accompanying of charge current can be generated. The structure proposed here is simple and can be realized in the present experiments.     

Absorption of ultrashort intense lasers in laser–solid interactions

With the advent of ultrashort high intensity laser pulses,laser absorption during the laser–solid interactions has received significant attention over the last two decades since it is related to a variety of applications of high intensity lasers,including the hot electron production for fast ignition of fusion targets,table-top bright X-ray and gamma-ray sources,ion acceleration,compact neutron sources,and generally the creation of high energy density matters.Normally,some absorption mechanisms found for nanosecond long laser pulses also appear for ultrashort laser pulses.The peculiar aspects with ultrashort laser pulses are that their absorption depends significantly on the preplasma condition and the initial target structures.Meanwhile,relativistic nonlinearity and ponderomotive force associated with the laser pulses lead to new mechanisms or phenomena,which are usually not found with nanosecond long pulses.In this paper,we present an overview of the recent progress on the major absorption mechanisms in intense laser–solid interactions,where emphasis is paid to our related theory and simulation studies.     

Simulation of laser–plasma interactions and fast-electron transport in inhomogeneous plasma

A new framework is introduced for kinetic simulation of laser–plasma interactions in an inhomogeneous plasma motivated by the goal of performing integrated kinetic simulations of fast-ignition laser fusion. The algorithm addresses the propagation and absorption of an intense electromagnetic wave in an ionized plasma leading to the generation and transport of an energetic electron component. The energetic electrons propagate farther into the plasma to much higher densities where Coulomb collisions become important. The high-density plasma supports an energetic electron current, return currents, self-consistent electric fields associated with maintaining quasi-neutrality, and self-consistent magnetic fields due to the currents. Collisions of the electrons and ions are calculated accurately to track the energetic electrons and model their interactions with the background plasma. Up to a density well above critical density, where the laser electromagnetic field is evanescent, Maxwell’s equations are solved with a conventional particle-based, finite-difference scheme. In the higher-density plasma, Maxwell’s equations are solved using an Ohm’s law neglecting the inertia of the background electrons with the option of omitting the displacement current in Ampere’s law. Particle equations of motion with binary collisions are solved for all electrons and ions throughout the system using weighted particles to resolve the density gradient efficiently. The algorithm is analyzed and demonstrated in simulation examples. The simulation scheme introduced here achieves significantly improved efficiencies.     

Radiography of a K_α X-ray source generated through ultrahigh picosecond laser–nanostructure target interaction

Laser-driven ultrafast X-ray sources are widely used for diagnostic radiography. However, there is a large divergence of fast electrons when they are generated by an intense short-pulse laser interacting with a foil target.We design a nanowire array target to achieve a more compact point X-ray source. Fast electrons are confined and guided by the nanowire array structure in order to generate a Kαsource with a small spot size. In our work, the smallest measured source size is comparable to the laser spot size, while the conversion efficiency can reach2.4 × 10-4.     

Analysis of directional emission in square resonator lasers with an output waveguide

Square microcavity laser with an output wavegnide is proposed and analyzed by the finite-difference time-domain(FDTD)technique.For a square resonator with refractive index of 3.2,side length of 4μm,and output wavegnide of 0.4-μm width,we have got the quality factors(Q factors)of 6.7×10~2 and 7.3×10~3 for the fundamental and first-order transverse magnetic(TM)mode near the wavelength of 1.5μm,respectively.The simulated intensity distribution for the first-order TM mode shows that the coupling efficiency in the waveguide reaches 53%.The numerical simulation shows that the first-order transverse modes have fairly high Q factor and high coupling efficiency to the output waveguide.Therefore the square resonator with an output wavegnide is a promising candidate to realize single-mode directional emission microcavity lasers.     

Self-focusing of Hermite–Gaussian laser beams in plasma under plasma density ramp

Self-focusing of Hermite–Gaussian laser beams in plasma under plasma density ramp has been investigated. It is known that a laser beam shows an oscillatory self-focusing and defocusing behavior with the propagation distance. To overcome the defocusing, localized upward plasma density ramp is introduced, so that the laser beam attains a minimum spot size and maintains it with only a mild ripple. The density ramp could be important for the self-focusing of a Hermite–Gaussian laser by choosing the laser and plasma parameters appropriately. Self-focusing becomes stronger as the propagation distance increases. The behavior of beam-width parameters with the distance of propagation is presented graphically.     

Formation of ring-patterned nanoclusters by laser–plume interaction

Journal of Nanoparticle Research - Nanocrystalline SnS powder has been prepared using tin chloride (SnCl2) as a tin ion source and sodium sulfide (Na2S) as a sulfur ion source with the help of...     

Long duration light emission from femtosecond laser–target interactions

Time resolved emission from the interaction of ultra-short (∼200 fs) laser pulses with aluminum and copper targets was investigated. Measurements show that emission from the laser produced plasma in air is significantly more intense than in near vacuum conditions and that the emission in air can extend for periods exceeding 100 ns. Modeling the laser–target–air coupling shows that the laser–target interaction can lead to blast wave shock waves being launched in the ambient air and that the emission from the shocked air dominates over emission from the target surface. The long term emission measurements in air are in agreement with the modeling results.