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A one-dimensional steady-state model for stimulated Raman backscatter (SRS) and stimulated Brillouin backscatter (SBS) processes in laser-irradiated plasmas is presented. Based on a novel “predictor-corrector” method, the model is capable to deal with broadband scattered light and inhomogeneous plasmas, exhibiting robustness and high efficiency. Influences of the electron density and temperature on the linear gains of both SRS and SBS are investigated, which indicates that the SRS gain is more sensitive to the electron density and temperature than that of the SBS. For the low-density case, the SBS dominates the scattering process, while the SRS exhibits much higher reflectivity in the high-density case. The nonlinear saturation mechanisms and competition between SRS and SBS are included in our model by a phenomenological method. The typical anti-correlation between SRS and SBS versus electron density is reproduced in the model. Calculations of the reflectivities are qualitatively in agreement with the typical results of experiments and simulations.

High-quality protons have many potential applications in tumor therapy, proton radiography, fast ignition and nuclear physics. Over recent decades, significant progress has been made in generating such proton beams with lasers. However, it is still challenging to achieve a compact proton beam with an energy of hundreds of MeV under the laser intensities currently available. Here, we propose a novel scheme to produce above-100 MeV protons in nonuniform near-critical-density (NCD) plasmas driven by a Laguerre–Gaussian (LG) laser pulse. When a linearly-polarized LG laser pulse with the intensity of ∼1020 W cm−2 enters the NCD plasmas with a trapezoidal density profile, a donut-like double-channel structure with an electron column on the axis is produced behind the laser pulse, together with a unique, strong magnetic field. At the end of the density plateau, the magnetic field begins to expand in both longitudinal and transverse directions. The magnetic pressure force displaces the electrons with regard to the protons, resulting in a quasi-static electric field E x S . This electrostatic field can remain robust and be sustaintained for a long time since the magnetic field decreases very slowly, i.e. E x S ∝ ▽ B z 2 . Thus, the protons can be accelerated by the longitudinal electrostatic field and confined by the generated transverse focusing field. Three dimensional particle-in-cell simulations confirmed that a well-defined above-100 MeV proton beam with a cut-off energy at least 3.5 times larger than that of the normal Gaussian laser could be obtained, making the proton beams a potential source for tumor therapy in the future.

A new scheme is proposed to enhance the conversion efficiency (CE) of soft x-rays (0.1–1.5 keV) generated by irradiating a double-gold-foil target using double laser pulses to collide gold plasmas. A detailed analysis of the hydrodynamic evolution of the colliding plasmas is performed by using one-dimensional radiation hydrodynamic simulations. The results show that the total soft x-ray CE can be enhanced up to 71.5% by setting the foil thickness as d1 = 0.3 μm, and this is 14.5% higher than that for a single laser irradiating a single-gold-foil target. Most of the enhanced soft x-ray CEs are generated from a stagnation layer formed by plasma collisions. Such a scheme is simple and practical, which is beneficial for their applications in astrophysical opacity calculations, inertial confinement fusion, and hydrodynamic instability analyses.

High-gain magnetized liner inertial fusion (MagLIF) is a possible way to realize fusion. To investigate electrothermal effects on the transport of magnetic flux and thermal flux in MagLIF, this study has developed a resistive magnetohydrodynamics (MHD) module including an axial magnetic field using the code MULTI-IFE. MagLIF driven by a peak current of 60 MA releases 1080 MJ of fusion energy for a 1-cm-long liner, corresponding to an energy gain of approximately 180. The magnetic field is decompressed by electrothermal effects owing to the great temperature gradient in the fuel. This papers shows that a Nernst flux limiter of between 0.1 and 0.3 prevents the Nernst velocity from significantly decompressing the axial magnetic field and achieving a relative high yield. Compared with that of a simple gas target, the magnetic flux loss in the high-gain MagLIF target can be reduced from 70% to 30% owing to the magnetic insulation in the cryogenic deuterium-tritium. Most of the cryogenic DT layer in a high-gain MagLIF target is burned, resulting in a significant increase in the fusion yield.

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

The generation and stopping of a laser-driven two-component ion beam are investigated by three-dimensional particle-in-cell simulation and an extended stopping model. It is found that a two-component ion beam with the tunable composition ratio can be obtained from the two-species-ions sandwich target interacting with a relativistic laser pulse. When the generated proton beam mixed with a certain portion of carbon ions is transporting in fully ionized dense plasma, the beam stopping power is significantly enhanced compared to a pure proton beam. The maximum penetration depth is sharply reduced, and the Bragg peak with a higher magnitude appears earlier at the end of their paths, which is beneficial for achieving more localized energy deposition. The effect of heavy ion mixing on proton beam driven fast ignition is also discussed. A simple theoretical model is established, indicating that the required ignition time is relatively delayed for a heavy-ion doping case. For a small hot-spot size, it is possible to achieve the fusion ignition for a low mixed ratio. However, it is difficult to maintain a high fuel temperature due to the growing energy loss originating from mechanical work and thermal conduction.

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

Multiple lasers interacting with a deuterated (D) pitcher-catcher target and neutron generation are investigated using two-dimensional hybrid particle-in-cell and Monte Carlo simulations. It is found that when multiple laser pulses are focused on the front surface of the pitcher layer, D+ ion acceleration by target normal sheath acceleration (TNSA) is enhanced by the interfering overlapped light fields and the resulting periodic target-surface structure. With three laser pulses each of 4.5×1019 W/cm2 intensity, 33 fs duration and ~160 mJ energy, focusing at suitable angles on the pitcher layer, one can obtain 15 MeV D+ ions and ~25% laser-to-D+ ions energy conversion efficiency. As the resulting high-energy-density D+ ions bombard the catcher layer, D-D fusion reactions are triggered. About 3.6×107 neutrons can be produced, with the maximum neutron production rate as high as 3.1×1036 m-3s-1, almost an order of magnitude higher than that from a single laser of the same total energy.

By using three-dimensional particle-in-cell simulations, we demonstrate that an energetic ring-shaped ion beam can be generated by an ultra-intense circularly polarized Laguerre–Gaussian laser pulse interacting with micro-structure targets. The electron and ion dynamics of three different targets including a sleeve–wire target, wire target, and common planar target are investigated. It is found that an optimized sleeve–wire target can provide a remarkable increase in the maximum ion energy and laser-to-ion energy conversion efficiency. The reason can be attributed to the matched transverse profiles between the electric-field distribution of Laguerre–Gaussian laser and sleeve–wire structure, resulting in efficient laser-target energy coupling. In fact, using a laser pulse of intensity 2.74 × 1 0 20 W/cm2, duration 66.7 fs, and energy ∼1 J, one can obtain ∼35 MeV protons, ∼5.8 MeV/u carbon ions, and ∼15% laser-to-ion energy conversion.

Ionization-induced electron injection in a chirped laser pulse driven wakefield acceleration is investigated through particle-in-cell simulations. We find that negative (positive) chirp can longitudinally stretch (compress) laser pulse, and the wakefield amplitude can be enhanced regardless of the chirp sign. On the other hand, the rapid compression of a positively chirped pulse leads to the decrease of the wakefield potential difference, which breaks the ionization injection condition and consequent injection truncation occurs. The effective injection only lasts for a few hundred micrometers and the detailed injection length is determined by the chirp parameter. As a result, a highly tunable quasi-monoenergetic electron beam with narrow energy spread, hundreds of MeV central energy and tens of pC charge can be stably produced by this scheme.

We propose a compact scheme for high-density and high-energy positron generation. In the scheme, the generation, compression and acceleration of positrons are encompassed in a plasma channel irradiated by an ultra intense laser. In about 20 μm, positrons are accelerated with their energy beyond 20 GeV, and are compressed with a maximal density of 4.1n c . The obtained positron beam with a wave-like density profile is accelerated via the direct laser acceleration.

A scheme for generating high-flux positrons by two counter-propagating laser pulses colliding in near-critical-density plasmas is proposed, which might be realized with current laser facilities. Positrons of number 2.79 × 105 and with a maximum density of 9.63 × 1024 m−3 can be generated for a laser with an intensity of 1022 W cm−2. This is attributed to the increase in the cross sections for photon radiation and positron generation in the colliding scheme. In order to improve the positron generation, the relevant parameters are discussed in detail. This scheme will facilitate the observation of the Breit-Wheeler positrons in the laboratory.

We show that a ring-shaped hollow electron beam can be injected and accelerated by using a Laguerre-Gaussian laser pulse and ionization-induced injection in a laser wakefield accelerator. The acceleration and evolution of such a hollow, relativistic electron beam are investigated through three-dimensional particle-in-cell simulations. We find that both the ring size and the beam thickness oscillate during the acceleration. The beam azimuthal shape is angularly dependent and evolves during the acceleration. The beam ellipticity changes resulting from the electron angular momenta obtained from the drive laser pulse and the focusing forces from the wakefield. The dependence of beam ring radius on the laser-plasma parameters (e.g., laser intensity, focal size, and plasma density) is studied. Such a hollow electron beam may have potential applications for accelerating and collimating positively charged particles.

By using a two-dimensional particle-in-cell simulation, we demonstrate a scheme for high-energy-density electron beam generation by irradiating an ultra intense laser pulse onto an aluminum (Al) target. With the laser having a peak intensity of 4 × 1023 W cm−2, a high quality electron beam with a maximum density of 117nc and a kinetic energy density up to 8.79 × 1018 J m−3 is generated. The temperature of the electron beam can be 416 MeV, and the beam divergence is only 7.25°. As the laser peak intensity increases (e.g., 1024 W cm−2), both the beam energy density (3.56 × 1019 J m−3) and the temperature (545 MeV) are increased, and the beam collimation is well controlled. The maximum density of the electron beam can even reach 180nc. Such beams should have potential applications in the areas of antiparticle generation, laboratory astrophysics, etc.

High-energy-density electron–positron pairs play an increasingly important role in many potential applications. Here, we propose a scheme for enhanced positron production by an ultra intense laser irradiating a gas-Al compound target via the multi-photon Breit–Wheeler (BW) process. The laser pulse first ionizes the gas and interacts with a near-critical-density plasma, forming an electron bubble behind the laser pulse. A great deal of electrons are trapped and accelerated in the bubble, while the laser front hole-bores the Al target and deforms its front surface. A part of the laser wave is thus reflected by the inner curved target surface and collides with the accelerated electron bunch. Finally, a large number of γ photons are emitted in the forward direction via the Compton back-scattering process and the BW process is initiated. Dense electron–positron pairs are produced with a maximum density of m−3. Simulation results show that the positron generation is greatly enhanced in the compound target, where the positron yield is two orders of magnitude greater than that in only the solid slab case. The influences of the laser intensity, gas density and length on the positron beam quality are also discussed, which demonstrates the feasibility of the scheme in practice.

The dynamics of electrons forming the boundary layer of a highly nonlinear laser wakefield driven in the so called bubble or blowout regime is investigated using particle-in-cell simulations. It is shown that when the driver pulse intensity increases or the focal spot size decreases, a significant amount of electrons initially pushed by the laser pulse can detach from the bubble structure at its tail, middle, or front and form particular classes of waves locally with high densities, referred to as the tail wave, lateral wave, and bow wave. The tail wave and bow wave correspond to real electron trajectories, while the lateral wave does not. The detached electrons can be ejected transversely, containing considerable energy, and reducing the efficiency of the laser wakefield accelerator. Some of the transversely emitted electrons may obtain MeV level energy. These electrons can be used for wake evolution diagnosis and producing high frequency radiation.

By using two-dimensional particle-in-cell simulations, we demonstrate high-flux dense positrons generation by irradiating an ultra-intense laser pulse onto a tapered hollow target. By using a laser with an intensity of 4 × 1023 W/cm2, it is shown that the Breit-Wheeler process dominates the positron production during the laser-target interaction and a positron beam with a total number >1015 is obtained, which is increased by five orders of magnitude than in the previous work at the same laser intensity. Due to the focusing effect of the transverse electric fields formed in the hollow cone wall, the divergence angle of the positron beam effectively decreases to ∼15° with an effective temperature of ∼674 MeV. When the laser intensity is doubled, both the positron flux (>1016) and temperature (963 MeV) increase, while the divergence angle gets smaller (∼13°). The obtained high-flux low-divergence positron beam may have diverse applications in science, medicine, and engineering.

A scheme for enhancing the electron injection charge in a laser-driven bubble acceleration is proposed. In this scheme, a thin foil target is placed in front of a gas target. Upon interaction with an ultra-intense laser pulse, the foil emits electrons with large longitudinal momenta, allowing them to be trapped into the transmitted shaped laser-excited bubble in the gaseous plasma target. Two-dimensional particle-in-cell simulation is used to demonstrate this scheme, and an electron beam with a total electron number of 4.21×108 μm−1 can be produced, which is twice the number of electrons produced without the foil. Such scheme may be widely used for applications that require high electron yields such as positron and gamma ray generation from relativistic electron beams interacting with solid targets.

Laser shaping of a relativistic circularly polarized laser pulse in ultra-intense laser thin-foil interaction is investigated by theoretical analysis and particle-in-cell simulations. It is found that the plasma foil as a nonlinear optical shutter has an obvious cut-out effect on the laser temporal and spatial profiles. Two-dimensional particle-in-cell simulations show that the high intensity part of a Gaussian laser pulse can be well extracted from the whole pulse. The transmitted pulse with longitudinal steep rise front and transverse super-Gaussian profile is thus obtained which would be beneficial for the radiation pressure acceleration regime. The Rayleigh-Taylor-like instability is observed in the simulations, which destroys the foil and results in the cut-out effect of the pulse in the rise front of a circularly polarized laser.