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We examine electron kinetic effects in broadband-laser-driven back-stimulated Raman scattering (BSRS) bursts using particle-in-cell simulations. These bursts occur during the nonlinear stage, causing reflectivity spikes and generating large numbers of hot electrons. Long-duration simulations are performed to observe burst events, and a simplified model is developed to eliminate the interference of the broadband laser's random intensity fluctuations. Using the simplified model, we isolate and characterize the spectrum of electron plasma waves. The spectrum changes from a sideband structure to a turbulence-like structure during the burst. A significant asymmetry in the spectrum is observed. This asymmetry is amplified and transferred to electron phase space by high-intensity broadband laser pulses, leading to violent vortex-merging and generation of hot electrons. The proportion of hot electrons increases from 6.76% to 14.7% during a single violent burst event. We demonstrate that kinetic effects profoundly influence the BSRS evolution driven by broadband lasers. [ABSTRACT FROM AUTHOR]

The ion kinetic effect on the formation of intense laser-driven collisional shock waves is investigated via hybrid fluid-particle-in-cell simulations. It is found that the ion heat flux dominates the shock formation, which is considerably larger than the electron heat flux in the shock region. The rise of the temperature due to the laser energy deposition drives a heatwave into the overdense plasma, creating an electron–ion energy exchange zone between the critical surface and heat wave front. The heated ions, which are generated at the electron–ion energy exchange zone via the friction force, are found to travel to the high-density region and cause a tail distribution gain. Despite the small quantity, the heated tail ions contribute most of the ion heat flux during the shock formation. Additionally, as the electron heat flux decreases, the population of the heated tail ions is reduced, leading to a fall in the ion heat flux. This results in the delay or even suppression of the shock formation, because the ions are in a non-equilibrium state in the vicinity of the shock region, the ratio of the downstream ion temperature to the upstream ion temperature tends to a modestly decrease in comparison to the theory. The study provides a clear picture of the formation process of laser-driven shock waves. [ABSTRACT FROM AUTHOR]

The competition between forward stimulated Raman side scatter (FSRSS) and backward stimulated Raman side scatter (BSRSS) is investigated in inhomogeneous plasma using particle-in-cell (PIC) simulations. Experimental observations at the National Ignition Facility have demonstrated the significance of stimulated Raman side scatter or backscatter instability compared to two-plasmon-decay under ignition-scale conditions for various laser beam geometries. Side scatter refers to the geometry where the Raman scattered light is generated in a direction perpendicular to the local density gradient. For an obliquely incident pump, the scattered light can either copropagate (FSRSS) or counter propagate (BSRSS) with respect to the pump. Under ignition-scale conditions, linear analysis shows that both BSRSS and FSRSS are absolutely unstable (temporally growing) at higher densities ( n e ∼ 0.2 n c ), whereas at lower densities ( n e ∼ 0.1 n c ), BSRSS becomes convective with substantial gain, while FSRSS remains absolute. Two-dimensional PIC simulations demonstrate that the competition between BSRSS and FSRSS is sensitive to the density. BSRSS tends to dominate at higher densities, while FSRSS becomes dominant at lower densities. At moderate densities ( n e ∼ 0.15 n c ), FSRSS and BSRSS coexist. Furthermore, an increase in laser intensity leads to enhanced electron trapping, which kinetically strengths and then saturates BSRSS in the lower density region. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical simulations are carried out to investigate the nonlinear bubble growth of ablative Rayleigh–Taylor instability (ARTI) seeded by localized perturbations (LPs), where the LPs are described by a Gaussian mode. It is found that the nonlinear bubble-front penetration of LP-seeded ARTI follows the self-similar scaling law α b A T ∫ g d t 2 , different from the classical case, where the self-similar behavior is not observed. It is also found that the quadratic growth coefficient α b in the LP-seeded ARTI mainly depends on the initial perturbation amplitude and initial perturbation width. When the perturbation amplitude is small, α b has a value of ∼0.03, which is not sensitive to the perturbation width. As the perturbation amplitude increases, the value of α b increases, and the phenomenon is more significant when the perturbation width is narrower. It is shown that the increase in α b is due to the spike-induced upward jet and the ablation-generated vorticity inside the bubble. [ABSTRACT FROM AUTHOR]

The effect of ablation on the nonlinear spike growth of single-mode ablative Rayleigh–Taylor instability (RTI) is studied by two-dimensional numerical simulations. It is shown that the ablation can reduce the quasi-constant velocity and significantly suppress the reacceleration of the spike in the nonlinear phase. It is also shown that the spike growth can affect the ablation-generated vorticity inside the bubble, which further affects the nonlinear bubble acceleration. The vorticity evolution is found to be correlated with the mixing width (i.e., the sum of the bubble and spike growths) for a given wave number and ablation velocity. By considering the effects of mass ablation and vorticity, an analytical model for the nonlinear bubble and spike growth of single-mode ablative RTI is developed in this study. It is found that the nonlinear growth of the mixing width, induced by the single mode, is dominated by the bubble growth for small-scale ablative RTI, whereas it is dominated by the spike growth for classical RTI. [ABSTRACT FROM AUTHOR]

A new evolution pattern for broadband laser excited stimulated Raman back-scattering (BSRS) in the kinetic regime is proposed by numerical simulations. It is found that the change of coherence of different frequency beamlets will cause the fluctuation of laser intensity, generating an ensemble of random intensity pulses and leading to an intermittent excitation of BSRS. The kinetic inflation and intense amplification of scattered light are observed due to the synergism between these pulses, which cause a burst of instantaneous reflectivity. The synergistic effect is highly bandwidth-dependent. Under the bandwidth similar to the existing broadband laser facilities, these bursts will generate over-expected scattered light and hot electrons. Fortunately, a large bandwidth laser can still inactivate the synergy mechanism and mitigate the scattering effectively. We formulated a theoretical model to predict the inactivate point, and the calculation Δ ω / ω 0 = 2.57 % is in good agreement with the numerical results. [ABSTRACT FROM AUTHOR]

The evolution of collisional planar plasma shocks is investigated by using a hybrid fluid-particle-in-cell code, which enables the analysis of the impact of ion kinetic effects. The structure and propagation of shocks in multi-component plasmas with moderate density ( 10 22 ∼ 10 24 cm − 3 ) are found to be strongly dependent on the electron thermal conduction and ion kinetic effects. In high electron heat flux cases, the electron thermal conduction creates a preheat layer ahead of the shock front, allowing the energetic ions to stream through the upstream plasma. It is found that the shock velocity drops by about 4.67% and the heatwave velocity increases about 47% when the electron flux limiter increases from f e = 0.05 to 0.15. Furthermore, the inhibiting effect of high electron heat flux on the species separation induced by the shock in multi-component plasmas is observed. These results provide a new dataset valuable for benchmarking and improving radiation hydrodynamic models. [ABSTRACT FROM AUTHOR]

We report on the growth and characterization of epitaxial (010)- and (100)-YMn0.5Cr0.5O3 films on the corresponding YAlO3 substrates. It is found that the magnetic transition temperature increases from 75K for the bulk sample to 120K for the compressive-strained films. Along the c- and aaxes, ferroelectric polarization develops below 120K with the saturation values of 0.17 and 0.05 μC/cm², respectively, while it remains zero along the b-axis. Furthermore, it is demonstrated that the ferroelectric polarization can be modulated by magnetic field. The displayed ferroelectricity is explainable by considering coexistence of the predominant bc-cycloidal and minor E-type antiferromagnetic orders. [ABSTRACT FROM AUTHOR]

Three-dimensional particle-in-cell simulation is used to investigate radiation-pressure driven acceleration and compression of small solid-density plasma by intense laser pulses. It is found that multiple impacts by presently available short-pulse lasers on a small hemispheric shell target can create a long-living tiny quasineutral monoenergetic plasma bunch of very high energy density. [ABSTRACT FROM AUTHOR]

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

The impact of a laser-accelerated micron-size projectile on a dense plasma target is studied using two-dimensional particle-in-cell simulations. The projectile is first accelerated by an ultraintense laser. It then impinges on the dense plasma target and merges with the latter. Part of the kinetic energy of the laser-accelerated ions in the projectile is deposited in the fused target, and an extremely high concentration of plasma ions with a mean kinetic energy needed for fusion reaction is induced. The interaction is thus useful for laser-driven impact fusion and as a compact neutron source. [ABSTRACT FROM AUTHOR]

Intense short-petawatt-laser driven relativistic electron beams in a hollow high-Z plasma fiber embedded in low-Z plasmas of different densities are studied. When the plasma is of lower density than the hollow fiber, resistive filamentation of the electron beam is observed. It is found that the electron motion and the magnetic field are highly correlated with tens of terahertz oscillation frequency. Depending on the material property around the hollow fiber and the plasma density, the beam electrons can be focused or defocused as it propagates in the plasma. Relativistic electron transport and target heating are also investigated. [ABSTRACT FROM AUTHOR]

Proton acceleration by ultra-intense laser pulse irradiating a target with cross-section smaller than the laser spot size and connected to a parabolic density channel is investigated. The target splits the laser into two parallel propagating parts, which snowplow the back-side plasma electrons along their paths, creating two adjacent parallel wakes and an intense return current in the gap between them. The radiation-pressure pre-accelerated target protons trapped in the wake fields now undergo acceleration as well as collimation by the quasistatic wake electrostatic and magnetic fields. Particle-in-cell simulations show that stable long-distance acceleration can be realized, and a 30 fs monoenergetic ion beam of >10 GeV peak energy and <2° divergence can be produced by a circularly polarized laser pulse at an intensity of about 1022 W/cm2. [ABSTRACT FROM AUTHOR]

A 2D analytical model is presented for the generation of plasma wakes (or bubbles) with an ultra-intense laser pulse by taking into account the response of plasma ions. It is shown that the effect of ion motion becomes significant at the laser intensity exceeding 1021 W/cm2 and plasma background density below 1019 cm-3. In this regime, ion motion tends to suppress the electrostatic field induced by charge separation and makes the electron acceleration less effective. As a result, the assumption of immobile ions overestimates the efficiency of laser wake-field acceleration of electrons. Based on the analytical model, the dynamics of plasma ions in laser-induced wake field is investigated. It is found that only one bubble appears as the plasmas background density exceeds the resonant density and the deposited laser energy is concentrated into the bubble, resulting in the generation of an ion bunch with extremely high energy density. [ABSTRACT FROM AUTHOR]