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We investigate the behaviour of backward stimulated Raman scattering (BSRS) in inhomogeneous plasma with a transverse external magnetic field via 1D and 2D particle-in-cell simulations and theoretical analysis. The external magnetic field causes nonlinear damping of large-amplitude electron plasma waves (EPWs), and significantly limits the reflectivity of stimulated Raman scattering (SRS) even if is very weak. For a single resonance point, limits the kinetic nonlinear frequency shift, impedes the autoresonance behaviour of SRS, and narrows the resonance region. We also exhibit distorts the distribution function of electrons evidently, and alters the wavefronts of EPWs in high dimensional cases. The simulation results indicate a moderate can significantly reduce the trapped electron induced gain of SRS reflectivity, and make the BSRS reflectivity stay at its linear convective saturation level steadily. [ABSTRACT FROM AUTHOR]

Enhanced stimulated Brillouin scattering and stimulated Raman scattering are shown in comparable-mass plasma by considering charge state through a simplified theoretical model and particle-in-cell simulations. We present a more general dispersion relation of plasma wave with arbitrary mass ratio and charge state compared to the previous electron–positron plasma study. Further, the ion-bulk mode is responsible for Brillouin amplification from kinetic analysis. Higher intensities of seed laser pulse amplification through stimulated Brillouin scattering are observed in βZ = 1 type plasmas and β = 1 type plasmas as Z increases; the former is caused by kinetic effects, while the latter is due to the higher growth rates of instability. Our results will have potential application in pulse amplification in the comparable-mass plasmas with charge state Z > 1, such as plasma of. [ABSTRACT FROM AUTHOR]

Phase space vortex merging in electron plasma waves is researched by applying a periodically changing electric field in plasma space with Maxwellian velocity distribution. The pump electric field excites the electron plasma wave with large amplitude and electron phase space vortices are formed. The electron plasma wave can be transited to longer wave modes through vortex merging processes. Excepting the fundamental mode of electron plasma waves, two or more modes appear and grow exponentially before vortex merging. After merging, another mode replaces the initial pumping wave as the strongest wave mode. It is verified that the growth of this new strong mode is related to trapping particle instability. Vortex merging is one of the saturation mechanisms of trapped particle instability. [ABSTRACT FROM AUTHOR]

A new saturation mechanism, beam plasma instability with trapped electrons resulting in saturation of stimulated Raman backscattering (SRBS), is presented. The beam plasma instability can be excited due to interaction between beam electrons and background electrons and it will produce a new mode. Here, the trapped electrons can be considered as beam electrons with velocity vp (phase velocity of the plasma wave) and density nt (density of the trapped electrons normalized by nc, nc is the critical density for the incident laser light). This new mode will induce a sinusoidal density modulation and then this modulation will produce harmonic wave with modulation wave-number period δk to enhance this new mode. As reported previously, the density modulation can increase Landau damping of the plasma wave and decrease the resonant region where SRBS occurs. Therefore, SRBS will saturate due to beam plasma instability induced by the trapped electrons. [ABSTRACT FROM AUTHOR]

We propose a nonlinear mechanism for transition from convective to absolute in stimulated Raman backscattering instability due to the effect of trapped particles in the plasma wave. Convective instability saturates at the low level, yet it is sufficient to trap electrons near the plasma phase velocity. The trapped electrons tend to flatten the distribution function. With spatial averaging over the trapped region, we find that the flattened distribution function reduces the damping rate due to bounce resonance of the plasma wave and then decreases the threshold for absolute instability. So the transition from a weak, convective instability to a strong, absolute instability can occur, leading to exponential growth everywhere and inflation of reflectivity of several orders of magnitude as observed in the experiment, once the threshold is exceeded. [ABSTRACT FROM AUTHOR]

The effect of a static sinusoidal density modulation on the temporal growth rate of backward stimulated Raman scattering (BSRS) is discussed by kinetic theory and Vlasov simulation. One-dimensional Vlasov simulations show that the temporal growth rate will decrease with the increasing density modulation amplitude ε, which is consistent with the kinetic theory. Moreover, the transition from an absolute instability to a convective instability via the density modulation is also observed from the variation of the temporal growth rate of BSRS. The temporal growth rate in the case of density modulation wave-number ks = 0.1kl (kl is wave-number of Langmuir wave) decreases faster with increasing ε than that in the case of ks = 0.5kl because of the generation of more modes, which will make Landau damping of Langmuir waves stronger. In addition to the generation of other modes, the decrease of the resonant region where BSRS occurs with the increasing density modulation amplitude ε is also a reason for the reduction of the temporal growth rate. [ABSTRACT FROM AUTHOR]

With the help of particle-in-cell simulations, we show a stably propagating train of attosecond (10−1810−18 s) electron bunches which are generated along the target back surface via laser-solid interactions. The electron bunches are generated by the oscillating electric fields of the surface plasma wave. Because of the combinational effects of the electrostatic field and the static magnetic field on the target back surface, the electron bunches are stably propagating along the target back surface, which means they are totally separated from the laser pulse. The averaged energy of these electron bunches is over 20 MeV20 MeV, the maximum averaged density is about 6nc (where nc ≈ 1.1 × 1021 cm−3 is the critical density of the incident laser), and the averaged duration is less than 200 as. Such electron bunches are easily applied to the generation of attosecond x-rays via Compton backscattering. The energy conversion efficiency from the laser to the attosecond electron bunches is about 1.5%. [ABSTRACT FROM AUTHOR]

Synchrotron radiation is strongly enhanced by the resonant excitation of surface plasma waves (SPWs). Two-dimensional particle-in-cell simulations show that energy conversion efficiency from laser to radiation in the case of SPWs excitation is about 18.7%, which is improved by more than 2 orders of magnitude compared with that of no SPWs excitation. Besides the high energy conversion efficiency, the frequency spectrum and the angular distribution of the radiation are also improved in the case of SPWs excitation because of the quasi-static magnet field induced by surface plasma waves excitation. [ABSTRACT FROM AUTHOR]

The propagation of light waves in an underdense plasma is studied using one-dimensional Vlasov–Maxwell numerical simulation. It is found that the backward stimulated Raman scattering will be enhanced by electron-ion collisions. With appropriate electron-ion collision rate the Langmuir waves, driven via SRS, can be made to propagate for a long distance. [ABSTRACT FROM AUTHOR]

One-dimensional Vlasov–Maxwell simulations of laser-plasma interactions are presented. It is shown that plasma gratings and density cavitons are formed sequentially. There are strong electromagnetic fields in the cavitons and the electromagnetic structures are nearly standing and long-lived. The formation of gratings and cavitons can be explained by a nonlinear second-order differential equation. The electromagnetic fields trapped in cavitons have both subcycle and cycle structures. Plasma whose density is higher than the critical density can be formed around the cavitons. Gratings and high density plasmas can reflect light in a very high level. This may be detrimental to the inertial confinement fusion. [ABSTRACT FROM AUTHOR]