The influence of sinusoidal density modulation on the stimulated Raman scattering (SRS) reflectivity in inhomogeneous plasmas is studied by three-wave coupling equations, fully kinetic Vlasov simulations and particle in cell (PIC) simulations. Through the numerical solution of three-wave coupling equations, we find that the sinusoidal density modulation is capable of inducing absolute SRS even though the Rosenbluth gain is smaller than π, and we give a region of modulational wavelength and amplitude that the absolute SRS can be induced, which agrees with early studies. The average reflectivity obtained by Vlasov simulations has the same trend with the growth rate of absolute SRS obtained by three-wave equations. Instead of causing absolute instability, modulational wavelength shorter than a basic gain length is able to suppress the inflation of SRS through harmonic waves. And, the PIC simulations qualitatively agree with our Vlasov simulations. Our results offer an alternative explanation of high reflectivity at underdense plasma in experiments, which is due to long-wavelength modulation, and a potential method to suppress SRS by using the short-wavelength modulation.
We study stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) in shock ignition by comparing fluid and PIC simulations. Under typical parameters for the OMEGA experiments [Theobald \emph{et al}., Phys. Plasmas \textbf{19}, 102706 (2012)], a series of 1D fluid simulations with laser intensities ranging between 2$\times$10$^{15}$ and 2$\times$10$^{16}$ W/cm$^2$ finds that SBS is the dominant instability, which increases significantly with the incident intensity. Strong pump depletion caused by SBS and SRS limits the transmitted intensity at the 0.17n$_c$ to be less than 3.…
Using particle-in-cell simulations, we examine hot electron generation from electron plasma waves excited by stimulated Raman scattering and rescattering in the kinetic regime where the wavenumber times the Debye length (kλ_D) is greater than 0.3 for backscatter. We find that for laser and plasma conditions of possible relevance to experiments at the National Ignition Facility (NIF), anomalously energetic electrons can be produced through the interaction of a discrete spectrum of plasma waves generated from SRS (back and forward scatter), rescatter, and the Langmuir decay of the rescatter-gene…
A novel regime of self-compression is proposed for plasma-based backward Raman amplification(BRA) upon flying focus. By using a pumping focus moving with a speed equal to the group velocity of stimulated Raman backscattering(SRBS), only a short part of SRBS which does always synchronize with the flying focus can be amplified. Due to the asymmetrical amplification, the pulse can be directly compressed in the linear stage of BRA. Therefore, instead of a short pulse, the Raman spontaneous or a long pulse can seed the BRA amplifiers. The regime is supported by the 2D particle-in-cell(PIC) simulati…
The coupling evolutions of stimulated Raman scattering (SRS) and Raman rescattering (re-SRS) are studied under the parameter conditions of relevance to the gas-filled hohlraum experiments at the National Ignition Facility by a nonenveloped fluid code for the first time. It is found that re-SRS works as a frequency filter of backscattered light of SRS in the gas region. The low frequency modes originated from density points higher than about 0.1nc would stimulate re-SRS and be heavily depleted by re-SRS at the region of their effective quarter critical density region. Due to the high collisiona…
Higher intensity of strong-coupling stimulated Brillouin scattering (SC-SBS) amplification is achieved by supplementary Raman amplification. In the new scheme, a Raman pump laser first amplifies the seed pulse in the homogeneous plasma, then a SC-SBS pump laser continues the amplification in the inhomogeneous plasma in order to suppress the spontaneous instability of pump lasers. The intensity of seed laser gets higher and the duration of seed laser gets shorter than that in the pure SC-SBS scheme with the same incident energy, while the energy conversion effciency is not significantly reduced…
We show via particle-in-cell simulations that small normalized magnetic fields ($ω_c/ω_p \ll 1$) applied perpendicularly to a light wave can significantly modify the evolution of backward stimulated Raman scattering (SRS) in the kinetic regime. The presence of the magnetic field increases the threshold for kinetic inflation and decreases the amount of reflectivity when SRS is driven significantly above threshold. Analysis indicates this arises because trapped electrons are accelerated as they surf across the wave, leading to the continual dissipation of the electron plasma waves over a wider r…
Strongly coupled large-angle stimulated Raman scattering (LA SRS) of a short intense laser pulse proceeds in a plane plasma-filled capillary differently than in a plasma with open boundaries. Oblique mirror reflections off capillary walls partly suppress the lateral convection of scattered radiation and increase the growth rate of the instability: the convective gain of the LA SRS falls with an angle much slower than in an unbounded plasma and even for the near-forward SRS can be close to that of the direct backscatter. The long-term evolution of LA SRS in the interior of the capillary is domi…
Backward stimulated Raman scattering (BSRS) with Langmuir decay instability (LDI) and Langmuir collapse has been researched by Vlasov simulation for the first time. The decay productions of LDI cascade and their evolution with time is clearly demonstrated, which occurs simultaneously with Langmuir collapse. The BSRS reflectivity will be decreased largely through LDI cascade and Langmuir collapse. In CH plasmas, when $T_i/T_e=1/3$, the Landau damping of the slow ion-acoustic wave (IAW) is lower than that in H plasmas. Therefore, the BSRS can be further suppressed through LDI cascade by the way …
In this paper, we address the theoretical resolution of the Vlasov-Gauss system from the linear regime to the strongly nonlinear one, when significant trapping has occurred. The electric field is that of a sinusoidal electron plasma wave (EPW) which is assumed to grow from the noise level, and to keep growing at least up to the amplitude when linear theory in no longer valid (while the wave evolution in the nonlinear regime may be arbitrary). The ions are considered as a neutralizing fluid, while the electron response to the wave is derived by matching two different techniques. We make use of …
Interaction of a high intensity short laser pulse with near-critical plasmas allows to achieve extremely high coupling efficiency and transfer laser energy to energetic ions. One dimensional Particle-In-Cell (PIC) simulations are considered to detail the processes involved in the energy transfer. A confrontation of the numerical results with the theory highlights a key role played by the process of stimulated Raman scattering in the relativistic regime. The interaction of a 1 ps laser pulse (I $\sim$ 6.10$^{18}$ W.cm$^2$) with an under-critical (0.5 $n_c$) homogeneous plasma leads to a very hi…
We developed a new nonlinear fluid laser-plasma-instability code (FLAME) using a multi-fluid plasma model combined with full electromagnetic wave equations. The completed one-dimensional (1D) version of FLAME was used to study laser-plasma instabilities in shock ignition. The simulations results showed that absolute SRS modes growing near the quarter-critical surface were saturated by Langmuir-wave Decay Instabilities (LDI) and pump depletion. The ion-acoustic waves from LDI acted as seeds of stimulated Brillouin Scattering (SBS), which displayed a bursting pattern and caused strong pump deple…
In this paper, we derive coupled envelope equations modeling the growth of stimulated Raman scattering (SRS) in a multi-dimensional geometry, and accounting for nonlinear kinetic effects. In particular, our envelope equations allow for the nonlinear reduction of the Landau damping rate, whose decrease with the plasma wave amplitude depends on the rate of side-loss. Account is also made of the variations in the extent of the plasma wave packet entailed by the collisionless dissipation due to trapping. The dephasing between the electron plasma wave (EPW) and the laser drive, as well as the self-…
In the shock-ignition inertial confinement fusion scheme, high-intensity lasers propagate through an inhomogeneous coronal plasma, driving a shock designed to cause fuel ignition. During the high-intensity ignitor laser pulse, SRS backscatter in the long scale-length coronal plasma is likely to be in the kinetic regime. In this work, we use one-dimensional particle-in-cell simulations to show that there is a non-linear frequency shift caused by kinetic effects, resulting in the growth of Stimulated Raman Scattering (SRS) in an inhomogeneous plasma far exceeding the predictions of the fluid the…
Absolute instability modes due to rescattering of SRS in a large nonuniform plasma are studied theoretically and numerically. The backscattered light of convective SRS can be considered as a pump light with a finite bandwidth. The different frequency components of the backscattered light can be coupled to develop absolute stimulated Raman scattering (SRS) and two plasmon decay (TPD) instability near their quarter-critical densities via rescattering process. The absolute SRS mode develops a Langmuir wave with a high phase velocity about $c/\sqrt{3}$ with $c$ the light speed in vacuum. Given tha…
We present a scheme for amplifying an extreme-ultraviolet (XUV) seed isolated attosecond pulse via stimulated Raman scattering of a pulse-train pump. At sufficient seed and pump intensity, the amplification is nonlinear, and the amplitude of the seed pulse can reach that of the pump, one order of magnitude higher than the initial seed amplitude. In the linear amplification regime, we find that the spectral signature of the pump pulse train is imprinted on the spectrum of the amplified seed pulse. Since the spectral signature is imprinted with its frequency downshifted by the plasma frequency, …
In a laser-irradiated plasma, the Langdon effect makes the electron energy distribution function (EEDF) tend to a super-Gaussian distribution, which has important influences on laser plasma instabilities. In this work, the influences of a super-Gaussian EEDF on the convective stimulated backward Raman scattering (SRS) and stimulated backward Brillouin scattering (SBS) are studied systematically for a wide range of typical plasma parameters in the inertial confinement fusion (ICF). Distinct behaviors are found for SRS and SBS in the variation trend of the peak spatial growth rate and the corres…
We study theoretically the exchange of angular momentum between electromagnetic and electrostatic waves in a plasma, due to the stimulated Raman and Brillouin backscattering processes. Angular momentum states for plasmon and phonon fields are introduced for the first time. We demonstrate that these states can be excited by nonlinear wave mixing, associated with the scattering processes. This could be relevant for plasma diagnostics, both in laboratory and in space. Nonlinearly coupled paraxial equations and instability growth rates are derived.
Stimulated Raman scattering (SRS) in its strongly nonlinear, kinetic regime is controlled by a technique of deterministic, strong temporal modulation and spatial scrambling of laser speckle patterns, called Spike Trains of Uneven Duration and Delay (STUD pulses) [B. Afeyan and S. Hüller, Phys. Rev. Lett. (submitted)]. Kinetic simulations show that use of STUD pulses may decrease SRS reflectivity by more than an order of magnitude over random-phase-plate (RPP) or induced-spatial-incoherence (ISI) beams of the same average intensity and comparable bandwidth.
Stimulated Raman backscattering (RBS) of intense laser radiation confined by a single-mode plasma channel with a radial variation of plasma frequency greater than a homogeneous-plasma RBS bandwidth is characterized by a strong transverse localization of resonantly-driven electron plasma waves (EPW). The EPW localization reduces the peak growth rate of RBS and increases the amplification bandwidth. The continuum of non-bound modes of backscattered radiation shrinks the transverse field profile in a channel and increases the RBS growth rate. Solution of the initial-value problem shows that an el…
The hybrid Stimulated Raman Scattering - Two Plasmon Decay instability had shown to play the dominant role for plasma waves excitation and energy absorption at interaction of very intense femtosecond obliquely incident $p$-polarized laser pulse (intensity $10^{17}-5\cdot10^{19}$ W/cm$^2$) with a steep gradient ($L\sim λ$) plasma near the quarter critical surface, i.e. within the typical for modern experiments conditions. The plasmons are excited as two wave packets confined near this surface with very wide $\approx ω_0/c$ spatial spectra along its normal. Hence, phase matching conditions for 3…
Stimulated Raman scattering (SRS) in plasma in the degenerate regime is studied theoretically and numerically. Different from normal SRS with the non-degenerate eigen electrostatic mode excited, the degenerate SRS is developed at plasma density $n_e>0.25n_c$ when the laser amplitude is larger than a certain threshold. To satisfy the phase-matching conditions of frequency and wavenumber, the excited electrostatic mode has a constant frequency around half of the incident light frequency $ω_0/2$, which is no longer the non-degenerate eigenmode of electron plasma wave $ω_{pe}$. Both the scattered …
