Stimulated Raman Backscattering Research Articles

Role of spontaneous thermal emissions in inflationary laser Raman instability

The role of thermal fluctuations on the stimulated Raman backscattering instability is investigated by means of Vlasov and particle-in-cell (PIC) simulations in a regime of strong linear Landau damping of the Langmuir wave. The instability is initially convective and amplifies thermal noise, leading to a low-amplitude back-scattered laser sideband. Linear Landau damping of the Langmuir sideband modifies and flattens the electron velocity distribution function at the resonant velocity, leading to a gradual decrease in the Landau damping rate and an increase in the convective amplification. The Langmuir wave traps electrons resulting in a rapid nonlinear absolute instability and large amplitude flashes of backscattered light off large amplitude Langmuir waves with trapped electrons, leading to the production of hot electrons. Conditions for simulating realistic thermal noise with Vlasov and PIC simulations are discussed and defined.

Eigenmodes and eigenfunctions of high-power electromagnetic waves in inhomogeneous magnetized plasmas and their stability to stimulated Raman backscattering

Eigenmodes and eigenfunctions of high-power electromagnetic waves in inhomogeneous magnetized plasmas are investigated. Considering the dielectric permittivity in the presence of an external magnetic field and relativistic nonlinearity and considering the inhomogeneity of the electron density in the linear and rippled structures, two nonlinear wave equations are obtained. In the linear regime, eigenmodes and eigenfunctions of the electric field for two-electron densities are obtained. In the nonlinear regime, results indicate that by increasing the external magnetic field, the amplitude of the electric field increases, which affects the dielectric permittivity. The conditions of the stimulated Raman backscattering and the equation of the space-charge potential are obtained. Results also indicate that in the presence of the external magnetic field, plasma inhomogeneity, and relativistic nonlinearity, the stimulated Raman backscattering becomes unstable.

Suppressing stimulated Raman side-scattering with vector light

Recent observations of stimulated Raman side-scattering (SRSS) in different laser inertial confinement fusion ignition schemes have revealed that there is an underlying risk of SRSS on ignition. In this paper, we propose a method that uses the nonuniform nature of the polarization of vector light to suppress SRSS, and we give an additional threshold condition determined by the parameters of the vector light. For SRSS at 90°, where the scattered electromagnetic wave travels perpendicular to the density profile, the variation in polarization of the pump will change the wave vector of the scattered light, thereby reducing the growth length and preventing the scattered electromagnetic wave from growing. This suppression scheme is verified through three-dimensional particle-in-cell simulations. Our illustrative simulation results demonstrate that for linearly polarized Gaussian light, there is a strong SRSS signal in the 90° direction, whereas for vector light, there is very little SRSS signal, even when the conditions significantly exceed the threshold for SRSS. We also discuss the impact of vector light on stimulated Raman backscattering, collective stimulated Brillouin scattering and two-plasmon decay.

Stimulated Raman side and backscatter instabilities of crossed laser beams in plasma

Stimulated Raman scattering (SRS) instability due to two nonlinearly coupled laser beams propagating in homogeneous, thermal plasma is analyzed. The temporal evolution equations for pump, scattered and Langmuir waves excited due to the interaction are set up. Using numerical techniques temporal evolution of amplitudes of pump, scattered and resultant Langmuir waves for side and back SRS instabilities have been obtained in the non-relativistic regime. The temporal growth rate, saturation time and saturation amplitude of waves excited by two crossed (perpendicular) lasers are studied and compared with the single beam case. The model considers pump depletion to be the dominant saturation mechanism. The study points towards a new concept for generation of high amplitude Langmuir waves.

Stimulated Raman scattering coupled with decay instability in a magnetized plasma with hot drifting electrons

We examine the impact of the hot drifting electron on stimulated Raman backscattering, which is coupled to decay instability, in a magnetized plasma. Through parametric coupling, this produces a plasma wave that moves forward and an electromagnetic wave that shifts downward. The plasma wave produced by stimulated Raman scattering (SRS) in this process decays into an ion-acoustic wave and a secondary, longer-wavelength Langmuir wave traveling opposite to the ion-acoustic wave. This energy diversion and attenuation of the primary Langmuir wave by drifting electrons limit the amplitude of SRS. In the presence of drifting electrons, the SRS is suppressed significantly and the plasma wave is attenuated resonantly at a higher rate.

Investigation of influence of Kerr and detuning parameters over instability gain in the three wave resonant interaction of stimulated Raman backscattering

Investigation of influence of Kerr and detuning parameters over instability gain in the three wave resonant interaction of stimulated Raman backscattering

Non-linear stimulated Raman back-scattering burst driven by a broadband laser

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 Δω/ω0=2.57% is in good agreement with the numerical results.

Self-compression of stimulated Raman backscattering by a flying focus.

The regime of self-compression has been proposed for plasma-based backward Raman amplification upon a 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 always synchronizes with the flying focus can be amplified. Therefore, instead of a short pulse, plasma noise or a long pulse can seed the BRA amplifiers. Here we demonstrate the regime by 2D particle-in-cell simulations, showing that the pump pulse is compressed from 26 ps to 116 fs, with an output amplitude comparable with the case of a well-synchronized short seed. As only one laser pulse is used in the simulation, the results present a significant path to simplify the Raman amplifiers.

Study of growth rate of backward Raman scattering in plasma with density ripple

The recent studies proved that stimulated backward Raman scattering of a laser is affected significantly by the existence of magnetic field and density rippled plasma. At lower hybrid frequencies, the localized radial and azimuthal modes can be supported by the magnetized plasma. The density ripple interacts with the Raman process’s main Langmuir wave, producing in a secondary Langmuir wave with a higher wave number that is highly Landau damped on the electrons. Numerically, the influence of various modes on the growth rate revealed that the ripple and local effects greatly decrease the growth rate of stimulated Raman backscattering. As a result, the Raman process is controlled by the magnetized density rippled plasma and the growth rate is reduced significantly.

Pump-depletion dynamics and saturation of stimulated Brillouin scattering in shock ignition relevant experiments.

As an alternative inertial confinement fusion scheme, shock ignition requires a strong converging shock driven by a high-intensity laser pulse to ignite a precompressed fusion capsule. Understanding nonlinear laser-plasma instabilities is crucial to assess and improve the laser-shock energy coupling. Recent experiments conducted on the OMEGA EP laser facility have demonstrated that such instabilities can ∼100% deplete the first 0.5ns of the high-intensity laser. Analyses of the observed laser-generated blast wave suggest that this pump-depletion starts at ∼0.02 critical density and progresses to 0.1-0.2 critical density, which is also confirmed by the time-resolved stimulated Raman backscattering spectra. The pump-depletion dynamics can be explained by the breaking of ion-acoustic waves in stimulated Brillouin scattering. Such pump depletion would inhibit the collisional laser energy absorption but may benefit the generation of hot electrons with moderate temperatures for electron shock ignition [Phys. Rev. Lett. 119, 195001 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.195001].

Single and double shell ignition targets for the national ignition facility at 527 nm

Converting and using the National Ignition Facility (NIF) to deliver 527 nm light instead of its current 351 nm would allow the laser to deliver more energy and power to ignition targets. We update previous 527 nm target design work to reflect more contemporary target designs using high-density carbon capsules and low density helium gas filled Hohlraums. We extend single shell capsule designs based on current experimental results to higher energy and power and also explore double shell capsules, both driven by green light. These studies were completed using detailed pulse shapes found for targets that converged with acceptable 2D implosion symmetries and then used the Lava Lamp II code to confirm their feasibility at NIF. A 1.2× dimensional scaleup of one tuned NIF target at the limit of its current 351 nm capabilities and shot 170827 uses 3.3 MJ, at the limit of the current NIF's 527 nm capability. With the less-structured pulse of a double shell target, 3.7 MJ could be delivered by the laser. Our LPI calculations do not preclude operation at 527 nm, particularly for low fill Hohlraums, and suggest that the stimulated Raman backscatter may be no worse than the small quantities seen in 170827; stimulated forward Raman scattering may be present. If Stimulated Brillouin Scattering is too great, the much greater laser bandwidth available at 527 nm could be used to decrease backscatter. These larger targets with higher energy and power may offer a better chance of achieving ignition with only modest changes to the NIF laser.

Low mode implosion symmetry sensitivity in low gas-fill NIF cylindrical hohlraums

Achieving an efficient capsule implosion in National Ignition Facility indirect-drive target experiments requires symmetric hohlraum x-ray drive for the duration of the laser pulse. This is commonly achieved using two-sided two-cone laser irradiation of cylindrical hohlraums that, in principle, can zero the time average of all spherical harmonic asymmetry modes <6 as well as the time dependence of the usually dominant mode 2. In practice, experimental evidence indicates that maintaining symmetric drive becomes limited late in the pulse due to the inward expansion of the hohlraum wall and outward expansion of the capsule ablator plasmas impairing the propagation of the inner-cone laser beams. This effect is enhanced in hohlraums employing low gas-fill, now used almost exclusively as these provide the highest performing implosions and reduce Stimulated Brillouin and Raman backscatter losses, since the gas plasma provides less back pressure to limit blow-in of the hohlraum wall and capsule ablator plasmas. In order to understand this dynamic behavior, we combined multi-keV X-ray imaging of the wall and imploded fuel plasmas as we changed a single parameter at a time: hohlraum gas-fill, laser outer cone picket energy, radius of high density carbon capsules used, and laser beam polar and azimuthal pointing geometry. We developed a physics-based multi-parameter experimental scaling to explain the results that extend prior scalings and compare those to radiation hydrodynamic simulations to develop a more complete picture of how hohlraum, capsule, and laser parameters affect pole vs equator drive symmetry.

One-dimensional steady-state model for stimulated Raman and Brillouin backscatter processes in laser-irradiated plasmas

AbstractA 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.

Saturation of stimulated Raman backscattering due to beam plasma instability induced by trapped electrons

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.

Plasma modulator for high-power intense lasers

A type of plasma-based optical modulator is proposed for the generation of broadband high-power laser pulses. Compared with normal optical components, plasma-based optical components can sustain much higher laser intensities. Here we illustrate via theory and simulation that a high-power sub-relativistic laser pulse can be self-modulated to a broad bandwidth over 100% after it passes through a tenuous plasma. In this scheme, the self-modulation of the incident picoseconds sub-relativistic pulse is realized via stimulated Raman forward rescattering in the quasi-linear regime, where the stimulated Raman backscattering is heavily dampened. The optimal laser and plasma parameters for this self-modulation have been identified. For a laser with asub-relativistic intensity of I ∼ 1017W/cm2, the time scale for the development of self-modulation is around 103 light periods when stimulated Raman forward scattering has been fully developed. Consequently, the spatial scale required for such a self-modulation is in the order of millimeters. For a tenuous plasma, the energy conversion efficiency of this self-modulation is around 90%. Theoretical predictions are verified by both one-dimensional and two-dimensional particle-in-cell simulations.

Compression of laser pulses by near-forward Raman amplification in plasma

We propose a new scheme for compression of ultrashort laser pulses enabled by Near-Forward Raman Amplification (NFRA) in plasma. In contrast to traditional forward Raman amplification in plasma, the seed pulse in NFRA propagates faster than the pump through manipulation of the spatiotemporal properties of the pump beam. A “π-pulse” solution similar to that of stimulated Raman backscattering amplification is obtained, indicating that NFRA can be used for compression of ultrashort laser pulses. NFRA is numerically demonstrated with a 2D simulation using a front-tilt near-forward pump pulse.

A High-Order Perturbation Method for an Investigation into Relativistic Stimulated Raman Back Scattering of a High Power Laser in Plasma

The relativistic stimulated Raman back scattering (SRBS) of a high power linearly polarized laser pulse in the plasma is investigated. The nonlinear coupled equations of Raman instabilities in the presence of combined effects of relativistic and ponderomotive nonlinearities are derived. Using high-order perturbation method in relativistic fluid-Maxwell model, a set of coupled non-linear equations is obtained which describes the evolutions of two scattered electromagnetic waves and a modified plasma wave. The temporal evolution of SRBS process is then studied using numerical solutions of the set of coupled equations. The results showed that by using the linear approximation, the relativistic mass correction cannot be employed accurately and it leads to over-estimation of the perturbed current density and Raman Scattering. Our analysis shows that, in the relativistic regime, it is crucial to employ higher order perturbation method to obtain reliable results.

Stimulated Raman backscattering amplification with a low-intensity pump

The efficiency transfer from the pump to the seed of stimulated Raman backscattering in plasma was optimized at a pump intensity below 1014 W/cm2. Two ways were employed to obtain high-quality but low-intensity pump beams. First, the pump focus was moved away from the plasma entrance to optimize guiding the uniform part of the beam in the plasma channel. The seed was amplified from 50 μJ to 1.1 mJ after 2-mm interaction, with an effective Raman transfer efficiency over 4.7%. Second, an aperture was set to make the pump focus pass through the plasma channel. An output seed energy of 1.02 mJ was obtained when the pump energy decreased to 20 mJ, showing a transfer efficiency of 5.1%. The experimental results indicate that the transfer efficiency may be mainly suppressed by plasma heating but not spontaneous Raman scattering.

ПЛАЗМА ИЗ КЛИНОВИДНОЙ СТРУИ ГАЗА ДЛЯ РАМАНОВСКОЙ КОМПРЕССИИ ЛАЗЕРНЫХ ИМПУЛЬСОВ

Two processes of shortening an intense laser pulse are discussed in a transparent plasma: self-compression at wake wave excitation (Balakin et. al, 2013) and at stimulated Raman backscattering (Malkin et. al, 1999). Studying the possibility of amplification and compression of ultrashort (up to several field periods) laser pulses in a plasma based on the process of stimulated Raman backscattering is an important task aimed at creating next-generation superpower laser systems that generate ultrashort petawatt and exawatt laser pulses. However, there is a number of negative physical processes that may limit the effectiveness of Raman amplification. Most of these negative processes have been studied and ways are suggested to neutralize them. Among the most dangerous is the nonlinear frequency shift near the threshold for the overturning of the plasma wave (Balakin et. al, 2018). The use of a highly inhomogeneous jet plasma gives a significant density gradient along the jet. Accordingly, it is possible to compensate an excessively large frequency modulation of the pump due to the use of density inhomogeneity along the gas jet. In this case, Raman compression occurs without a significant loss of energy efficiency. Using a nozzle for a gas jet in the form of a long slit allows one create a long and uniform plasma in one of the directions having a wedge shape. The possibility of obtaining a high-energy output signal using wide-aperture laser pulses in a wedge-shaped plasma is predicted. Optimal parameters of the gas jet and laser pulses are proposed to ensure high efficiency and focusability, close to the ideal case. This research was supported by the Russian Science Foundation (Project 17-72-20111).

Nonlinear transition from convective to absolute Raman instability with trapped electrons and inflationary growth of reflectivity

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.