Stimulated Raman Backscattering Research Articles

Stimulated Raman backscattering from a laser wakefield accelerator

Experiments were performed using the HERCULES laser system at the University of Michigan to study backward stimulated Raman scattering (BSRS) from a laser wakefield accelerator driven with a 30 fs pulse. 'The spectrum of backscattered light was found to be significantly broadened and red-shifted in cases where electrons were accelerated. BSRS broadening (red-shifting) was found to increase with respect to both plasma density and accelerated electron charge for laser powers exceeding 100 TW. Two-dimensional Particle-in-Cell simulations reveal temporal dynamics for the BSRS emission, which ceases as the wakefield bubble is evacuated of plasma electrons because of relativistic self-focusing. The intensity and duration of the BSRS signal was found to vary with plasma density and laser intensity. Both experimental and simulation results indicate that backward SRS is associated with plasma electron density within the wakefield bubble. This measurement can serve as a diagnostic of bubble dynamics, and is correlated with trapped electron charge in this regime.

Numerical study of bandwidth effect on stimulated Raman backscattering in nonlinear regime

Nonlinear behaviors of stimulated Raman scattering driven by finite bandwidth pumps are studied by one dimensional particle-in-cell simulations. The broad spectral feature of plasma waves and backscattered light reveals the different coupling and growth mechanisms, which lead to the suppression effect before the deep nonlinear stage. It causes nonperiodic plasma wave packets and reduces packet and etching velocities. Based on the negative frequency shift and electron energy distribution, the long-time evolution of instability can be divided into two stages by the relaxation time. It is a critical time after which the alleviation effects of nonlinear frequency shift and hot electrons are replaced by enhancement. Thus, the broadband pump suppresses instability at early time. However, it aggravates in the deep nonlinear stage by lifting the saturation level due to the coupling of the incident pump with each frequency shifted plasma wave. Our simulation results show that the nonlinear effects are valid in a bandwidth range from 2.25% to 3.0%, and the physics are similar within a nearby parameter space.

Simulation of double-pass stimulated Raman backscattering

Experiments on Stimulated Raman Backscattering (SRBS) in plasma have demonstrated significantly higher energy conversion in a double-pass amplifier where the laser pulses go through the plasma twice compared with a single-pass amplifier with double the plasma length of a single pass. In this paper, the improvement in understanding recent experimental results is presented by considering quite in detail the effects of plasma heating on the modeling of SRBS. Our simulation results show that the low efficiency of single-pass amplifiers can be attributed to Landau damping and the frequency shift of Langmuir waves. In double-pass amplifiers, these issues can be avoided, to some degree, because pump-induced heating could be reduced, while the plasma cools down between the passes. Therefore, double-pass amplifiers yield considerably enhanced energy transfer from the pump to the seed, hence the output pulse intensity.

Prospects for femtosecond ultrahigh intensity laser system towards Exawatt level

The new generation system for ultrahigh-power laser running over Exawatt (EW, 10 18 W) level is emerging recently, which is under a mechanism called Raman backscattering (RBS) in plasma. The main advantage of using plasma is that it can tolerate much higher laser intensities, 10 17 W cm −2 , more than five order of solid-state devices limited, 10 12 W cm −2 . Although Petawatt (PW, 10 15 W) laser pulses have been realized by some groups based on the chirped pulse amplification scheme (CPA), many cutting-edge scientific researches and technical applications, such as inertial confinement fusion (ICF), plasma physics, astrophysics, plasma-based particle accelerators, and X-ray lasers, need even higher laser power than EW level. For these purposes, huge laser projects like the extreme light infrastructure (ELI) have been proposed to offer new paradigm in EW class laser power. However, such an ultrahigh intensity laser system could only be achieved by CPA using very large (beyond 1 m 2 ) and expensive compressor gratings. In addition, to extrapolate CPA to the EW power range, hundreds of such gratings would be required. Even if there is enough budget, the problem of energy restriction on the last grating is still under question. Therefore, it is crucial to find a new medium or new technology for generating femtosecond ultra-intense laser pulses. This paper introduces a solution for generating laser intensities many orders of magnitude higher than currently results. This technology of optical amplification that a process known as Raman backscattering amplification and compression could enable the generation of femtosecond pulses of 20000 times the original seed intensity in a column of plasma just a few millimeters in length and less than a millimeter in width, without stretcher and compressor. Such sufficient high intensity and lower frequency of the pulse amplification indeed have got into super-radiant amplification regime (SRA) or stimulated Raman backscattering (SRBS). It has revealed that an unprecedented large pulse intensity amplification could be realized. The seed pulse will be compressed to 25 fs and its unfocused intensity increased from 1×10 15 to 4×10 17 W cm −2 . Furthermore, it could be increased further to around 1×10 23 W cm −2 by focusing the resulting beam to 1 μm. Therefore, it might be possible to increase this power up to EW in a larger plasma and using more powerful optical pumping.

On the stimulated Raman sidescattering in inhomogeneous plasmas: revisit of linear theory and three-dimensional particle-in-cell simulations

Stimulated Raman sidescattering (SRSS) in inhomogeneous plasma is comprehensively revisited on both theoretical and numerical aspects due to the increasing concern of its detriments to inertial confinement fusion. Firstly, two linear mechanisms of finite beam width and collisional effects that could suppress SRSS are investigated theoretically. Thresholds for the eigenmode and wave packet in a finite-width beam are derived as a supplement to the theory proposed by Mostrom and Kaufman (1979 Phys. Rev. Lett. 42 644). Collisional absorption of SRSS is efficient at high-density plasma and high-Z material, otherwise, it allows emission of sidescattering. Secondly, we have performed the first three-dimensional particle-in-cell simulations in the context of SRSS to investigate its linear and nonlinear effects. Simulation results are qualitatively agreed with the linear theory. SRSS with the maximum growth gain is excited at various densities, grows to an amplitude that is comparable with the pump laser, and evolutes to lower densities with a large angle of emergence. Competitions between SRSS and other parametric instabilities such as stimulated Raman backscattering, two-plasmon decay, and stimulated Brillouin scattering are discussed. These interaction processes are determined by gains, occurrence sites, scattering geometries of each instability, and will affect subsequent evolutions. Nonlinear effects of self-focusing and azimuthal magnetic field generation are observed to be accompanied with SRSS. In addition, it is found that SRSS is insensitive to ion motion, collision (low-Z material), and electron temperature.

Transition from convective to absolute Raman instability via the longitudinal relativistic effect by using Vlasov-Maxwell simulations

The longitudinal relativistic effect on stimulated Raman backscattering (SRBS) is investigated by using one-dimensional (1D) Vlasov-Maxwell simulations. Using a short backscattered light seed pulse with a very small amplitude, the linear gain spectra of SRBS in the strongly convective regime is presented by combining the relativistic and non-relativistic 1D Vlasov-Maxwell simulations, which is in agreement with the steady-state linear theory. More interestingly, by considering transition from convective to absolute instability due to electron trapping, we successfully predict the critical duration of the seed which can just trigger the kinetic inflation of the excited SRBS after the seed leaves the simulation box. The critical duration in the relativistic case is much shorter than that in the nonrelativistic case, which indicates that the kinetic inflation more easily occurs in the relativistic case than in the nonrelativistic case. In the weakly convective regime, the transition from convective to absolute instability for SRBS can directly occur in the linear regime due to the longitudinal relativistic modification. For the same pump, our simulations first demonstrate that the SRBS excited by a short and small seed pulse is a convective instability in the nonrelativistic case but becomes an absolute instability due to the decrease of the linear Landau damping from the longitudinal relativistic modification in the relativistic case. In more detail, the growth rate of the backscattered light is also in excellent agreement with theoretical prediction.

Pulse splitting of stimulated Raman backscattering with a chirped pump

Raman amplified seed splitting was demonstrated in the backscattering scheme in a plasma, when a significant chirp was introduced into the pump. Particle-in-cell simulations have shown that a single seed laser pulse gradually splits into two, self-compressed pulses in the nonlinear amplification regime. This is in difference to previous studies of the pump chirp, which mainly have focused on its compensation for the plasma density gradient, in order to maximize the resonant amplification. The splitting, as revealed by the spectral evolution of the amplified seed, is attributed to resonance slipping, which is the result of the spatio-temporal distribution of the gain of the chirped pump.

Mitigation of stimulated Raman backscattering by elliptical laser beam in collisionless plasma

This paper presents theoretical and numerical study of the propagation of an elliptical laser beam in collisionless plasma with relativistic-ponderomotive nonlinearities and its effect on the excitation of electron plasma wave (EPW) and resulting stimulated Raman backscattering (SRBS) process. Nonlinear interaction between an intense elliptical laser beams with the electron plasma wave leads stimulated Raman backscattering. Following Wentzel Kramers Brillouin (WKB) and paraxial-ray approximations, non-linear differential equations for the beam width parameters of elliptical laser beam, electron plasma wave and back-scattered beam are set up and solved numerically. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on the focusing of elliptical laser beam in plasma and further its effect on the excitation of electron plasma wave and backreflectivity of stimulated Raman scattering (SRS). The focusing of an elliptical laser beam, EPW and scattered wave are reduced at higher values of laser intensity in plasma. The amplitude of excited electron plasma wave, which depends on the focusing of main laser beam and electron plasma wave reduce for higher values of incident laser intensity. The results indicate that the focusing of waves mitigates the backreflectivity of SRS. The results are also compared with only relativistic nonlinearity and the Gaussian profile of laser beam. It is observed that an elliptical laser beam mitigates the backreflectivity of SRS at higher values of laser intensity. This study is relevant to inertial confinement fusion scheme, where SRS plays very important role.

The relationship between gas fill density and hohlraum drive performance at the National Ignition Facility

Indirect drive inertial confinement fusion experiments were conducted at the National Ignition Facility to investigate the performance of the hohlraum drive as a function of hohlraum gas fill density by imploding high-density-carbon capsules using a 2-shock laser pulse. Measurements characterized the backscatter behavior, the production of hot electrons, the motion and brightness of the laser spots on the hohlraum wall, and the efficiency of the hohlraum x-ray drive as a function of gas fill density ρgf between 0.03 mg/cc (“near vacuum”) and 1.6 mg/cc. For hohlraums with ρgf up to 0.85 mg/cc, very little stimulated Raman backscatter (SRS) was observed. For higher ρgf, significant SRS was produced and was observed to occur during the rise to peak laser power and throughout the main pulse. The efficiency with which laser energy absorbed by the hohlraum is converted into drive energy was measured to be the same for ρgf ≥ 0.6 mg/cc once the laser reached peak power. However, for the near vacuum case, the absorbed energy was converted to drive energy more efficiently throughout the pulse and maintained an efficiency ∼10% higher than the gas filled hohlraums throughout the main pulse.

High Orbital Angular Momentum Harmonic Generation.

We identify and explore a high orbital angular momentum (OAM) harmonics generation and amplification mechanism that manipulates the OAM independently of any other laser property, by preserving the initial laser wavelength, through stimulated Raman backscattering in a plasma. The high OAM harmonics spectra can extend at least up to the limiting value imposed by the paraxial approximation. We show with theory and particle-in-cell simulations that the orders of the OAM harmonics can be tuned according to a selection rule that depends on the initial OAM of the interacting waves. We illustrate the high OAM harmonics generation in a plasma using several examples including the generation of prime OAM harmonics. The process can also be realized in any nonlinear optical Kerr media supporting three-wave interactions.

Experimental study on the incident-angle-dependent laser coupling features of polystyrene targets

Laser-produced plasmas have attracted great interest due to their potential utility in wide-ranging applications, especially in the field of inertial confinement fusion (ICF). For direct-driven ICF, laser coupling with polystyrene targets is a crucial and fundamental problem. In addition, oblique incidence is also a common phenomenon for laser facilities with multiple beams. It is necessary to evaluate the effects of oblique incidence on the laser coupling features relevant to the direct-driven ICF. Experiments using an intense nanosecond flat-top laser at around W cm–2 to irradiate polystyrene planar targets from three different incidence angles have been performed on the Shenguang-III prototype laser facility. The time-integrated absolute values of the full aperture backscatter (FABS), near backscatter scattering (NBS), and the x-ray conversion efficiency (CE) have been measured quantitatively. According to the experimental results, with the increase of the incidence angle, the percentage of the stimulated Brillouin backscatter and the overall x-ray CE decreased while the stimulated Raman backscatter fraction rose. Theoretical analyses based on hydrodynamic simulations and linear theory were qualitatively consistent with the experimental results. In addition, the specularly reflected light was also observed at 30° laser oblique incidence.

Vlasov–Maxwell simulations of backward Raman amplification of seed pulses in plasmas

AbstractWe study the problem of the amplification of an ultra-short seed pulse via stimulated Raman backscattering (SRB) from a long pump pulse (assumed to have an envelope with a constant amplitude), in an underdense plasma. The SRB interaction couples the pump light wave to a daughter light seed wave propagating in the opposite direction, scattered off an electron plasma wave. In recent numerical simulations, it has been observed that besides stimulated Raman backward scattering (SRBS) and stimulated Raman forward scattering, other high-frequency kinetic instabilities can occur when modified distribution functions exist during the evolution of the system. In particular, we showed the prominent role played by kinetic electrostatic electron nonlinear (KEEN) waves (Afeyan et al., 2004). We continue this work by applying a relativistic Vlasov–Maxwell code to study stimulated KEEN wave scattering (SKEENS) and its role in the SRBS short pulse amplification processes. An analysis of the full spectrum of waves participating in the amplification processes is presented. The absence of spurious noise in grid-based Vlasov codes allows us to follow the evolution of the system with a kinetic (collisionless) description. This affords us a glimpse at the intricate phase-space structures such as trapped particle orbits, which coexist and interact nonlinearly in the electron distribution function.

Broadband stimulated Raman backscattering

Broadband amplification employing stimulated Raman backscattering is demonstrated. Using seed pulses with a bandwidth of about 200 nm, we study the amplification in a wide spectral range in a single laser shot. With chirped pump pulses and a Ne gas jet, we observed under optimized conditions, amplification in a range of about 80 nm, which is sufficient to support the amplification of sub-20 fs pulses. This broad amplification range is also in excellent agreement with PIC simulations. The conversion efficiency is at certain wavelengths as high as 1.2% and was measured to be better than 6 × 10−3 on average.

Control and ultra-short pulse generation in stimulated Raman backscattering

A stimulated Raman scattering (SRS) is a great concern for laser fusion, causing an energy and symmetry loss and target preheating. In particular, for dominating backward- SRS (BRS), a complexity of nonlinear saturation was revealed. Recently, to develop lasers at multi-exawatts and beyond, relevant to high-energy physics, a proposed Raman amplification based on BRS met a restrictive operation window. Here, we stress BRS generics that due to a nonlinear frequency shift (relativistic/trapping effect), it nonlinearly saturates with intermittent pulsations. A “break up” of Manley-Rowe invariants explains a non-steady saturation. Further, a coherent pulsation BRS regime is proposed for femto-sec pulse generation in a thin exploding foil plasma, with parameter scalings investigated by analytics and particle simulations.

Amplification and generation of ultra-intense twisted laser pulses via stimulated Raman scattering.

Twisted Laguerre–Gaussian lasers, with orbital angular momentum and characterized by doughnut-shaped intensity profiles, provide a transformative set of tools and research directions in a growing range of fields and applications, from super-resolution microcopy and ultra-fast optical communications to quantum computing and astrophysics. The impact of twisted light is widening as recent numerical calculations provided solutions to long-standing challenges in plasma-based acceleration by allowing for high-gradient positron acceleration. The production of ultra-high-intensity twisted laser pulses could then also have a broad influence on relativistic laser–matter interactions. Here we show theoretically and with ab initio three-dimensional particle-in-cell simulations that stimulated Raman backscattering can generate and amplify twisted lasers to petawatt intensities in plasmas. This work may open new research directions in nonlinear optics and high–energy-density science, compact plasma-based accelerators and light sources.

The analysis of Raman scattering in a free-electron laser with a rectangular hybrid wiggler

A one dimensional theory of the stimulated Raman backscattering process in a free electron laser with rectangular hybrid wiggler (RHW) is analyzed. The dispersion relation in the rest frame of the electron beam and also a formula for the lab-frame spatial growth rate are derived. A numerical computation of the growth rate for RHW is conducted and a comparison with that for coaxial hybrid wiggler is made away from the resonance. The growth rate is found larger for the rectangular wiggler than for the coaxial wiggler. A much narrower magnetoresonance associated with the third spatial harmonic is also obtained compared to the first one.

Chirped pulse Raman amplification in warm plasma: towards controlling saturation.

Stimulated Raman backscattering in plasma is potentially an efficient method of amplifying laser pulses to reach exawatt powers because plasma is fully broken down and withstands extremely high electric fields. Plasma also has unique nonlinear optical properties that allow simultaneous compression of optical pulses to ultra-short durations. However, current measured efficiencies are limited to several percent. Here we investigate Raman amplification of short duration seed pulses with different chirp rates using a chirped pump pulse in a preformed plasma waveguide. We identify electron trapping and wavebreaking as the main saturation mechanisms, which lead to spectral broadening and gain saturation when the seed reaches several millijoules for durations of 10’s – 100’s fs for 250 ps, 800 nm chirped pump pulses. We show that this prevents access to the nonlinear regime and limits the efficiency, and interpret the experimental results using slowly-varying-amplitude, current-averaged particle-in-cell simulations. We also propose methods for achieving higher efficiencies.

The development of laser-plasma interaction program LAP3D on thousands of processors

Modeling laser-plasma interaction (LPI) processes in real-size experiments scale is recognized as a challenging task. For explorering the influence of various instabilities in LPI processes, a three-dimensional laser and plasma code (LAP3D) has been developed, which includes filamentation, stimulated Brillouin backscattering (SBS), stimulated Raman backscattering (SRS), non-local heat transport and plasmas flow computation modules. In this program, a second-order upwind scheme is applied to solve the plasma equations which are represented by an Euler fluid model. Operator splitting method is used for solving the equations of the light wave propagation, where the Fast Fourier translation (FFT) is applied to compute the diffraction operator and the coordinate translations is used to solve the acoustic wave equation. The coupled terms of the different physics processes are computed by the second-order interpolations algorithm. In order to simulate the LPI processes in massively parallel computers well, several parallel techniques are used, such as the coupled parallel algorithm of FFT and fluid numerical computation, the load balance algorithm, and the data transfer algorithm. Now the phenomena of filamentation, SBS and SRS have been studied in low-density plasma successfully with LAP3D. Scalability of the program is demonstrated with a parallel efficiency above 50% on about ten thousand of processors.

Generation of multi-millijoule red-shifted pulses for seeding stimulated Raman backscattering amplifiers.

The efficient generation of redshifted pulses from chirped femtosecond joule level Bessel beam pulses in gases is studied. The redshift spans from a few 100 cm⁻¹ to several 1000 cm⁻¹ corresponding to a shift of 50-500 nm for Nd:glass laser systems. The generated pulses have an almost perfect Gaussian beam profile insensitive of the pump beam profile, and are much shorter than the pump pulses. The highest measured energy is as high as 30 mJ, which is significantly higher than possible with solid state nonlinear frequency shifters.

Nonlinear nature of kinetic undamped waves induced by electrostatic turbulence in stimulated Raman backscattering

The influence of low-frequency waves of kinetic nature induced by electron trapping in backward Stimulated Raman Scattering (SRS) is investigated. Semi-lagrangian Vlasov-Maxwell simulations are carried out not only for periodic boundary conditions but also in the case of an open plasma with parabolic shape, in optical mixing. We provide a numerical example of generation of KEEN (kinetic electrostatic electron nonlinear) waves nonlinearly induced from the SRS through a mechanism we first here elucidate. In particular we identify a process of backward scattering of the SRS probe light from the so generated KEEN waves, which may provide a mechanism for the possible experimental observation and measurement of such nonlinear structures.