Langmuir Decay Instability Research Articles

Development of a full aperture backscatter system for the Orion laser.

A full aperture backscatter system (FABS) is currently in development on the Orion laser at AWE to measure scattered light from the stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) processes. The light is to be collected through the full aperture of the final optic assembly and traverse back down the beam path, with fractions of this light being directed to an optics table. By measuring the energy of this backscattered light, it is possible to gain insight into some of the laser-plasma instabilities that are present on the laser system and should give an indication of some of the scattered light losses due to the SBS and SRS processes. The uncontrolled scattered light can lead to hotter electrons, which then preheat the target causing a degradation in compression and can inhibit ignition in inertial confinement fusion experiments, as well as secondary instabilities whereby the scattered light may in itself cause further LPIs, such as two-ion decay or the Langmuir decay instability. The FABS diagnostic on Orion is planned to enhance the plasma diagnostics suite available and provide quantitative guidance on increasing the energy coupling. Current progress includes the characterization of filters and, hence, a broadband xenon lamp to be used in measuring the transmission efficiency of the optics chain, desktop alignment of the backscatter optics, and characterization of the streak cameras.

Anomalous hot electron generation via stimulated Raman scattering in plasma with up-ramp density profiles

We investigate the evolution and propagation of the electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) in the inhomogeneous plasma theoretically and numerically with particle-in-cell (PIC) simulations. A theoretical model of EPWs in inhomogeneous plasmas is presented, which shows that the evolution of the EPW wavenumber is mainly related to the plasma density profile rather than the plasma electron temperature, in agreement with PIC simulations. When the density gradient is positive along the propagation direction of an EPW, its wavenumber decreases with time and consequently its phase velocity increases continuously, causing the trapped electrons to be accelerated to anomalous high energy. Furthermore, it is found that the Langmuir decay instability tends to reduce the levels of SRS saturation and electron acceleration and produce hot electrons in the opposite direction. This work provides a new understanding of electron heating due to SRS excitation.

Nonlinear models for coupling the effects of stimulated Raman scattering to inertial confinement fusion codes

Laser plasma instabilities (LPI) reduce driver-target coupling, alter implosion symmetry, and therefore can fundamentally limit fusion performance in inertial confinement fusion (ICF). Developing a predictive modeling capability for LPI effects can critically advance the success of the field. We perform vector particle-in-cell simulations of multi-speckled laser beams undergoing stimulated Raman scattering (SRS) at various densities and intensities relevant to mainly indirectly driven and a subset of parameter space for directly driven ICF systems, focusing on the regimes with intensities above the onset of electron trapping. Based on the wavenumber of the SRS daughter electron plasma wave, we identify several regions with underpinning SRS saturation physics: the electron-trapping dominated region with intermediate kλD values, the strong Landau damping region at larger kλD values, and the region where the Langmuir decay instability arises at lower kλD values. We develop a nonlinear SRS reflectivity model that features the base trapping-dominated scaling of (kλD)−4 and its modifications. Electron trapping deforms the initialized electron distribution functions, and we have developed a new δf-Gaussian-mixture algorithm for an accurate characterization of the trapped hot electron population. With this SRS hot electron description, we construct a nonlinear energy deposition model and a hot electron source model—based on a modified Manley–Rowe relation—suitable for including SRS effects as a sub-grid module in a high-fidelity ICF design code.

Investigation of the Langdon effect on the nonlinear evolution of SRS from the early-stage inflation to the late-stage development of secondary instabilities

In a laser-irradiated plasma, the Langdon effect can result in a super-Gaussian electron energy distribution function (EEDF), imposing significant influences on stimulated backward Raman scattering (SRS). In this work, the influence of a super-Gaussian EEDF on the nonlinear evolution of SRS is investigated by the three wave coupling model simulation and Vlasov–Maxwell simulation for plasma parameters covering a wide range of kλ De from 0.19 to 0.48 at both high and low intensity laser drives. In the early stage of SRS evolution, it is found that besides the kinetic effects due to electron trapping (2018 Phys. Plasmas 25 100702), the Langdon effect can also significantly widen the parameter range for the absolute growth of SRS, and the time for the absolute SRS to reach saturation is greatly shortened by the Langdon effect within certain parameter regions. In the late stage of SRS, when secondary instabilities such as decay of the electron plasma wave to beam acoustic modes, rescattering, and Langmuir decay instability become important, the Langdon effect can influence the reflectivity of SRS by affecting secondary instabilities. The comprehension of the Langdon effect on nonlinear evolution and saturation of SRS would contribute to a better understanding and prediction of SRS in inertial confinement fusion.

Suprathermal electrons from the anti-Stokes Langmuir decay instability cascade.

The study of parametric instabilities has played a crucial role in understanding energy transfer to plasma and, with that, the development of key applications such as inertial confinement fusion. When the densities are between 0.11n_{c}≲n_{e}≲0.14n_{c} and the electron temperature is in inertial confinement fusion-relevant temperatures, anomalous hot electrons with kinetic energies above 100keV are generated. Here a new electron acceleration mechanism-the anti-Stokes Langmuir decay instability cascade of forward stimulated Raman scattering-is investigated. This mechanism potentially explains anomalous energetic electron generation in indirectly driven inertial confinement fusion experiments, it also provides a new way of accelerating electrons to higher energy for applications such as novel x-ray sources.

Nonlinear development of absolute Raman backscatter in ignition-scale direct-drive coronal conditions

Motivated by a resurgence of interest in stimulated Raman scattering (SRS) in direct-drive inertial confinement fusion, we use Vlasov–Maxwell simulations to investigate the nonlinear saturation and associated hot electron production for absolute SRS backscatter near the quarter-critical density. The simulated plasma conditions are relevant to recent planar target experiments on the National Ignition Facility [Rosenberg et al., Phys. Rev. Lett. 120, 055001 (2018)]. It is discovered that wave–wave interactions dominate over kinetic effects in the nonlinear development of instability with clear signatures of the Langmuir decay instability (LDI) cascade and modulational instability of the primary electron plasma wave. Spectral broadening of the scattered light is shown to occur by two mechanisms: via a long wavelength modulation of the electron density—caused by the ponderomotive action of the near-turning point Airy-like structure of the SRS light—and by seeding of SRS by the LDI cascade. Hot electrons are produced at a level that is ≲1% of the incident laser energy by acceleration in the turbulent spectrum associated with the LDI cascade. Despite the high electron plasma temperature, collisions are shown to be important and have the effect of moderating the LDI cascade. The effect of collisions is shown to have a more dramatic effect on hot electron production than on the SRS reflectivity.

Numerical study of Langmuir wave coalescence in laser-plasma interaction

Type-III-burst radio signals can be mimicked in the laboratory via laser-plasma interaction. Instead of an electron beam generating Langmuir waves (LWs) in the interplanetary medium, the LWs are created by a laser interacting with a millimeter-sized plasma through the stimulated Raman instability. In both cases, the LWs feed the Langmuir decay instability which scatters them in several directions. The resulting LWs may couple to form electromagnetic emission at twice the plasma frequency, which has been detected in the interplanetary medium, and recently in a laboratory laser experiment [Marquès et al., Phys. Rev. Lett. 124, 135001 (2020)]. This article presents the first numerical analysis of this laser configuration using particle-in-cell simulations, providing details on the wave spectra that are too difficult to measure in experiments. The role of some parameters is addressed, with a focus on laser intensity, in order to illustrate the behavior of the electromagnetic emission's angular distribution and polarization.

Stimulated Brillouin scattering of backward stimulated Raman scattering

The rescattering of backward stimulated Raman scattering (BSRS) by stimulated Brillouin scattering (SBS) is found in the high electron density region by relativistic Vlasov-Maxwell simulation and particle-in-cell (PIC) simulation, where the BSRS is in the regime of absolute instability and dominates in all the scatterings. Both one dimension (1D) Vlasov simulation and two dimension (2D) PIC simulation have been given to verify that there exists SBS of BSRS in the regime of absolute instability for BSRS. The SBS of BSRS will be even stronger than forward stimulated Raman scattering (FSRS) and SBS in regime of absolute instability for BSRS. Thus, besides Langmuir decay instability and laser energy absorption, the SBS of BSRS is also an important saturation mechanism of BSRS in high electron density region.

Saturation of Raman scattering in nonlinear regime

Saturation of Raman scattering of laser pulses in plasma, which is expected for hohlraum targets in ICF, is investigated. Trapping and Langmuir decay instability are two major candidates for saturation of Raman scattering in nonlinear regime. Coupled equations of plasma and scattered waves are derived and solved analytically in nonlinear regime where the effect of Landau damping and nonlinear frequency shift is considered. The solution leads to derivation of scattered waves growth and their reflectivity. Plots of reflectivity of scattered waves under various laser beam and plasma conditions are presented. The effects of plasma temperature and its density, as well as, laser frequency, its intensity and pulse duration on reflectivity are discussed.

Anti-Langmuir decay instability in Langmuir decay instability cascade

Backward stimulated Raman scattering (BSRS) with Langmuir decay instability (LDI) and anti-Langmuir decay instability (ALDI or anti-LDI) has been researched by Vlasov simulation. The decay productions of anti-LDI in LDI cascade and their evolution with time are demonstrated for the first time. The BSRS reflectivity will be decreased largely through LDI cascade and ALDI in the small wave-number region. Different mechanisms to saturate BSRS in CH (or H) and C plasmas have been demonstrated. The dominant saturation mechanism of BSRS in CH (or H) plasmas is LDI cascade and ALDI. However, in C plasmas, due to very weak Landau damping of ion acoustic waves, LDI cascade will promote stimulated Brillouin scattering (SBS) excitation, then SBS will compete with BSRS and saturate BSRS in the later stage. The proportion of the hot electrons is decreased largely through LDI cascade and ALDI. These results give an effective mechanism to suppress BSRS and hot electron generation in the small wave-number region, which are of important significance in the inertial confinement fusion.

Parametric instabilities in strongly correlated plasma

Parametric instabilities (Stimulated Brillouin Scattering, parametric decay instability, and Langmuir decay instability) in laser plasmas with strongly correlated/coupled ions are examined. It is shown that strong correlation/coupling effects lead to a slowing down of the ion acoustic wave which, in turn, leads to a significant destabilization of parametric instabilities and concomitant enhancement of the scattered light. Applications of these results to inertial confinement fusion experiments are discussed.

Competition between stimulated Raman scattering and two-plasmon decay in inhomogeneous plasma

We demonstrate competitions between stimulated Raman scattering (SRS) and two-plasmon decay (TPD) in the laser polarization plane in inhomogeneous near quarter-critical density plasma by using linear convective gain analysis and two-dimensional (2D) particle-in-cell (PIC) simulations. Linear theoretical analysis implies that convective SRS occurs in a wider and lower density region than absolute SRS and has a shared occurrence region with convective TPD. This convective SRS prefers a parameter space with the laser intensity larger than the order of 1015 W/cm2 and the density scale length about several hundreds microns, which may be common in large scale direct-drive scheme, shock ignition scheme, and hybrid-drive scheme. A convective nature and saturation mechanism under these parameter regions are identified to be Langmuir decay instability and strong pump depletion. The significance of this convective SRS is shown in our 2D PIC simulations that hot electrons are reduced through suppressing the electron staged acceleration by TPD in the lower density region due to its high phase velocity. Temperature induced competitions are also studied using a relativistic modification to the Langmuir wave dispersion relation when Te>5 keV. Both absolute and convective SRS are observed to be dominant in the simulations when the temperature is as high as 10 keV.

Raman–Brillouin interplay for inertial confinement fusion relevant laser–plasma interaction

The co-existence of the Raman and Brillouin backscattering instability is an important issue for inertial confinement fusion. The present paper presents extensive one-dimensional (1D) particle-in-cell (PIC) simulations for a wide range of parameters extending and complementing previous findings. PIC simulations show that the scenario of reflectivity evolution and saturation is very sensitive to the temperatures, intensities, size of plasma and boundary conditions employed. The Langmuir decay instability is observed for rather small $k_{epw}{\it\lambda}_{D}$ but has no influence on the saturation of Brillouin backscattering, although there is a clear correlation of Langmuir decay instability modes and ion-fractional decay for certain parameter ranges. Raman backscattering appears at any intensity and temperature but is only a transient phenomenon. In several configurations forward as well as backward Raman scattering is observed. For the intensities considered, $I{\it\lambda}_{o}^{2}$ above $10^{15}~\text{W}~{\rm\mu}\text{m}^{2}/\text{cm}^{2}$ , Raman is always of bursty nature. A particular setup allows the simulation of multi-speckle aspects in which case it is found that Raman is self-limiting due to strong modifications of the distribution function. Kinetic effects are of prime importance for Raman backscattering at high temperatures. No unique scenario for the saturation of Raman scattering or Raman–Brillouin competition does exist. The main effect in the considered parameter range is pump depletion because of large Brillouin backscattering. However, in the low $k_{epw}{\it\lambda}_{D}$ regime the presence of ion-acoustic waves due to the Langmuir decay instability from the Raman created electron plasma waves can seed the ion-fractional decay and affect the Brillouin saturation.

Direct observation of the two-plasmon-decay common plasma wave using ultraviolet Thomson scattering.

A 263-nm Thomson-scattering beam was used to directly probe two-plasmon-decay (TPD) excited electron plasma waves (EPWs) driven by between two and five 351-nm beams on the OMEGA Laser System. The amplitude of these waves was nearly independent of the number of drive beams at constant overlapped intensity, showing that the observed EPWs are common to the multiple beams. In an experimental configuration where the Thomson-scattering diagnostic was not wave matched to the common TPD EPWs, a broad spectrum of TPD-driven EPWs was observed, indicative of nonlinear effects associated with TPD saturation. Electron plasma waves corresponding to Langmuir decay of TPD EPWs were observed in both Thomson-scattering spectra, suggesting the Langmuir decay instability as a TPD saturation mechanism. Simulated Thomson-scattering spectra from three-dimensional numerical solutions of the extended Zakharov equations of TPD are in excellent agreement with the experimental spectra and verify the presence of the Langmuir decay instability.

Temperature dependence of parametric instabilities in the context of the shock-ignition approach to inertial confinement fusion

The role of the coronal electron plasma temperature for shock-ignition conditions is analysed with respect to the dominant parametric processes: stimulated Brillouin scattering, stimulated Raman scattering, two-plasmon decay (TPD), Langmuir decay instability (LDI) and cavitation. TPD instability and cavitation are sensitive to the electron temperature. At the same time the reflectivity and high-energy electron production are strongly affected. For low plasma temperatures the LDI plays a dominant role in the TPD saturation. An understanding of laser–plasma interaction in the context of shock ignition is an important issue due to the localization of energy deposition by collective effects and hot electron production. This in turn can have consequences for the compression phase and the resulting gain factor of the implosion phase.

Stimulated scattering in laser driven fusion and high energy density physics experiments

In laser driven fusion and high energy density physics experiments, one often encounters a kλD range of 0.15 < kλD < 0.5, where stimulated Raman scattering (SRS) is active (k is the initial electron plasma wave number and λD is the Debye length). Using particle-in-cell simulations, the SRS reflectivity is found to scale as ∼ (kλD)−4 for kλD ≳ 0.3 where electron trapping effects dominate SRS saturation; the reflectivity scaling deviates from the above for kλD < 0.3 when Langmuir decay instability (LDI) is present. The SRS risk is shown to be highest for kλD between 0.2 and 0.3. SRS re-scattering processes are found to be unimportant under conditions relevant to ignition experiments at the National Ignition Facility (NIF). Large-scale simulations of the hohlraum plasma show that the SRS wavelength spectrum peaks below 600 nm, consistent with most measured NIF spectra, and that nonlinear trapping in the presence of plasma gradients determines the SRS spectral peak. Collisional effects on SRS, stimulated Brillouin scattering (SBS), LDI, and re-scatter, together with three dimensional effects, are examined. Effects of collisions are found to include de-trapping as well as cross-speckle electron temperature variation from collisional heating, the latter of which reduces gain, introduces a positive frequency shift that counters the trapping-induced negative frequency shift, and affects SRS and SBS saturation. Bowing and breakup of ion-acoustic wavefronts saturate SBS and cause a dramatic, sharp decrease in SBS reflectivity. Mitigation of SRS and SBS in the strongly nonlinear trapping regime is discussed.

Possible origins of a time-resolved frequency shift in Raman plasma amplifiers

Raman amplification is a resonant process in which the energy of a long pump pulse is transferred to a short seed pulse by a plasma wave. There has been a significant effort to identify a window in parameter space within which the interaction is expected to be highly efficient and not degraded by competing instabilities or excessive damping. However, experimental results have thus far failed to approach the theoretical limits. Recent amplified signal spectra display a characteristic blue shift, which evolves within the seed pulse duration and suggests that the mechanism responsible for this shift is also limiting amplification in these experiments. We present the evidence and explore different hypotheses for the origins of the shift—namely localization in density minima along the axis of laser propagation induced by an ion acoustic wave that could arise from the Langmuir decay instability, filamentation which could also modulate the plasma density but in the plane transverse to laser propagation, particle trapping, and additional ionization induced by the amplified seed field.

The reduced-description particle-in-cell model for the two plasmon decay instability

Recently, motivated by the resurgent interest in suprathermal electron generation by two plasmon decay (TPD) in direct-drive laser-fusion [J. A. Delettrez et al., Bull. Am. Phys. Soc. 53, 248 (2008); V. A. Smalyuk et al., Phys. Rev. Lett. 100, 185005 (2008); B. Yaakobi et al., Phys. Plasmas 12, 062703 (2005)], the fully kinetic, reduced-description, particle-in-cell (RPIC) methodology has been extended to include TPD. It provides a computationally efficient fully kinetic simulation tool, especially in nonlinear regimes where the Langmuir decay instability (LDI) is a dominant saturation mechanism. This RPIC methodology is an extension of the modeling of laser-plasma instabilities in underdense plasmas reported previously [H. X. Vu, B. Bezzerides, and D. F. DuBois, J. Comput. Phys. 156, 12 (1999)]. The relationship between RPIC and the extended Zakharov model previously used for TPD [D. F. DuBois, D. A. Russell, and H. A. Rose, Phys. Rev. Lett. 74, 3983 (1995)] is explored theoretically and tested in simulations. The modification of the electron velocity distribution—in particular, the generation of hot electrons—as calculated in RPIC leads to weakening of the wave turbulence excited by TPD compared to the Zakharov model predictions but the locations in wave vector space of important spectral features, e.g., arising from the LDI, of the nonlinear wave fluctuations are exactly the same in the two approaches. New results involving two oblique, overlapping laser beams, a common geometrical feature in direct-drive schemes, are presented. The two laser beams can cooperatively excite common primary Langmuir waves which initiate the LDI process.

Kinetic dispersion of Langmuir waves. I. The Langmuir decay instability

We derive a fully kinetic, three-dimensional dispersion relation for Langmuir waves with a focus on the Langmuir decay instability (LDI). The kinetic dispersion is compared to the standard fluid dispersion found with an equation of state (EOS) closure. The EOS closure fails to capture the intricacies of the nonlinear pressure when high frequency electron plasma waves (EPWs) couple to low frequency ion acoustic waves (IAWs). In particular, we find discrepancies in the kλd scaling of the LDI growth rate, where k is the wavenumber of the incident EPW and λd is the Debye length. As a result, the kinetic dispersion relation for LDI results in instability thresholds that can be in excess of twice those predicted by the fluid theory. Both the fluid and kinetic dispersion relations predict a nonlinear frequency shift due to the beating of the pump and scattered EPWs, but again the kλd scaling of these frequency shifts differ. In addition, the kinetic dispersion predicts a nonlinear reduction in the IAW damping from the three-wave interaction.

Saturation of stimulated Raman scattering in laser-plasma interaction

The saturation of stimulated Raman scattering （SRS） through Langmuir decay instability （LDI） and electron trapping in laser-plasma interaction process is studied. Analytical expressions for the saturation time of SRS are obtained for both the LDI and electron trapping schemes in 1-D homogeneous plasma irradiated by unrelativistic laser beams. The simple analytical model agrees well with numerical simulation and experiment in respect of the dependence of the saturation time of SRS on the parameters of lasers and plasmas.