Session CO7: Laser-Plasma Instabilities

Laser--Plasma Interaction Experiments at Direct-Drive Ignition-Relevant Plasma Conditions at the National Ignition Facility

Laser--plasma interaction (LPI) instabilities, such as stimulated Raman scattering (SRS) and two-plasmon decay, can be detrimental for direct-drive inertial confinement fusion because of target preheat by the high-energy electrons they generate. The radiation--hydrodynamic code \textit{DRACO} was used to design planar-target experiments at the National Ignition Facility that generated plasma and interaction conditions relevant to ignition direct-drive designs $(I_{\mbox{L}} \sim 10^{15}\mbox{\thinspace W/cm}^{2},$ $T_{\mbox{e}} >3$ keV, density gradient scale lengths of $L_{\mbox{n}} \sim 600\;\mu \mbox{m).}$ Laser-energy conversion efficiency to hot electrons of $\sim $0.5{\%} to 2.5{\%} with temperature of $\sim $45 to 60 keV was inferred from the experiment when the laser intensity at the quarter-critical surface increased from $\sim $6 to $15\times 10^{14}\,\mbox{W/cm}^{2}.$ LPI was dominated by SRS, as indicated by the measured scattered-light spectra. Simulations of SRS using the LPI code \textit{LPSE }have been performed and compared with predictions of theoretical models. Implications for ignition-scale direct-drive experiments will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Measurements and modeling of Raman side-scatter in ICF experiments.

Raman side-scatter, whereby the Raman scattered light is resonant at its turning point in a density gradient, was identified experimentally in planar-target experiments at the National Ignition Facility (NIF) in conditions relevant to the direct-drive scheme of inertial confinement fusion (ICF). This process was found to be one of the principal sources of supra-thermal electrons in such conditions, which can preheat the target and reduce its compressibility. We have developed a new semi-analytical model of the instability, which describes both its convective and absolute aspects; we derived quantitative estimates of the amplification region in typical ICF regimes, which highlights the need for sufficiently large laser spots to allow the instability to develop. Full-scale simulations of these experiments using the laser-plasma interaction code ``pF3d'' show SRS side-scatter largely dominating over back-scatter, and reproduce the essential features observed in the experiments and derived in the theory; we provide extrapolations to the case of spherical geometries relevant to direct-drive and discuss implications for indirect-drive ICF experiments.   

Absolute Stimulated Raman Scattering Sidescatter in Direct-Drive Irradiation Geometries

As the plasmas involved in direct-drive laser-fusion experiments increase in scale length and temperature, the relative importance of absolute stimulated Raman scattering (SRS) relative to two-plasmon decay as a source of hot electrons is expected to increase, owing to the stronger dependence on scale length and weaker dependence on temperature of the former.\footnote{ C. S. Liu, M. N. Rosenbluth, and R. B. White, Phys. Fluids \textbf{17}, 1211 (1974)} Absolute SRS backscatter occurs only at quarter-critical density, while absolute SRS sidescatter can occur throughout the sub-quarter-critical density plasma. In this talk a formalism is presented to investigate thresholds for absolute SRS in the context of direct-drive irradiation, comprising multiple beams having varying angles of incidence and polarization. Representative examples will be presented to illustrate the behavior of absolute SRS under these conditions, with emphasis on sidescatter at wavelengths below the half-harmonic of the laser. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Modeling Stimulated Raman Scattering in Direct-Drive Inertial Confinement Fusion Plasmas for National Ignition Facility Conditions

In the plasmas of direct-drive inertial confinement fusion (ICF), the coupling of laser power to the target plasma is strongly influenced by the laser--plasma interaction (LPI) processes driven by multiple crossing laser beams.\footnote{J. F. Myatt \textit{et al.}, Phys. Plasmas \textbf{21}, 055501 (2014).} For the plasma parameters relevant to the conditions of the experiments at the National Ignition Facility (NIF), the threshold of the stimulated Raman scattering (SRS) is usually well exceeded because of the large scale length of the plasma density, making the study of SRS vital for the NIF ICF program. The SRS evolution starts as a convective or absolute instability,\footnote{H. Wen \textit{et al.}, Phys. Plasmas \textbf{22}, 052704 (2015).} and the nonlinear saturation is determined by the ion-acoustic perturbations and kinetic effects. The LPI processes of cross-beam energy transfer and two-plasmon decay also drive the ion-acoustic modes and their interplay with SRS is analyzed. This work was supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Nonlinear Fluid Simulation Study of Stimulated Raman and Brillouin Scatterings in Shock Ignition

We developed a new nonlinear fluid laser-plasma-instability code \textit{FLAME} using a multi-fluid plasma model combined with full electromagnetic wave equations. The completed one-dimensional (1D) version of \textit{FLAME} was used to study laser-plasma instabilities in shock ignition. The simulations results showed that absolute Stimulated Raman Scattering (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 depletion. Re-scattering of SRS was also observed in a high temperature case. These results largely agreed with corresponding Particle-in-Cell simulations.   

Mitigation of cross-beam energy transfer in direct-drive inertial-confinement-fusion implosions with enhanced laser bandwidth

Cross-beam energy transfer (CBET) is a special category of stimulated Brillouin scattering in which two overlapping laser beams exchange energy by means of an ion acoustic wave in an under-dense expanding plasma [C.J. Randall \textit{et al.}, Phys. Fluids \textbf{24}, 1474 (1981)]. CBET can cause the incident laser energy to be misdirected in direct-drive inertial-confinement-fusion (ICF) implosions, thereby reducing both the maximum ablation pressure achieved and the overall symmetry of the implosion [J.F. Myatt \textit{et al}., Phys. Plasmas \textbf{21}, 055501 (2014)]. One strategy for mitigating CBET may be to increase the bandwidth of the laser light, thereby disrupting the coherent wave-wave interactions underlying this resonant parametric process. In this presentation, we report on results of two-dimensional planar simulations performed with the code LPSE-CBET that demonstrate a significant reduction in CBET for bandwidths between 2 and 5 THz. Although large compared to OMEGA and NIF values (about 1 and 0.3 THz, respectively), it may be possible to reach such bandwidths with existing ICF lasers using a technique based on stimulated rotational Raman scattering [D. Eimerl \textit{et al}., Phys. Rev. Lett. \textbf{70}, 2738 (1993)], which is a subject that we also briefly discuss.   

Polarization Rotation Caused by Cross-Beam Energy Transfer in Direct-Drive Implosions

The first evidence of polarization rotation caused by cross-beam energy transfer (CBET) during direct-drive implosions has been provided by a new beamlets diagnostic that was fielded on OMEGA. Beamlet images are, in essence, the end points of beamlets of light originating from different regions of each beam profile and following paths determined by refraction through the coronal plasma. The intensity of each beamlet varies because of absorption and many CBET interactions along that path. The new diagnostic records images in two time windows and includes a Wollaston prism to split each beamlet into two orthogonal polarization images recording the polarization of each beamlet. Only the common polarization components couple during CBET so when each beam is linearly polarized, CBET rotates the polarization of each beam. A 3-D CBET postprocessor for hydrodynamics codes was used to model the beamlet images. The predicted images are compared to the images recorded by the new diagnostic. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Using Wave-Based Cross-Beam Energy Transfer Simulations to Improve the Ray-Based Models Used in Inertial Confinement Fusion Applications

Ray-based models of cross-beam energy transfer (CBET) are used in radiation--hydrodynamics codes to calculate laser-energy deposition for inertial confinement fusion (ICF) experiments. In direct-drive ICF, calculations suggest that CBET is responsible for a 10{\%} to 20{\%} reduction in laser energy absorption.\footnote{I. V. Igumenshchev \textit{et al.}, Phys. Plasmas \textbf{19}, 056314 (2012).} In indirect drive, ray-based calculations predict full pump depletion of the outer cone beams.\footnote{P. Michel \textit{et al.}, Phys. Plasmas \textbf{20}, 056308 (2013).} Ray-based CBET models require artificial limiters to give quantitative agreement with experimental observables. The recent development of a 3-D wave-based solver (\textit{LPSE} CBET) that does not rely on the paraxial or eikonal approximations allows the limitations of ray-based CBET models to be studied at conditions relevant to laser-driven ICF. The accuracy of ray-based CBET models is limited by uncertainties in the approximations used to account for the experimental realities of beam speckle, polarization smoothing, and interactions at caustics. A physics-based technique is proposed for including the effect of beam speckle in existing ray-based models that gives excellent agreement with the wave-based calculations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Early Computed Hard X-Ray Emissions from Two-Plasmon--Decay Fast Electrons Not Observed in Experiments Point to Discrepancies in the Two-Plasmon--Decay Source Model

The temporal source of two-plasmon--decay (TPD) fast-electron transport in the 1-D hydrodynamic code \textit{LILAC}, based on the measured, integrated hard x-ray (HXR) emission as a function of laser intensity, depends exponentially on the TPD threshold parameter up to about 0.9 and saturates above it. This model, along with \textit{LPSE} simulations, produces HXR emissions much earlier than observed for certain shots. The amount of early emission depends on the rise time of the drive pulse and varies from a small shoulder to an early peak much larger than measured as the rise time decreases. The cause of this discrepancy could be that faster rise times limit the population of the thermal electron distribution near 10 keV, from which electrons are accelerated by the TPD plasma waves. Electron kinetic simulations will be performed with the Vlasov--Fokker--Planck code \textit{FPI }to address the issue of the rise time of the 10-keV electron population as a function of the intensity rise time. Another cause could be an $\sim $20{\%} overestimate of the threshold parameter from the hydrodynamic conditions that would disappear over time. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

First Experimental Comparisons of Laser-Plasma Interactions between Spherical and Cylindrical Hohlraums at SGIII Laser Facility

In thish report, we introduce our recent laser-plasmas instability (LPI) comparison experiment at the SGIII laser facility between the spherical hohlraums and the cylindrical hohlraums. Three kinds of filling are considered: vacuum, gas-filling with or without a capsule inside. The experiment has shown that the LPI level in the spherical hohlraum is close to that of the outer beam in the cylindrical hohlraum, while much lower than that of the inner beam. The experiment is further simulated by using our 2-dimensional radiation hydrodynamic code LARED-Integration, and the laser back-scattering fraction and the SRS spectrum are post-processed by the high efficiency code of laser interaction with plasmas HLIP. According to the simulation, the plasma waves are strongly damped and the SRS is mainly developed at the plasma conditions of electron density from 0.08 {\$}n\textunderscore c{\$} to 0.1 {\$}n\textunderscore c{\$} and electron temperature from 1.5 keV to 2.0 keV inside the hohlraums. However, obvious differences between the simulation and experiment are found, such as that the SRS back-scattering is underestimated, and the numerical SRS spectrum peaks at a larger wavelength and at a later time than the data.   

Modeling Laser-Plasma Interaction over a Suite of NIF Experiments

We systematically study laser-plasma interaction (LPI) on NIF indirect-drive experiments, namely backscatter and cross-beam energy transfer. LLNL’s best practice radiation-hydrodynamic simulation methodology$^1$ in the Lasnex simulation code$^2$ is employed without ad-hoc tuning to match experimental data. This entails converged numerical resolution, an improved DCA model for coronal ($n_e1$ keV) gold opacity, electron heat flux strongly limited to $0.03n_eT_e^{3/2}m_e^{-1/2}$, and the inline CBET model$^3$. The rad-hydro plasma conditions are used for LPI analysis, namely linear instability gains, and the paraxial-envelope code pF3D$^4$. Simulated scattered-light spectra are also compared to measurements. We initially focus on shots with low backscatter, so its self-consistent treatment should not be important. These shots have low hohlraum fill density and short laser pulses, and the only significant backscatter is outer-beams Brillouin. Our long-term goals are to understand reflectivity trends to guide target design and develop LPI mitigation strategies. 1 O Jones, L Suter et al., PoP 24, 056312 (2017) 2 G Zimmerman, W Kruer, Comments PPCF 2, 85 (1975) 3 D Strozzi, D Bailey et al., PRL 118, 025002 (2017) 4 R Berger, C Still et al., PoP 5, 4337 (1998)   

Vlasov-Fokker-Planck and PIC with Collisions Modeling of Stimulated Raman Scattering in the presence of Inverse-Bremsstrahlung heating in plasmas relevant to Inertial Confinement Fusion

Laser energy is absorbed in inertial fusion plasmas through the inverse-bremsstrahlung process. Theoretical work has predicted the evolution of non-Maxwellian electron distribution functions in the presence of inverse-bremsstrahlung heating [1], where the velocity gradient of the distribution function is relaxed at $v \gtrsim 2 v_{th}$ resulting in lower-than-Maxwellian Landau damping rates for electron plasma waves relevant to inertial fusion[2]. Here, we present the first self-consistent modeling of this process using OSHUN, a Vlasov Fokker Planck code, for conditions relevant to inertial fusion. We find enhanced SRS growth rates due to this effect in the collisional electron plasmas in the Trident experiments [3], as well as in the laser entrance hole of hohlraums during the picket pulse at the National Ignition Facility [4]. In hotter or low Z, low density plasmas, the effect is muted due to the lower collisionallity. Preliminary comparison of results from SRS simulations between OSHUN and OSIRIS with collisions will also be presented. [1] Langdon, Phys. Rev. Lett. 1980 [2] Afeyan et. al Phys. Rev. Lett. 1998 [3] Montgomery et. al Phys. Plasmas 2002 [4] Dewald et. al Phys. Rev. Lett. 2017   

Anti-Stokes scattering and Stokes scattering of stimulated Brillouin scattering cascade in high-intensity laser-plasmas interaction

The anti-Stokes scattering and Stokes scattering in stimulated Brillouin scattering (SBS) cascade have been researched by the Vlasov-Maxwell simulation. In the high-intensity laser-plasmas interaction, the stimulated anti-Stokes Brillouin scattering (SABS) will occur after the second stage SBS rescattering. The mechanism of SABS has been put forward to explain this phenomenon. In the early time of SBS evolution, only the first stage SBS appears, and the total SBS reflectivity comes from the first stage SBS. However, when the high-stage SBS and SABS occur, the SBS reflectivity will appear a burst behavior, and the total reflectivity comes from the SBS cascade and SABS superimposition. The SABS will compete with the SBS rescattering to determine the total SBS reflectivity. Thus, the SBS rescattering including the SABS is an important saturation mechanism of SBS, and should be taken into account in the high-intensity laser-plasmas interaction.   

Investigation of longitudinal relativistic effect on stimulated Raman backscattering by using one-dimensional 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 strongly convective regime is presented by combining the relativistic and non-relativistic 1D Vlasov-Maxwell simulations, which is in excellent agreement with the steady-state linear theory. Meanwhile, we successfully predict the critical duration of the seed which can just trigger the kinetic inflation of the excited SRBS after the\textbf{ }seed leaves the simulation box. In weakly convective regime, the transition from convective to absolute instability for SRBS can directly occur in linear regime due to the longitudinal relativistic modification. For the same pump, our simulations clearly 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.   

Theory and simulation of multibeam stimulated Raman scattering in inertial confinement fusion

Collective effects of multibeam laser plasma interactions are of great interests in recent years [1,2,3]. Here, we have demonstrated a general theory of multibeam stimulated Raman scattering (SRS) with arbitrary beam number and beam polarization incident in a cone geometry in both homogeneous and inhomogeneous plasma. Scattering geometry with a shared Langmuir wave or a shared electromagnetic wave are the ones that with the maximum growth rates as predicted by DuBois et al [4]. In addition to those scattering geometries, cone scattering where scattered lights perpendicular to the cone axis is dominant over common wave geometry in the nonlinear stage. In particle-in-cell simulations we have verified the effects of three scattering geometries, and obtained the density and incident angle dependence of the effects of multibeam SRS relevant to the indirect-drive ignition conditions. [1] S. Depierreux et al., Phys. Rev. Lett. {\bf 117}, 235002 (2016). [2] P. Michel et al., Phys. Rev. Lett. {\bf 115}, 055003 (2015). [3] E. L. Dewald et al., Phys. Rev. Lett. {\bf 116}, 075003 (2016). [4] D. F. DuBois, B. Bezzerides, and H. A. Rose, Phys. Fluids B {\bf 2}, 241 (1992).