Session TO08: ICF: Laser-Plasma Interactions

Live

Inertial confinement fusion (ICF) or high energy density (HED) experiments typically involve a large number of laser beams overlapping in plasmas, in conditions often prone to laser-plasma interaction (LPI) processes such as backscattering instabilities. Here we investigate how backscatter from one beam is affected by the presence of other overlapping beams, and under which conditions the collective behavior (i.e. multiple overlapped beams acting together as one) dominates over the sum of the individuals (i.e. each beam’s backscatter independent of the others). Understanding collective LPI behavior in ICF or HED can help with the interpretation of on-going experiments at large-scale laser facilities and guide the design of future lasers, for example regarding the optimization of the number of beams and their separation in the near-field.   

Live

The tunable OMEGA port 9 (TOP9) laser on OMEGA was used to perform cross-beam energy transfer (CBET) experiments in a gas-jet plasma. The TOP9 laser is a wavelength{\-}tunable UV beam ($\Delta \lambda $\textasciitilde 3nm) that enables CBET experiments in a stationary plasma. TOP9 was interacted with either one or four other 351-nm UV pump beams to study CBET using the time-resolved transmitted beam diagnostic (TBD). The frequency of the TOP9 beam was set so that the beat frequency generated with the UV pump beams was resonant with the ion-acoustic wave frequency of the gas-jet plasma. At low TOP9 intensities, where ion-wave amplitudes were small ($\delta n$/$n$ \textless 1.0{\%}), TBD measurements agreed well with linear CBET theory. At high TOP9 intensities, TBD measurements show ion-wave amplitudes that are initially large ($\delta n$/$n$ \textgreater 3.0{\%}) and then decrease to smaller amplitudes ($\delta n$/$n \quad \approx $ 1.0{\%}) over approximately 300 ps.   

Live

The performance of laser-driven inertial confinement fusion (ICF) implosions relies critically on the coupling of laser energy to the target plasma. Cross beam energy transfer (CBET), the resonant exchange of energy between overlapped beams mediated by ponderomotively excited ion-acoustic waves, inhibits this coupling by scattering light into unwanted directions. Here, we show that CBET can saturate through a resonance detuning that results from modifications to the velocity distribution functions due to trapping in the ion-acoustic wave. Particle-in-cell simulations of Tunable OMEGA Port 9 (TOP9) experiments at the Laboratory for Laser Energetics exhibit an initial stage of saturation in which ion-acoustic waves undergo transverse breakup [1]. This is followed by a second longer time scale saturation due to modified distribution functions. Results from these simulations can inform the reduced CBET models implemented in radiation hydrodynamics simulations, improving their predictivity capability, and reveal pathways towards CBET mitigation. 1. L. Yin et al. Phys. Plasmas 26, 082708 (2019).   

Live

Cross-beam energy transfer (CBET) allows crossing laser beams to exchange energy. The growth and saturation of CBET can involve complex, nonlinear electron and ion dynamics. Recent Tunable OMEGA Port 9 (TOP9) CBET experiments at the OMEGA facility provide a simplified, well diagnosed setting in which a single seed laser beam can interact with up to four pump beams, enabling detailed comparison with particle-in-cell modeling of the experiments in order to examine nonlinear CBET saturation. Two-dimensional (2D) VPIC simulations show that the experimental setting is stable to forward stimulated Raman scattering (FSRS) shown in prior studies to interfere with the CBET process. This allows for the examination of ion nonlinear effects on CBET in the absence of FSRS. VPIC simulations of one seed beam interacting with one pump are performed. It is found that collisional de-trapping of ions in the ion acoustic wave (IAW) is important near the linear regime at the lowest seed intensity, whereas at higher seed beam intensities, collisional de-trapping effects are small. Preliminary results from 3D studies will also be presented.   

Live

Planar target experiments performed at the National Ignition Facility (NIF) suggest that stimulated Raman scattering (SRS) is dominant at the ignition scale [M.J. Rosenberg {\it et al}., PRL {\bf 120} 055001 (2018)], and that the subsequent preheat can be near levels which degrade the performance of directly driven implosions [A.A. Solodov {\it et al}., Phys. Plasmas {\bf 27} 052706 (2020)]. Definitive conclusions require a valid extrapolation of the observed scattered light and a detailed understanding of the contributions of several instability processes. To help resolve this, recent experiments on the OMEGA EP laser in which similar SRS signatures were observed at scale-lengths intermediate between NIF and OMEGA were analyzed using a new ray-tracing model. The model allows the various SRS contributions to the observed scattered light spectrum to be determined. The relevance to ignition-scale plasmas and implications for preheat will be presented.   

Live

In the context of direct-drive inertial confinement fusion (ICF), it has been acknowledged that the levels of cross-beam energy transfer (CBET) found in current experiments would likely preclude ignition in a full-scale shot[1]. In experiments on the OMEGA laser, CBET reduces laser-target coupling by up to 30\%[2], with even higher losses on the NIF[1]. Increased laser driver bandwidth offers a promising route to mitigate CBET and other laser-plasma instabilities, which would allow for a significant expansion of the ICF design space. Here we present VPIC[3] particle-in-cell simulations investigating the efficacy of bandwidth in reducing CBET. We compare the PIC results with linearized fluid simulations performed with the LPSE code[4] and discuss the significance of nonlinear kinetic and fluid effects and their response to bandwidth. [1] V. N. Goncharov et al. (2017). Plasma Physics and Controlled Fusion, 59(1), 014008. [2] I. V. Igumenshchev et al. (2010). Physics of Plasmas, 17(12), 21–26. [3] K. J. Bowers et al. (2008). Physics of Plasmas, 15(5), 055703. [4] J. F. Myatt et al. (2019). Journal of Computational Physics, 399, 108916.   

Live

Laser--plasma instabilities such as cross-beam energy transfer, stimulated Raman scattering (SRS), and two-plasmon--decay (TPD) present a major challenge for laser-driven inertial confinement fusion (ICF). Quantitatively predicting the severity of these instabilities requires a model that captures the complex, 3-D interaction of multiple laser beams, including effects such as speckle, polarization, and bandwidth. Here, we employ the laser plasma simulation environment (\textit{LPSE}) to investigate the multibeam nature and mitigation of these instabilities with broadband lasers for conditions relevant to direct-drive ICF. While multibeam coupling plays a critical role in both absolute TPD and SRS, the coupling for SRS is weaker. The threshold for both instabilities can be increased significantly by using drive lasers with \textasciitilde 1{\%} relative bandwidth. A broadband laser based on optical parametric amplification, with sufficient energy and bandwidth to validate these predictions, is currently in development at the Laboratory for Laser Energetics.   

Live

In the shock-ignition inertial confinement fusion (ICF) scheme, high-intensity lasers propagate through a long scale-length coronal plasma where back-scattered stimulated Raman scattering (bSRS) is likely to be in the kinetic regime. For a monochromatic laser source, the authors have found that inflationary SRS (iSRS) has an intensity threshold that depends on the density scale-length. This threshold is always below $4\times 10^{15} \mathrm{W/cm}^2$ for shock-ignition plasma parameters. Previous work[1] has shown that bandwidth of the order $\Delta \omega_0 \sim 10^{-2}\omega_0$ reduces SRS reflectivity in large scale inhomogeneous plasmas. In this work, the \textit{EPOCH} particle-in-cell code is used to perform fully kinetic simulations, to examine the effect of broadband lasers on iSRS. Since potential shock-ignition designs vary widely across facilities, we focus on how the effects of a broadband driver on inflationary SRS scale with density scale-length and laser intensity. [1] Yao Zhao \textit{et al.} 2019 \textit{Plasma Phys. Control. Fusion} \textbf{61} 115008   

Live

Recent experiments at the National Ignition Facility (NIF) have characterized the laser energy coupling to the strong shock at shock-ignition--relevant laser intensities. Analysis of these experiments has shown that a significant amount of energy is scattered away from the target due to cross-beam energy transfer (CBET). This effect has generally not been accounted for in previous simulations of shock-ignition implosions. This talk will present results from 2-D DRACO simulations of these experiments using a pump-depletion model of CBET to compare with data from these experiments. Implications for previous shock-ignition designs for the NIF will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

Higher levels of hot electrons from the two-plasmon-decay instability are observed when smoothing by spectral dispersion (SSD) is turned off in directly driven inertial confinement fusion experiments at the Omega Laser Facility. This finding is explained using a hot-spot model based on speckle statistics and simulation results from the laser-plasma simulation environment. The model accurately reproduces the relative increase in hot-electron activity at two different drive intensities, although it slightly overestimates the absolute number of hot electrons in all cases. Extrapolating from the current 360-GHz system while adhering to the logic of the hot-spot model suggests that larger SSD bandwidth should significantly mitigate hot-electron generation, and legacy 1-THz OMEGA experiments appear to support this conclusion. These results demonstrate that it is essential to account for laser speckles and spatiotemporal smoothing to obtain quantitative agreement with experiments.   

Live

In direct-drive inertial confinement fusion (ICF), laser--plasma instabilities (LPI's) such as two-plasmon decay (TPD) can significantly degrade implosion performance. The major threat of TPD has traditionally been considered hot electrons, which can preheat the fusion fuel, reducing its compressibility. However, recent data analysis of ICF experiments on the OMEGA laser and improved LPI modeling have revealed that TPD can deplete a significant fraction of laser energy, modifying the balance of scattered and absorbed light.\footnote{ D. Turnbull \textit{et al.}, Phys. Rev. Lett. \textbf{124}, 185001 (2020).} Here we explore the spatial profile of these modifications by coupling the results of wave-based LPI simulations to a ray trace in plasma profiles extracted from radiation-hydrodynamics simulations. The wave-based LPI simulations, which model TPD for realistic OMEGA beam configurations, including speckle and polarization smoothing, provide a scaling of laser absorption as a function of beam incidence angle and intensity. Using this scaling, the ray trace provides the spatial profile of absorption and scattering. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

Cross-beam energy transfer (CBET) is the process by which two crossing laser beams transfer energy between one another through stimulated Brillouin scattering (SBS). Understanding the nonlinear saturation of CBET, including the effects of wave-particle interaction, the excitation of secondary instabilities such as stimulated Raman scattering (SRS) and forward SRS (FSRS), and speckle geometry, is important to controlling low-mode asymmetry in ICF implosions. In this work, particle-in-cell simulations using VPIC are performed to characterize the SRS and FSRS in a CBET-amplified multi-speckled beam across a range of plasma densities that commonly occur in ICF experiments. Furthermore, variations in crossing angle between the pump and probe beams alter ion trapping in the ion acoustic wave as well as ponderomotive effects. The influence of the above kinetic and fluid processes on CBET saturation will be presented.   

Live

Cross-beam energy transfer (CBET) is an undesirable energy-loss mechanism in directly-driven inertial-confinement-fusion (ICF) implosions. An earlier study [J.W. Bates et al., High Energy Density Phys. 36, 100772 (2020)] using a 2D version of the code LPSE [J.F. Myatt et al., J. Comp. Phys. 399, 108919 (2019)] demonstrated that for two, crossed, frequency-tripled, Nd:glass laser beams modeled with random speckle patterns, distributed phase plates and polarization smoothing, Gaussian bandwidths of about 1{\%} (8 THz) are effective at eliminating CBET in a plasma under ICF-relevant conditions. Here, we report on 2D and 3D LPSE simulations that examine CBET suppression in the multiple-laser-beam configuration used in direct-drive implosion on the OMEGA laser. In addition to Gaussian bandwidth, we also model the effects of laser ``detuning'' and smoothing by spectral dispersion and compare the efficacy of all three approaches for suppressing CBET between multiple, overlapped laser beams. Additionally, we will discuss in this presentation two possible techniques for CBET suppression in direct-drive ICF: i.) stimulated rotational Raman scattering; and ii.) excimer laser drivers.   

On Demand

The stimulated Raman scattering (SRS) instability can inhibit the performance of laser-driven inertial confinement fusion (ICF) implosions by scattering light into unwanted directions or by generating hot electrons that preheat the target fuel. In principle, ICF target designs can avoid parameter regimes conducive to large, linear SRS gains. In practice, kinetic inflation---the autoresonant enhancement of SRS due to electron trapping in the excited plasma wave---makes this difficult. Here we show that laser bandwidth in the form of frequency modulation can either decrease or increase the intensity threshold for kinetic inflation depending on the maximum chirp of the laser pulse. The threshold, mapped out by a series of particle-in-cell simulations, exhibits a minimum when the frequency change within the pulse cancels the spatial detuning due to density inhomogeneities along the trajectory of the scattered light. By tuning the chirp away from this minimum, the threshold can be larger than at zero bandwidth, providing a complementary approach to avoiding SRS. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.