Session NO06: ICF: Fundamental

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Carbon and other impurities can enter inertial confinement fusion fuel in jets. The higher Z material is more radiative, quenching the fusion burn. There are some experimental indications that this mix is spreading rapidly across the fuel hot-spot at close to bang time. The mix jet contains very little fuel and has a low temperature, meaning fusion barely occurs within it. This results in a current of fast fusion alpha particles from the hotter region into the jet. This will cause a resistive return current that changes the plasma diffusion rate of these contaminants across the fuel. We explore this effect with Vlasov-Fokker-Planck simulations.   

Live

The growth of perturbations due to hydrodynamic instabilities at material interfaces, such as Richtmyer-Meshkov (RM), Rayleigh-Taylor (RT), and Kelvin-Helmholtz (KH), plays an important role in the evolution of high-energy-density systems (HED). Previous HED experiments have shown that the perturbation growth of coupled RM, RT, and KH can be measured by considering a perturbed interface tilted with respect to an incident shock wave. As the shock interacts with the interface, a sheet of vorticity is produced along the interface due to the baroclinic torque generated by the misalignment of the density gradient (across the interface) and pressure gradient (across the shock). In this study, we investigate the post-shock hydrodynamics using a vortex-sheet model. This approach enables us to relate the interfacial dynamics to the vorticity distribution. In particular, we examine the competition between shock- vs. shear-driven perturbation growth by considering different tilt angles and develop a scaling for the early time behavior. Additionally, the vortex-sheet can be used to predict time-dependent RT growth and interface decompression arising from laser turn-off.   

Live

Laser imprint mitigation using high-Z coating has been shown to have an order-of-magnitude reduction in the laser non-uniformity seeding of hydrodynamic instabilities [Obenschain, PoP 9, 2234 (2002); M. Karasik, to be published]. A thin ($\sim400\AA$) Pd or Au coating on the front of the ablator, pre-expanded to 100um provides an initial smooth soft x-ray drive, subsequently transitioning to efficient direct drive. Experiments on this hybrid indirect-direct drive have thus far been conducted with plastic ablator thicknesses comparable to that on ignition-scale capsules. Thinner ablators, such as 8um used in cryogenic implosions on OMEGA, may be more susceptible to the effects of x-ray preheat from the coating. In order to quantify the x-ray emission from the coating, NRL has installed an absolutely calibrated time-resolved soft x-ray transmission grating spectrometer on Omega EP similar to that [Weaver, PoP 8, 5230 (2001)] utilized on the Nike laser at NRL. The spectra show increased initial soft x-ray emission from the high-Z coating at the start of the laser pulse, followed by decay to the level of uncoated CH, as expected. The experiments also utilize VISAR to measure the speed of the shock in the ablator to be used for benchmarking shock timing with the high-Z coated targets.   

Live

The physics of shock heating as it applies to ICF implosions is a rich topic with many unanswered questions, including the nature of shock heating (collisional vs collisionless), the differential heating of ion species with different mass and charge and their subsequent equilibration, the impact of kinetic effects, and the impact of electron viscosity. Here, we address these questions with data from a shock-driven OMEGA experiment where the performance of thin CH-shell implosions filled with either 100{\%} D$_{\mathrm{2}}$ or 50:50 D$_{\mathrm{2}}$:N$_{\mathrm{2}}$ gas is compared. The D$_{\mathrm{2}}$-only implosions outperform the mix-fill implosions by a factor of 10 in yield, while the mix-fill implosions show 40{\%} higher ion temperature (Tion) than the pure D$_{\mathrm{2}}$ implosions as inferred from DD-neutron spectra. This is consistent with enhanced shock heating of the N than D ions. However, the Tion difference observed is smaller than predicted by average-ion HYADES simulations (80{\%}), and the Tions are also lower than predicted by HYADES (50{\%}). The impact of multi-ion effects or electron viscosity on these results is addressed.   

Live

High power lasers interacting with a flowing plasma can produce a plasma response that leads to beam bending and, by momentum conservation, to a drag force which slows down the plasma flow velocity [1]. When the plasma flow velocity is in the vicinity of the ion sound speed, where this plasma response is the strongest, the flow's interaction with the laser light can lead to shock formation. We report on progress in numerical and analytical studies of shock formation in the geometry of the proposed NIF experiments. Different targets have been examined in large scale hydrodynamic simulations to achieve the necessary plasma flow profile across the overlap region of crossing NIF beams. Wave interaction simulations in 2D examined the laser response, slowing of the flow and shock formation in the background plasma as described by a nonlinear hydrodynamic model. A scan of the parameter space, including the effects of temporal and spatial smoothing, and crossed beam geometry have been examined with the goal of designing an experiment on NIF. [1] H.A. Rose, Phys. Plasmas \textbf{3}, 1709 (1996).   

Live

A new source of ion-acoustic turbulence is investigated whose origin is the stimulated Brillouin scattering between two counter-propagating laser beams in inhomogeneous, flowing plasmas where the laser intensity is slightly higher than that usually encountered in inertial confinement fusion experiments, $I\sim 10^{15}\mbox{W/cm}^{2}$. Densities are $>n_{c} /4$, $ZT_{e} /T_{i} \gg 1$ and the flow scale-length is about 100$\mbox{\mu m.}$ Numerical Vlasov simulation shows that the resulting large-amplitude ion acoustic waves lead to harmonic generation which evolves into turbulence due to wave breaking. This SBS-driven turbulence generates chains of solitary waves which accelerate ions to energies that are dozens of times the energy of thermal electrons. Its ion acceleration mechanism and ion energy spectrum are described and its potential applications are discussed.   

Live

Using numerical simulations, we show that plasmas will absorb energy from lasers by the non-linear motion of electrons interacting with small-scale, random intensity non-uniformities. The necessary conditions for this mechanism arise when multiple speckled lasers overlap, as occurs in laser fusion hohlraums. Stochasticity emerges as a result of chaotic particle trajectories, leading to scattering of electrons in the laser fields at a rate which can exceed the Coulomb scattering rate for typical laboratory parameters. We focus our calculations on the laser overlap region of the National Ignition Facility, where the absorption rate is predicted to exceed the inverse bremsstrahlung rate by a factor of twenty-five at peak power. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344, and release LLNL-ABS-812057.   

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In the field of the nonlinear optics of plasmas, stimulated Raman scattering (SRS) has been actively investigated owing to its deleterious consequences for laser driven inertial confinement fusion (ICF). There has been little research on SRS for general Laguerre Gaussian laser modes which are characterized by radial and azimuthal mode numbers (p and l) and can have orbital angular momentum (OAM). SRS involving lasers with OAM has an l number matching condition in addition to the frequency and wave-number matching conditions. A given laser can scatter into a variety of different l (and p) modes and the resulting plasma waves can also carry OAM. The helical shape of a phase front of a wave with OAM changes the particle trapping dynamics. We present preliminary results of SRS of Laguerre Gaussian modes using the UPIC and OSIRIS particle-in-cell codes. We use electrostatic and Darwin PIC codes to isolate the evolution of driven plasma waves and the quasi-3D version of OSIRIS to study SRS of a single Laguerre Gaussian speckle for plasma conditions of interest to ICF.   

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New intense laser applications require increases in pulse energy, power, and intensity beyond the limitations of conventional solid-state media. Plasma optics are a promising solution due to their increased resiliency to damage but require characterization in the linear and nonlinear response regimes. The plasma amplifier project at NIF has demonstrated the combination of up to 21 frequency shifted beams via Cross-Beam Energy Transfer, achieving 10x amplification of a seed beam up to nearly 8 kJ in a 1 ns pulse. Full characterization of the plasma optic is underway, most recently by investigating power transfer (both seed amplification and pump depletion) with \textasciitilde 100 ps time resolution of seed beams from 1 ns down to 100 ps in duration. This investigation of the ion wave response timescale for \textasciitilde kJ scale transfers is a critical step toward achieving beam combination not just to high energy but also high power. Experimental details and supporting simulation results will be discussed.   

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Inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF) use laser beams targeted at hohlraum walls as the first step in driving a fusion~reaction. It has been observed [1] that in some cases, the laser beams pointed at the hohlraum waist can refract off the density gradient from the expanding hohlraum wall and escape through the opposite entrance hole; this process, known as ``glint'', is the current leading hypothesis for the drive deficit observed in ICF experiments. Here we show that as a laser beam refracting off a density gradient overlaps with itself, the incoming part can transfer its energy to the outgoing part, like in crossed-beam energy transfer (CBET) for direct-drive ICF.~~This can result in higher hohlraum coupling losses than if computed using absorption alone in hydrodynamics codes. We provide a 1D analytical and numerical description of this "self-amplified glint" and demonstrate that for an ideal rarefaction profile, a resonant surface always exists before the turning point where this self-amplification can occur. The amount of energy transfer is determined by a balance between absorption and CBET gain. [1]: D. P. Turnbull et al., PRL 114, 125001 (2015)   

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Recent experimental work is presented which was designed to observe the Landau-Darrieus Instability (LDI) for the first time in the context of laser-driven ablation fronts. The most recent experiments were conducted at the National Ignition Facility (NIF) and build upon previous work conducted at OMEGA EP. To avoid stabilizing the LDI, the conduction zone length D$_{\mathrm{c}}$must be smaller than the studied wavelength. This is done by reducing the drive intensity. To maximize ablation velocity and maintain adequate growth rates, low-density foams are used. Finally, to prevent the Rayleigh-Taylor instability from dominating the system while still maximizing the growth of the LDI, thick targets with long drive durations are fielded. The analysis of the OMEGA EP experiment is presented here which critically informs the subsequent design and interpretation of the recent NIF results.   

Live

Trapping of particles in large-amplitude Ion Acoustic Waves is a ubiquitous process in the nonlinear evolution of many plasma instabilities such as Stimulated Brillouin Scattering (SBS), Collisionless Shocks, and Ion Streaming Instabilities. Recent work$^1$ has proposed that multi-species plasmas composed of heavy, high-Z and light low-Z ions can be used in ICF experiments to suppress the growth of potentially damaging levels of SBS without significant loss of the radiation drive in indirect-drive hohlraums. This suppression relies on the validity of using linear Landau damping of the IAW by the light ions. However, if the SBS-driven IAW can trap the light ions, the suppression of SBS could be reduced. Here, we present kinetic simulations with NIF-relevant plasma conditions that show the scattering of light ions on the heavy ions maintains the Maxwell-Boltzmann form of the ion distribution and thereby the linear Landau damping assumption. \\ $^1$ Berger,\textit{et al} PoP 26,012709(2019)   

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Laser propagation in an under-dense plasma, and the instabilities generated during its transit are of fundamental interest for inertial confinement fusion. We present an experimental study of the influence that an external magnetic field (B) up to 20 T has on the propagation of a nanosecond, 9x10$^{\mathrm{14\thinspace }}$Wcm$^{\mathrm{-2}}$ intensity laser in a pre-formed plasma. The temporally-resolved transmitted light profile indicates more light transmission in the magnetized case, with a larger transmitted beam. Measurements of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), both in the backward direction, are also presented. As a function of B, an increase of SRS is detected while no change is detected for SBS. Rad-hydro and particle-in-cell simulations are employed to elucidate the dynamics resulting in this behavior.   

Live

Shock release from inertial confinement fusion (ICF) shells poses a great challenge to single-fluid hydrodynamic equations, especially for describing compounds such as polystyrene. This has been evidenced by a recent experiment [D. Haberberger et al., Phys. Rev. Lett. 123, 235001 (2019)], in which low-density plasmas (10\textasciicircum 19 to 10\textasciicircum 20 cm\textasciicircum -3) are measured to move far ahead of what standard hydro simulations predict. To understand such experimental observations, we have performed large-scale nonequilibrium molecular-dynamics simulations of polystyrene shocked to experimental conditions. These simulations revealed that upon shock release, hydrogen can stream out of the bulk of the CH foil. The released hydrogen, exhibiting a much broader velocity distribution than carbon, forms low-density plasmas moving ahead of the CH shell, which is in quantitative agreement with the experimental measurements. Such kinetic effect of species separation is currently missing in single-fluid radiation-hydrodynamics simulations, which could have profound implication to ICF target designs.   

Shock-Release Experiments on OMEGA EP.

In an inertial confinement fusion implosion, release of the shocked material from the inner side of the shell after the shock breakout is an important process that affects formation of the hot spot and implosion performance. Experiments on OMEGA EP at the Laboratory for Laser Energetics used 4$\omega $ interferometry to measure the spatial evolution of the low-density material ahead of an accelerated CH. The foil trajectory was measured using side-on x-ray radiography. Radiation-hydrodynamics simulations using \textit{DRACO} and \textit{LILAC} agree well with the trajectory measurements but generally exhibit less extended electron density position and shorter scale length. It was found that the release strongly depends on the conditions at the back surface before the shock breakout. Analysis of the most recent experiments and simulations will be presented and the sensitivity of the density profile to different physics will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.