Session NO5: ICF: Direct, Indirect, and Polar Drive I

Direct-Drive Measurements of Laser-Imprint--Induced Shock-Velocity Nonuniformities and Laser Imprint Mitigation

In laser-direct-drive inertial confinement fusion, nonuniformities in the laser drive caused by laser speckle and beam-to-beam intensity variations, as well as mass modulations in the target, can seed the Richtmyer--Meshkov and Rayleigh--Taylor hydrodynamic instabilities and adversely affect the compression of the imploding shell. The physical energy transfer of the laser-intensity modulations to the shock front, called laser imprint, depends strongly on the initial plasma formation. Perturbations in the velocity profile of a laser-ablation driven shock wave seeded by laser imprint were recorded using a 2-D high-resolution velocimeter. The measured results for experiments with one, two, and five overlapping beams incident on target demonstrate a reduction in long-wavelength (\textgreater 25-?m) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint, since they highlight a threefold underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation such as multiphoton ionization are neglected.   

Broadband Smoothing of Laser Pulses for Imprint Reduction in Direct-Drive Inertial Confinement Fusion

In direct-drive inertial confinement fusion, an ensemble of laser beams irradiates a spherical capsule of deuterium--tritium fuel encased in a thin outer ablator. Early in the drive, intense speckles within the beams locally heat the ablation surface imprinting small{\-}scale density nonuniformities. Ultimately, this imprint can severely limit the fusion yield by seeding hydrodynamic instabilities that cause the capsule to break up during compression. Broad{\-}bandwidth lasers can mitigate imprint with rapidly moving speckle patterns that smooth the intensity profile faster than the ablation surface can hydrodynamically evolve. Here we explore the efficacy of imprint{\-}mitigation techniques enabled by broad{\-}bandwidth lasers.   

Toward Optimizing Cryogenic Inertial Confinement Fusion Implosions

Current radiation-hydrodynamics codes do not have enough predictive capability to be used for optimizing experimental implosion designs. Furthermore, 2-D or 3-D codes are prohibitively computationally expensive for this application. With recent advances in the statistical mapping of 1-D simulation outputs to experimental results on OMEGA, optimizing experimental design is starting to become viable. Through an appropriate parametrization of the initial conditions, we can apply standard nonlinear optimization methods to statistical predictions of the 1-D radiation-hydrodynamics code LILAC to devise improved experimental designs at nontrivial but manageable computational cost. A method is proposed for utilizing performance data from implosions that lie close in parameter space to current best-performing designs in order to suggest new designs with incremental performance improvements.   

Improved Predictive Models and Further Progress in the Cryogenic Optimization Campaign on OMEGA

Previous work on OMEGA has established a framework for generating predictive models for cryogenic ICF implosions, which were then used to increase performance on the OMEGA laser system. Here, we present improved predictive models built using this framework for the neutron yield, areal density, minimum ion temperature and x-ray hotspot radius in cryogenic implosions, which are used to optimize implosion design. We also present a predictive model for suprathermal electrons from warm implosions, which are necessary to quantify the preheat levels in cryogenic implosions to fully understand their effect in direct-drive experiments. In addition to highlighting and quantifying the magnitude of potential degradation sources, the use of this ensemble of predictive models has led to the design of experiments that are expected to show an increase in the areal density compared to the previous best performing implosion on OMEGA, while keeping neutron yields constant.   

Anomalous Absorption by the Two-Plasmon-Decay Instability in Directly Driven Inertial Confinement Fusion Experiments

Simulations of directly driven inertial confinement fusion experiments on the OMEGA Laser System were significantly improved with the inclusion of an inline model for crossed-beam energy transfer along with a nonlocal model for heat transport. Absorption and shell-velocity time histories are accurately predicted when experiments are driven at relatively low overlapped laser intensity. Discrepancies appear at higher intensity, however, with higher-than-expected laser absorption on target. Strong correlation between those discrepancies and signatures of the two-plasmon-decay instability (TPDI)---including time-dependent half-harmonic emission and hard x-ray signals---indicate that TPDI is responsible for this anomalous absorption. The data suggest that up to $\sim30\%$ of the laser right reaching $n_c/4$ can be absorbed locally when the TPDI threshold is exceeded, which is consistent with LPSE simulations.   

Three-Dimensional Hydrodynamic Modeling of OMEGA Direct-Drive Cryogenic Implosions with the Highest Fusion Yield

A recent optimization experimental campaign on the OMEGA laser resulted in the highest fusion yield (\textasciitilde 1.6\texttimes 10$^{\mathrm{14}}$ neutrons) so far while achieving an areal density of \textasciitilde 160 mg/cm$^{\mathrm{2}}$ in cryogenic DT direct-drive implosions. One-dimensional hydrodynamic simulations overpredict the measured performance of these implosions, suggesting that various low- and high-mode asymmetries in imploding targets can be important. The effects of these asymmetries were studied using 3-D hydrodynamic simulations with the code \textit{ASTER}. Simulations assumed the following sources of asymmetries with measured or estimated magnitudes: laser-power imbalance, laser beam mispointing and mistiming, laser imprint, target offset, and target defects. Simulations suggest that an integral effect of these sources can explain the difference between the measured and predicted in 1-D performances of OMEGA implosions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Indirectly-driven ICF implosions in Advanced Hohlraums on the National Ignition Facility

New advanced hohlraum concepts, the I-Raum [1] and the Frustraum, has been experimentally tested on the NIF. I-Raum results show enhanced inner beam propagation compared to a typical cylindrical hohlraum. This enhanced propagation is achieved by recessing the location where the outer beam cones hit the hohlraum wall. This target modification delays when the Au wall material, driven by the outer beam cones, obstructs the inner beam and reduces propagation. Initial subscale Frustraum experiments have also been completed showing increased capsule coupling efficiency compared to a cylinder, but challenging symmetry control. X-ray images of the Au wall motion and measurements of the shape of the imploded capsule have been measured for each advanced hohlraum concepts and are compared to cylinder results. [1] H. F. Robey et al., ``The I-Raum: A new shaped hohlraum for improved inner beam propagation in indirectly-driven ICF implosions on the National Ignition Facility'', Phys Plasmas 25, 012711 (2018).   

Numerical Investigation of Shock-Release OMEGA EP Experiments

Release of shocked material from the inner side of the shell after the shock breakout is an important process in an inertial confinement fusion implosion that affects formation of the hot spot and implosion performance. Experiments on OMEGA EP at the Laboratory for Laser Energetics used the 4$\omega $ interferometry to measure the low-density profile of the plasma in the rarefaction wave that follows the shock breakout from the back side of a CH shell driven by two OMEGA EP beams. The shell trajectory was measured using x-ray radiography. Results of radiation-hydrodynamic code \textit{DRACO} simulations of the evolution of the density profile in the rarefaction wave will be presented and compared with experimental data. Sensitivity of the density profile to the equation of state, electron thermal transport, and other 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.   

Cross-Beam Energy Transfer in Offset Implosions on OMEGA

It has been shown\footnote{ K. S. Anderson \textit{et al.}, ``Effect of Cross-Beam Energy Transfer on Target Offset Asymmetry in Directly-Driven Inertial Confinement Fusion Implosions,'' submitted to Physical Review Letters.} that cross-beam energy transfer (CBET) mitigates the detrimental effect of initial target offset on implosion symmetry and fusion yield in room-temperature inertial confinement fusion implosions. This work was motivated by previous discrepancies in fusion yield between simulations and experiments with large target offsets. It was shown that simulations agreed better with experimental observables when CBET physics was modeled. This talk will expand the previous work to include cryogenic implosions and room-temperature implosions with higher laser intensity (\textgreater 10$^{\mathrm{15}}$ W/cm$^{\mathrm{2}})$. Simulated and experimental observables will be compared. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

\textbf{Pulse Designs Varying Hot-Electron Production for Direct-Drive Inertial Confinement Fusion Implosions OMEGA Utilizing the SG5-650 Phase Plates}

New ``SG5-650'' phase plates ($R_{\mathrm{95}}$~$=$~325 $\mu $m) to complement the ``SG5-850's'' (R$_{\mathrm{95}}$~$=$~425~$\mu $m) have been fielded on OMEGA. The SG5-650 phase plates allow for the reduction of cross-beam energy transfer (CBET) effects by decreasing the R$_{\mathrm{beam}}$/R$_{\mathrm{target}}$. However, preheating from increased hot-electron production can occur as the net overlapped intensity increases. To experimentally evaluate this trade-off's impact on cryogenic implosion performance, pulse shapes were designed that gave approximately equal 1-D performance but the main drive power history was adjusted to provide different quarter-critical intensity levels and therefore different levels of hot-electron production. Hot-electron production levels were experimentally verified by using the hard x-ray diagnostic when shooting the pulses on warm plastic targets. In a future experiment utilizing the SG5-650 phase plates, an intensity scan will be used to study the effect of preheating on a 0.8x hydro scale of the best-performing implosion that utilized the SG5-850 phase plates.   

Hot-electron preheat and energy deposition in direct-drive implosion experiments at the National Ignition Facility

Laser--plasma instabilities can degrade the performance of direct-drive inertial confinement fusion implosions by generating hot electrons that preheat the target. To assess the extent of hot-electron preheat in polar-direct-drive implosions an experimental platform at the National Ignition Facility has been developed and fielded to study the hot-electron energy deposition in an unablated shell. The target consists of an outer plastic ablator and an inner Ge-doped plastic layer (payload). Hot-electron transport and energy deposition in the imploded shell is studied by comparing hard x-ray production between the mass-equivalent plastic and multilayer implosions. The experiments demonstrate how the divergence of hot electrons and the extent to which they slow down in the ablator reduce the preheat. Measurements indicate that 0.28$\pm $0.05{\%} of laser energy is deposited in the unablated shell, with 0.1$\pm $0.04{\%} deposited in the outer 20{\%} portion and 0.18$\pm $0.03{\%} deposited in the inner 80{\%} of the imploding shell. This platform will be used to study hot-electron preheat mitigation using buried mid-$Z$ layers in the ablator.   

Hot Electron Generation Mechanisms in Ignition-Scale Direct-Drive Coronal Plasmas on the NIF

Planar experiments at the NIF have diagnosed laser-plasma interactions and hot-electron production at plasma conditions uniquely relevant to direct-drive ignition designs, in which hot electrons could potentially preheat the capsule. Stimulated Raman scattering (SRS) is observed at the quarter-critical density and at lower densities. Comparison of hard x-ray and SRS signatures indicates a correlation between underdense SRS and hot electron generation, though the saturated absolute SRS instability at quarter-critical may contribute as well. Measurements of 3$\omega $/2 emission provide information about SRS and two-plasmon decay plasma waves near quarter-critical. LPSE and PIC modeling support interpretation of the experimental findings. These results are used to assess where in the density profile and through which physical mechanisms hot electrons are produced, which guides hot-electron preheat mitigation strategies for direct-drive--ignition designs. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

\textbf{Mass and charge dependence of ion shock coupling and thermal equilibration in ICF shock phase}

During the shock-convergence phase of ICF implosions there are steep spatial gradients and the ion mean free path becomes long compared to the system size, indicating that multi-ion and kinetic effects may be important. It has been previously indicated that there is substantial thermal decoupling and possibly other kinetic effects in D$^{\mathrm{3}}$He plasmas with conditions relevant to the NIF shock-phase. In this presentation I will show recent work conducted on the Omega laser facility recreating these conditions in DT plasmas. Results indicate a system that is better captured by average-ion hydrodynamic simulations than the D$^{\mathrm{3}}$He case. We are working to understand this behavior. Combined DT and D3He burn averaged observables are consistent with an equilibrating two-temperature model. This has major implications for our understanding of kinetic and multi-ion plasma physics and our modeling of ICF implosions. This work was supported in part by the U.S. DOE, NLUF and LLE.   

A short-pulse plastic-ablator implosion design for ignition on the Laser Mega Joule facility

Recent work on ICF implosions at the National Ignition Facility (NIF) has focused on key optimizations relevant to plastic and high-density-carbon implosions. The final commissioning of the LMJ facility will use 40 quads to operate at full capacity. Because of inherent technical constraints, such as lower maximum energy and power, different laser beam balance and smaller phase plates than on NIF, a different configuration has to be investigated to reach ignition. Integrated 2D simulations have been performed using the TROLL radiation hydrodynamics code to guide for the first time a short-pulse plastic-ablator low-fill hohlraum design, given the laser energy/power available on LMJ. The trade-off between the P2 asymmetry, late-time inner beam propagation and the achievable peak velocity and performance is used to determine the optimal hohlraum shape, case-to-capsule ratio and laser pulse features (picket energy, foot drive and pulse length). These key parameters are also compared to recent NIF designs to assess the full potential of the LMJ design.