Session NO07: ICF: Compression and Burn II

Record Pressures in OMEGA Cryogenic Direct-Drive Experiments via Subcooling

Achieving high pressures over 100 Gbar has been a long-standing objective for the cryogenic direct-drive fusion program. High pressures are challenging for ICF implosions as they require highly convergent implosions, which in typical cryogenic designs require a reduction in the fuel adiabat. In practice, the performance of low adiabat implosions fails to live up to theoretical expectations and has substantially lower hot-spot pressures, likely due to shock mistiming and the growth of hydrodynamic instabilities. We report on experiments that produce record inferred hot-spot pressures using cryogenic sub-cooling to reduce the vapor density within the cryogenic target, allowing the convergence ratio to increase independently from the adiabat. We describe also the two temperature, non-isobaric hotspot model used to reconstruct the pressure of implosion experiments from a number of neutron and x-ray diagnostics, and which has been benchmarked against simulations, and show that the pressures of subcooled implosions are increased substantially compared to standard implosions. We find also that the yields and areal densities of subcooled implosions are well predicted by statistical models trained only on standard cryogenic experiments, which suggests that the convergence ratio dependence of the statistical model accounts for the effect of hydrodynamic instabilities.   

Statistical modeling of the fusion yield and areal density in DT-layered implosions on OMEGA

In recent years, the application of statistical modeling has enabled development of accurate predictive models of fusion yield in direct-drive inertial confinement fusion experiments on OMEGA. Through the choice of separable basis functions and a careful selection of parameters guided by physical considerations, the statistical model has been used to quantify the effect of all the major sources of fusion yield degradation on OMEGA. The strongest dependencies include the age of the deuterium tritium fuel from filling to shot time, the asymmetry of inferred ion temperatures as a proxy of the L=1 mode, the ratio of the laser beam radius to target radius as a proxy of the illumination nonuniformity of overlapping beams, and parameters related to the hydrodynamic stability such as the shell adiabat, in-flight aspect ratio and convergence ratio. Here, we present statistical models of the areal density, ion temperature and fusion yield trained on a large database of OMEGA implosions. It is shown that a reasonably accurate predictive capability can be developed enabling the design of DT-layered implosions with incrementally higher performance.   

The Impact of Target-Layer Imperfections on Performance Sensitivity to Laser Imprint for Direct-Drive Inertial Confinement Fusion (DDICF) Implosions on OMEGA.

Recent experimental results showed that the highest performing class of DDICF implosions on OMEGA at adiabat ~4.5 are not dominated by degradation from laser imprint¹, but are significant at adiabat~3.5 .  This was shown by measuring changes in neutron yield and neutron-averaged areal density (ρR) to varying levels of laser spot smoothing by spectral dispersion (SSD) and observing performance saturation for sufficiently high SSD bandwidth. We report on a 2-D simulation study with DRACO² seeking to reproduce these trends by introducing target imperfections via a surface-roughness spectrum, in addition to laser imprint.  While previous DRACO simulation results with laser imprinting alone did not show performance saturation at the SSD levels observed, a mechanism to reproduce it could be done through an additional, high-mode perturbation source that is independent of SSD. The perturbation added is characterized by <1 um amplitude, high-mode imperfections (l=60 to 120 spectrum suggested by 2D VISAR measurements³) at the fuel-ablator interface, in addition to a NIF standard surface roughness spectrum. The results from this study are valuable for determining what perturbation seeds need to be included in simulations in order to create scientifically credible performance extrapolations for future target designs.    

Mitigation of Low-Mode Perturbations with 3-D Hot-Spot X-Ray Emission Tomography on Laser-Direct-Drive Implosions on OMEGA

A demonstration of 3-D hot-spot x-ray emission tomography on OMEGA was conducted to diagnose and mitigate low-mode perturbations on spherically symmetric laser-direct-drive inertial confinement fusion implosions [K. Churnetski et al., Rev. Sci. Instrum. 93, 093530 (2022), K. M. Woo et al., Phys. Plasmas 29, 082705 (2022)]. This 3-D hot-spot x-ray emission tomography technique was initially developed on polar-direct-drive implosions [K. Churnetski et al., Rev. Sci. Instrum. 93, 093530 (2022)]. Mode-1 and -2 asymmetries were separately seeded in the initial targets by either offsetting spherically symmetric targets from target chamber center (mode 1) or by varying the mass distribution in the plastic targets to create a mode-2 asymmetry. The on-target spatial distribution of laser energy was iteratively varied to mitigate the hot-spot shape asymmetries. A mapping relation of hot-spot shape asymmetries as a function of laser-energy corrections will be presented. The upper limit of the spherical harmonics modes that can be resolved with the current x-ray diagnostic capabilities and with improved time-resolved diagnostics will be discussed. This limit was identified using single-mode perturbation simulations produced by the radiation-hydrodynamic code DEC3D [K. M. Woo et al., Phys. Plasmas 25, 052704 (2018)].   

Suggestions of Instability Driven Mass Injection in Direct-Drive Cryogenic Implosion Experiments

We show that distinct signatures in both the acceleration and stagnation phases of direct-drive cryogenic implosion experiments are consistent with 3-D modeling that induces the jetting of a peripheral mass into the nascent hot spot. The cause of such jetting remains speculative, but a candidate is the role of intrinsic target features. The signatures arise due to the injection of dense fuel and ablator material into the low-density center during early stages of hot-spot formation. First, we show that the defect modeling creates an advance of hot-spot emission during the implosion history, such as has been observed and which is not accounted for by the model with imprinting. Second, we have found a reduction of the discrepancy when the ablation front is separated from the hot spot by increased cryogenic payload, providing strong support for an origin in ablation front instability. Lastly, we show that measurements used to infer ion and electron temperatures indicate an equilibration exceeding 1-D models, an expected result of the mass injection.    

Evidence of Increased Laser Energy Coupling with Pure Silicon Ablators in Laser Direct-Drive Inertial Confinement Fusion Implosions

Hydrodynamic simulations with high-Z ablators indicate a uniform increase absorption that would lead to higher implosion velocities in laser-direct-drive (LDD) inertial confinement fusion (ICF) experiments. An increase in implosion velocity achieved exclusively by replacing the ablator (without having to intentionally adjust the IFAR or adiabat) could lead to higher primary nuclear yields in cryogenic implosions at the expense of additional radiation preheat by introducing a high-Z material. One candidate that has been studied recently is a pure Silicon 2-μm thick ablator where simulations do exhibit more laser energy absorption without an increase on hot-electron production. Preliminary experimental results will be shown that an increase in laser absorption and decrease in hot-electron production has been observed generating a higher yield-over-clean (YOC) as compared to tradition plastic CH ablators with a small fraction (2% and 6% atomic fraction) of Silicon. Simulations to explore an optimal high-Z dopant with minimal radiation preheat and increased implosion velocity will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Effects of self-generated magnetic fields on the stagnation phase of cryogenic direct-drive implosions and scaling to direct-drive NIF

In this talk, the magnitude and evolution of self-generated magnetic fields in an Omega implosion is studied using the deceleration-phase code DEC2D. DEC2D simulates the stagnation-phase of implosions using an Eulerian moving mesh solver and models the growth of the Rayleigh-Taylor instability at the shell hot-spot interface. This instability can cause temperature and density gradients to misalign, generating Biermann battery magnetic fields. In order to model these magnetic fields, MHD solvers have been added in DEC2D: this includes advection, resistive diffusion, Nernst, thermal suppression, and the Righi-Leduc effect. Maximum magnetic fields of ~200 MG are estimated, with corresponding Hall parameters of  ~1.5. In direct-drive implosions, we find that when Biermann fields are included, the Righi-Leduc effect dominates the MHD terms and causes the yield to decrease by  ~2.5%.  Additionally, preliminary results suggest scaling the implosion to a direct-drive 2 MJ implosion leads to far more significant decreases in yield, due to the alpha-heating magnifying small changes in temperature.   

Use of burn-through membranes for laser imprint mitigation*

Our previous experiments have found that high-Z coating is highly effective at reducing laser imprint when it is pre-expanded. On Nike KrF laser (λ=248nm) at NRL this is accomplished using a smoothed, low intensity laser prepulse [Karasik et al, PRL. 114, 085001 (2015)]  . On Omega EP, we used an externally generated low level soft x-ray prepulse of ~10 J/cm2 to accomplish the pre-expansion due to lack of suitable laser prepulse [Karasik et al., Phys. Plasmas 28, 032710 (2021) ] . The prepulse was generated using an x-ray converter foil(s) with additional beam(s). We are now able to generate the required prepulse using a burn-through membrane in the path of the drive beams in order to avoid use of auxiliary targets and beams. In this approach, the laser first interacts with the thin Au-coated membrane generating a prepulse on the inner target; after which the membrane and the coating are given time to expand. The membrane becomes under dense and transmits the subsequent laser pulse which then drives the inner target. Measurements of the membrane and coating expansion using 4ω probe, soft x-ray emission using an NRL transmission grating spectrometer, as well as areal mass radiography demonstrating successful imprint mitigation will be presented.    

Foam-Supported Hybrid Shock Drive for Low-Adiabat Direct-Drive Fusion Experiments at the Omega Laser Facility

Improving the performance of DT-layered laser direct-drive (LDD) fusion experiments hinges upon the growth of Rayleigh–Taylor (RT) hydrodynamic instabilities, which compromise the integrity of the imploding target. An important RT seed in LDD fusion implosions is laser imprint, which imparts high-ℓ-mode perturbations on the target. These grow rapidly with convergence increasing the adiabat floor at which a target can be driven. Presently, high-performance LDD implosions suppress the growth of laser imprint by imploding high-adiabat (α > 4) targets. This, however, does not eliminate the source of the imprint, and limits the maximum realizable yield, ρR, and Lawson parameter. One proposed method to effectively mitigate imprint is the hybrid shock drive. The concept indirectly drives the target with x rays via a burn-through converter foil and then switches to a direct-illumination scheme once a plasma has been established and high-ℓ-mode perturbations from the laser have been conductively smoothed. Our design simplifies engineering challenges found in other hybrid proposals by supporting the high-Z converter foil on top of an ultralow-density foam layer instead of the typical vacuum standoff. The proposed platform offers a more compact and robust assembly comparing to alternative hybrid designs, while simplifying fabrication, enabling permeation filling, and overall providing a pathway to field low-adiabat cryogenic hybrid implosions on OMEGA. Planar experiments on OMEGA EP are planned which will quantify the efficacy of this novel design.   

Computational Study of Thermonuclear Burn Propagation into a Liquid DT Wetted Foam Layer

Wetted foam ICF targets employ a low-density CH foam or a 3D printed lattice to support a spherical shell of DT liquid¹.  Wetted foam ICF experiments² have shown that a mixed EOS must be used to accurately simulate the implosion results.  This is largely due to the fact that a DT+CH mixed EOS is less compressible than pure DT.  In the present study, we explore an important additional feature of the wetted foam approach to ICF – the delay of burn propagation caused by the presence of CH in the dense DT layer at the time of ignition.  Three independent radiation-hydrodynamics codes – xRAGE, HYDRA, and LILAC -- using a variety of physics and burn models have been used in the study.  The overall conclusion is that, although ignition is delayed and burn propagation is slowed compared to a pure DT layer, it is feasible to obtain high fusion gain with a wetted foam target. 

¹R. E. Olson et al., Phys. Plasmas 28, 122704 (2021).

²R. E. Olson et al., Phys. Rev. Lett. 117, 245001 (2016).   

3D simulations of polar direct drive wetted-foam capsules

We investigate the feasibility to achieve high nuclear yields using a liquid DT-wetted layer capsule directly driven by the National Ignition Facility’s (NIF’s) current laser capabilities. The capsule is composed of a thin plastic shell used to enclose a thick annular 3D-printed matrix layer that contains the liquid DT fuel. Comparisons across several simulation codes indicate that a high level of laser absorption can occur that drives a central gas pocket convergence of 15 enabling higher levels of gain and the potential to robustly ignite (using the current laser energy available at NIF). High laser absorption is consistent with previous polar direct drive (PDD) MJ-class NIF experiments where >95% capsule absorption of the laser drive energy was achieved using a 5 mm diameter plastic capsule. The results of simulations using the HYDRA radiation-hydrodynamics code will be shown to elucidate the laser driven 3D asymmetries in the implosion of these capsules. These asymmetries arise both in polar angle (caused by the variation in laser polar angles) and in azimuthal angle (owing to the differences in laser azimuthal incidence angles) dominated by an M4 perturbation near the pole and an M8 perturbation near the equator.  Fabrication efforts using 3D printing techniques are currently underway to construct the hybrid capsule for initial NIF experiments later this year. An overview of these multi-Laboratory efforts will be presented.

Hot electron preheat mitigation in ignition-scale direct-drive ICF implosions on the NIF

Hot electron preheat and its mitigation using mid-Z dopants has been quantified in ignition-scale polar direct-drive (PDD) implosion on the National Ignition Facility (NIF). Hard x-ray emission from buried Ge-doped layers was measured in NIF implosions of 2.3 mm CH shells at 730 and 850 kJ (intensities 1 to 1.25x10¹⁵ W/cm²), some of which contained Si dopant in the outer portion of the ablator. The Si-doped ablators were found to reduce stimulated Raman scattering (SRS) and consequently the hot electron energy deposition in the unablated shell by 30%, close to levels (~0.2% of laser energy) considered tolerable for direct drive. The extension of these experiments to ignition-relevant 3.0-mm, 1.3-MJ CH implosions and the extrapolation of these results to cryogenic DT designs will be discussed. These results provide a promising hot-electron preheat mitigation strategy that can expand the operable design space to higher intensities and allow for acceptable preheat levels in cryogenic DT implosions at intensities around 10¹⁵ W/cm². This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Comparison of Simulations to Measurements in Direct-Drive Energy Coupling Experiments on the National Ignition Facility

Energy coupling is a critical determinant of implosion performance. In this talk, results from experiments that systematically study laser drive during different times of the laser pulse on the NIF are presented. Shock radiography using solid spheres is used to infer early-time coupling during the foot and rise to the main pulse. Since shocks decouple from the laser drive shortly after peak intensity, studying shock trajectories isolates the effect of laser drive during this time. Simulations that include the effects of non-local transport and CBET reproduce these trajectories very well for varying on-target intensities, though some uncertainties remain. Implosions are sensitive to coupling throughout the drive. Self-emission and backlit implosion trajectories will be presented for varying on-target intensities. Implications for direct-drive implosion performance will be discussed.   

Using Polar Direct Drive Experiments on the National Ignition Facility to Validate 2D Radiation-Hydrodynamic Energy Coupling Models

Polar-direct-drive (PDD) solid sphere experiments using 2.1-mm diameter CH targets were performed on the National Ignition Facility (NIF) to evaluate energy-coupling models in 2-D radiation-hydrodynamics codes. Data from the Scattered-Light Time-history Diagnostic (SLTD)[1] and the Full-Aperture Backscatter Stations (FABS) were used to measure time resolved scattered light at 350 to 351 nm (Stimulated Brillouin scattering). The trajectory of converging shocks and the resulting density profile was probed by a Cu backlighter and captured by gated x-ray detectors (GXD) to yield radiographs. A multiple-shock design was used to assess sensitivity to cross-beam energy transfer (CBET) during peak power. Comparison of the scattered light distribution and inferred density profiles to the radiation-hydrodynamics code DRACO, including cross beam energy transfer (CBET) and non-local heat transport models, will be discussed.

The Impact of the Beam-to-Target Ratio in Direct-Drive DT Cryogenic Implosions on OMEGA

The performance of laser-direct-drive cryogenic DT inertial confinement fusion implosions depends strongly on the ratio between the radius of the laser beams that illuminate the shell and the target radius (R_b/R_t). The analysis of previous experiments using targets of varying radii at constant beam radius, has shown that laser imprint is most likely the dominant cause of this dependency at moderate illumination intensity (∼5×10¹⁴ W/cm²). To study the effect of target composition on the observed scaling with R_b/R_t, a new series of experiments with targets that contain 5% of Si doping in the outer ∼5 µm of the plastic shell, which contains the DT ice layer. Targets of 780-µm, 870-µm, and 1010-µm diameter were used scanning R_b/R_t from ∼1.09 to ∼0.85. A detailed comparison of the experimental observables from both neutron and x-ray detectors with the theoretical modeling will be presented.