Session YO5: ICF: Compression and Burn II

Influence of In-Flight Shape on Stagnation Performance in Direct-Drive Laser Implosion Experiments

In laser-driven implosions modest input (drive) pressure is amplified $\sim$1000$\times$ by spherical convergence to reach fusion conditions. The impact of deviation from sphericity (typically described in terms of spherical harmonic decomposition) will depend on the presence of other performance limitations. Results are presented of ambient implosions conducted with a range of on-target laser fluence asymmetries, which were quantified by a 3-D measurement of the in-flight symmetry. The measured yield was observed to correlate to these input symmetry changes with $\sim$2$\times$ drop over $\sim$3\% $\ell$=1 asymmetry of the on-target laser fluence. The implosion experiments are modeled with 3-D radiation-hydrodynamic simulations which include both shape asymmetry and laser imprint. The agreement of yield between the modeling and experiment falls from 70\% to 40\% as the symmetry improves. The smaller discrepancy for the larger symmetry suggests a saturation of performance limiters present in the integrated experiment.   

\textbf{Hot-Spot Flow Velocity in Laser-Direct-Drive Inertial Confinement Fusion Implosions}

Multidimensional effects on the hot-spot formation of laser-direct-drive inertial confinement fusion implosions can lead to an anisotropic hot-spot flow velocity and incomplete stagnation. The first and second moments of the primary DT fusion neutron peak, recorded with four neutron time-of-flight detectors positioned around the target chamber in quasi-orthogonal diagnostic lines of sight, are analyzed to infer the hot-spot flow velocity and the ion temperature of implosions on the 60-beam, 30-kJ, 351-nm OMEGA laser. The possible physical degradation mechanisms (e.g., initial target offset, target stalk orientation, and nonuniformities in the target and the laser drive) leading to a hot-spot flow velocity in implosions of DT cryogenic targets and DT gas-filled plastic shells will be presented. Correlations of the hot-spot flow velocity with gated x-ray images of the hot spot recorded along multiple lines of sight and of in-flight asymmetries of the imploding shell will be examined. A hot-spot flow velocity of 50 to 150 km/s is observed to point in the direction of the maximum inferred ion temperature. This material is based upon work supported by the DOE National Nuclear Security Administration under Award Number DE-NA0003856.   

Understanding Laser-Imprint Effects on Cryogenic DT Implosions on OMEGA

Direct measurements of the disruption caused by laser-imprint--seeded Rayleigh--Taylor instability growth to the imploding shell and hot-spot formation can provide a clear picture of how laser nonuniformities cause target performance to degrade in direct-drive implosions. Such detailed studies on warm target implosions\footnote{ S. X. Hu \textit{et al.}, Phys. Plasmas \textbf{23}, 102701 (2016).}$^{\mathrm{,}}$\footnote{ D. T. Michel \textit{et al.}, Phys Rev. E \textbf{95}, 051202(R) (2017).} have previously been conducted by using the x-ray self-emission imaging technique developed at LLE. An experimental campaign has been dedicated to applying this technique to measure laser{\-}imprint effects on cryogenic DT implosions on OMEGA. The x-ray emission measurements with framing cameras provided clean images of both the ablation-front location and the lighting-up hot spot as a function of time, which have been compared with state-of-the-art \textit{DRACO} simulations. By varying the DT shell adiabat from $\alpha $~$=$~4 to $\alpha $~$=$~2.5, a systematic examination of the laser-imprint effects on the DT shell integrity before stagnation was performed. Comparisons between experiments and simulations will be presented and discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Effect of Laser Bandwidth on High-Performance, Cryogenic Implosions

The goal of the high-performance cryogenic implosions on the OMEGA laser is to find the largest generalized Lawson criterion\footnote{ R. Betti \textit{et al.}, Phys. Rev. Lett. \textbf{114}, 255003 (2015).} that is accessible with 30-kJ, direct-drive implosions ($\chi_{\mathrm{no}}_{\alpha }$ OMEGA). $\chi _{\mathrm{no}}_{\alpha }$ OMEGA can then be used to extrapolate to the laser{\-}driver parameters needed for ignition. A study of implosion limitations is needed to meet the above goal. The bandwidth imposed by Smoothing by Spectral Dispersion (SSD) was varied to change the laser imprint on target implosions. Neutron yield ($Y_{\mathrm{n}})$ and areal density ($\rho R)$ were used to measure the implosion performance. $Y_{\mathrm{n}}$ and $\rho R$ data clearly show an SSD dependence that increases as bandwidth increases then plateaus at the higher values of bandwidth. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE{\-}NA0003856.   

Self-Radiography of Imploded Shells on OMEGA Based on Additive-Free Multi-Monochromatic Continuum Spectral Analysis

Radiographs of pure-DT cryogenic imploding shells will help to validate progress toward ignition-scalable performance of inertial confinement fusion implosions. These can be radiographed with the core spectral emission near \textit{hv}~$\approx $ 2 keV, based on hydrogen continuum emissivity and opacity, without spectral additives, as used in previous applications of implosion self-radiography. A demonstration of this concept of additive-free self-radiography based on continuum spectroscopy has been attempted at higher spectral energy (\textit{hv}~$\approx $~3 to 5 keV) with warm CH shell implosions on OMEGA. Self-backlighting is performed near peak compression, unlike externally backlit radiography, where self-emission is a background signal that overwhelms the backlighter near peak compression. This technique is also surprisingly immune to some anticipated sources of systematic error.   

\textbf{Development of a Knock-on deuteron imaging diagnostic for cryogenic ICF implosions}

Charged particle diagnostics are important for characterizing crucial parameters for inertial confinement fusion implosions such as burn radius, fuel areal density and fusion yield. In particular, knock on deuteron spectroscopy allows for a measurement of an inferred directional fuel areal density. This work details an effort to extend this capability by developing a knock-on deuteron imager (koDI)~for use on cryogenic deuterium-tritium (DT) implosions at the OMEGA facility in Rochester, NY. Knock-on deuteron imaging provides a means to investigate and image the morphology of the high-density DT fuel region. We present the first proof-of-concept experiments to develop the~koDI~diagnostic, which is a CR-39 based penumbral imaging system. As a surrogate for cryogenic DT targets, DT filled capsules with thick CD shells were used~for this preliminary study. Three separate~koDI's~were fielded~at nearly orthogonal lines of sight in order to enable 3D reconstruction of the implosion. ~In addition, targets~were intentionally offset~on two shots to investigate if the large fuel areal density could be discerned on knock-on deuteron images. This talk presents the results of these studies and the diagnostic principles behind the~koDI~system. This work~was supported~in part by the U.S. DOE, LLE and NNSA LRGF program.   

\textbf{Inferring impact of stalk from measurements of fuel flows in OMEGA DT implosions with seeded asymmetries}

Low-mode asymmetries seed flows in the Inertial Confinement Fusion (ICF) implosions, which will manifest as modifications to the measured ion temperature ($T_{\mathrm{ion}})$ and as energy shifts of the primary neutron spectra. The effects are important to understand to mitigate asymmetries and to more closely capture thermal $T_{\mathrm{ion}}$ used in performance metric calculations. Comparison of data from a recent OMEGA DT experiment with a seeded mode 2 in the laser drive and 3D Chimera simulations not including the capsule stalk mount indicated the importance of interplay of flows seeded by various asymmetry seeds, in particular, between flows seeded by the imposed mode 2 and the stalk mount. In this presentation, results from efforts to further elucidate the impact of the stalk will be discussed, including results from 3D xRAGE simulations including the stalk mount, and results from a mode 1 experiment executed with 40-$\mu $m target offsets at various angles to the stalk; $T_{\mathrm{ion}}$ asymmetry and directional flow measurements from these experiments allow in-depth analysis of the stalk impact. The results highlight the complexity of hot-spot dynamics, which is a problem that must be mastered to achieve ICF ignition. This work was supported in part by the U.S. DOE, NLUF and LLE.   

Observed Variations in Areal Densities as Measured by Detectors Along Multiple Lines of Sight

The neutron energy spectrum generated from cryogenic deuterium--tritium inertial confinement fusion experiments is used to interpret the cold-fuel distribution at peak compression. The spatial distribution of areal density is indicative of the symmetry of the implosion, which affects the final particle yield and hydrodynamic parameters of an experiment. Various particle detectors can be used to diagnose the areal density along a given line of sight. For a more complete view of an implosion's areal density distribution, it is necessary to deploy detectors along different lines of sight. This work will show areal densities from recent cryogenic implosions as measured along several lines of sight in relation to the OMEGA laser target chamber. Measurements from a neutron detector that has been deployed along a newly constructed line of sight are included. This new detector allows for measurements of a previously unviewed region of the shell. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Robust Direct-Drive Implosions on OMEGA Scaled from Polar-Direct-Drive National Ignition Facility Implosions

High-yield polar-direct-drive National Ignition Facility (NIF) implosions are hydrodynamically scaled to OMEGA energies. The goal is to study the energetics and performance scaling between OMEGA and the NIF. For the high temperatures obtained in these types of implosions, neutron yield should hydrodynamically scale as $Y~$\textasciitilde ~$E^{\mathrm{4/3}}$, where $E$ is the laser energy. Deviations from this scaling can arise from several effects resulting from the larger NIF corona including different laser--target coupling and differing laser$-$plasma interactions such as cross-beam energy transfer. Spherical and polar{\-}drive designs will be presented including the effects of nonuniformity for OMEGA implosions. Kinetic effects on these types of implosions and their role in the scalability of these implosions will also be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE{\-}NA0003856.   

NIF Polar-Drive High DT-Yield Exploder-Pusher Designs Modeled Using Pump-Depletion in DRACO

Exploding-pushers (XP) using a 1.1-MJ pulse produced the highest polar-direct-drive (PDD) DT-yield (1.1 \texttimes 10$^{\mathrm{16}})$ on the National Ignition Facility (NIF). The NIF-PDD XP targets provide a high-yield neutron source and a platform to develop predictive inertial confinement fusion modeling. The XP designs revealed the necessity to enhance the cross{\-}beam energy transfer (CBET) algorithm to implement a scalable pump-depletion model in the 2-D code \textit{DRACO} that physically limits growth, naturally controlling CBET gain that would otherwise permit unbounded gain of the Randall formulation. The pump-depletion model accurately reproduces NIF-PDD XP implosions and serves as a design tool for enhanced performance XP designs predicted to yield \textgreater 3 \texttimes 10$^{\mathrm{16}}$ neutrons using NIF's current optics. Designs using enhanced NIF optics are predicted to produce DT-yields \textgreater 100~kJ. The CBET pump-depletion model will be described and the XP design simulations discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Large-scale implosions for Frustraum using green and blue light

A diamond-shaped hohlraum (Frustraum) proposed by Amendt[1] may provide adequate radiation symmetry for large capsules (1.5 mm radius) while requiring only about 1.8 MJ (460 TW) of laser energy (power). The advantages of large-scale capsules (LS) were earlier reported.[2] Here we present additional advantages of using LS. The high ignition margin of the LS allows: (1) high-yield implosions using shorter pulse than nominal-scales since LS can tolerate higher fuel adiabat resulted from steeper rise in Tr; (2) the use of liquid-DT foam as a viable option for high yield. The variation of YoC (yield over clean) with initial gas fill will be discussed. LS also allows DT-gas only capsules to achieve high neutron yield of 5e16. If green light is used, the capsule radius can be increased to 1.9 mm. Although the ablation front growth factor (GF) increases with capsule size, the performance is still acceptable because of the high ignition margin. To reduce the GF, we can use lower-Z ablator, e.g., Boron, to reduce the growth factor for capsule with radius larger than 1.9 mm. 1. Amendt et al., PoP 2019 accepted for publication. 2. Ho et al., APS-DPP 2018 GO6.10   

Early Prediction of the Simulation Outputs

Inertial confinement fusion (ICF) is routinely modeled using radiation-hydrodynamic computer simulations. Since such simulations are computationally expensive, researchers have made successful attempts to build surrogate models that predict simulation outputs from the inputs. Linear models fail to make accurate predictions, but neural networks have succeeded in mimicking the non-linear ICF process. In this paper, we run multiple types of simulations to investigate (i) what type of models can be used to make accurate predictions and (ii) how to build surrogate models more efficiently. Current results indicate that making predictions from time histories sampled a couple of nanoseconds before the energy production peak may allow us to use a simpler surrogate model than the model required for predicting from the simulation inputs. Leveraging this fact, we also propose an early-termination strategy for the simulations that are not bringing enough new information to the model. This strategy may potentially enable moving computational resources to more valuable simulations and building the surrogate model in a more efficient way.   

Two-dimensional kinetic simulations of hot-spot ablator mix

A kinetic Vlasov-Fokker-Planck model allows 2D simulations to assess the effect of carbon ablator mix in the ICF hot-spot. We compare the case of uniform diffusive mix with that of an equal carbon mass localized in a Rayleigh-Taylor spike. Steep temperature gradients lead to a kinetic reduction of fusion reactivity. Inflow towards the mix spike increases the alpha particle stopping power, until it acts as a barrier and heat sink for alpha particles. This makes localized mix more damaging than uniformly distributed mix. The neutron spectrum becomes broadened, with line of sight variations.   

Inference of Electron Density in the Hot Spot of NIF Compressed Capsules from Krypton Helium-$\beta $ Stark Line Shapes

The dHIRES x-ray spectrometer measures Kr He$\alpha $ and He$\beta $ spectra from NIF compressed capsules with 10-eV spectral and 30-ps temporal resolution. Theoretical calculations of the Stark-broadened line shape of the He$\beta $ complex (3 $^{\mathrm{3}}$P$_{\mathrm{1}}$, $^{\mathrm{1}}$P$_{\mathrm{1}}$, $^{\mathrm{1}}$D$_{\mathrm{2}})$ show monotonic variations with density of the line widths, line energies, and intensity of the 3 $^{\mathrm{3}}$P$_{\mathrm{1}}$ and 3 $^{\mathrm{1}}$D$_{\mathrm{2}}$ lines relative to the main, 3 $^{\mathrm{1}}$P$_{\mathrm{1}}$ peak. Comparison of the measured Kr He$\beta $ complex line profiles with the theoretical line shapes provides a measure of the time history of the electron density. These comparisons will be shown for four NIF shots with Kr-doped capsules.