Session BO09: ICF: Laser Direct Drive

Live

A study of laser imprint for laser direct drive is presented through measurements of the seeds of laser imprint, the associated growth rates of the hydrodynamic instabilities, and a study of the performance of imploded cryogenic DT ice and gas-filled shell targets. By varying the bandwidth on smoothing by spectral dispersion (SSD) the imprint level is varied in fine steps. The seeds were characterized using a 2-D VISAR diagnostic and compared to results from radiation-hydrodynamics simulations. The integrated experiments use measured data from nuclear, x-ray, and optical diagnostics to gauge the implosion performance versus SSD bandwidth. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

We present results of laser imprint mitigation experiments measuring shock velocity modulations induced by illumination nonuniformities of the Nike laser. 2-dimensional spatial profiles of the shock velocity fluctuations were directly measured by the high resolution 2D VISAR.$^{a,b}$ Planar CH targets with and without a thin high-Z (400A Au or 600A Pd) overcoat were irradiated by four, eight, and sixteen Nike beams overlapped to explore the imprint reduction. The uncoated target experiment confirmed that the velocity perturbations decreased with an increasing number of laser beams, precisely as anticipated by the beam averaging effect on laser imprint. The coated experiment observed the shock velocity fluctuations were significantly suppressed by a factor of 3--6, compared to their counterparts in the uncoated experiment. The experimental results are being compared with 3D radiation-hydrodynamics simulations of laser imprint. $^a$ P.M. Celliers, et al., Rev. Sci. Instrum. 81, 035101 (2010). $^b$ J. Oh, et al., Bull. Am. Phys. Soc., 6(11), GP11.119 (2018).   

Live

Efficient conversion of the shell kinetic energy to the hot-spot thermal energy is an essential requirement in inertial confinement fusion implosions. The hydrodynamic profile of the fusing deuterium--tritium (DT) plasma will result in a measureable difference in the DT and DD average temperatures. Additionally, a deviation from the DT and DD plasma temperature inferred from the second moment has been suggested as one method used to infer the residual kinetic energy (RKE) in the fusion plasma. An advanced forward-fit technique is used to interpret the spectral moments of the neutron energy distribution emitted from a fusing plasma along two lines of sight. Presented here are measurements of the DT and DD temperature from DT cryogenic drive-drive implosions. The contribution of RKE in these implosions is extracted with the consideration of the role of temperature gradient present in a 1-D model. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

The design of inertial confinement fusion (ICF) ignition targets requires radiation-hydrodynamics simulations with accurate models of the fundamental material properties (i.e., equation of state, opacity, and conductivity). A feasibility study of using spatially-integrated, spectrally-resolved, X-ray Thomson scattering (XRTS) measurements to diagnose the temperature, density, and ionization of the compressed DT shell and hot spot of a laser-direct-drive implosion at stagnation was conducted. Synthetic scattering spectra were generated using 1-D implosion simulations from the LILAC code that were post processed with the X-ray Scattering (XRS) code. The optimal configuration for simultaneous collective and non-collective scattering measurements to diagnose the different regions of the stagnated plasma will be presented.   

Live

The hot-spot--ignition concept in inertial confinement fusion utilizes laser-driven implosions of spherical shell targets with DT ice as a fuel. Estimates of the physical conditions before and during the formation of the center hot spot in OMEGA-scale implosions reveal that the Knudson number can approach unity in the low-density interior of targets, indicating the potential importance of kinetic effects. To investigate these effects, cryogenic OMEGA implosions were simulated using the 3D hydrodynamic code ASTER, which includes the ion viscosity model assuming the Spitzer ion free path. The dependences of simulations results on the exact implementation of the viscosity model, including the effects of momentum and heat-flux limitations and using the energy conservation scheme, are studied. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

In laser-direct-drive (LDD) inertial confinement fusion (ICF) experiments, the hydrodynamic instabilities seeded by laser imprint and target features (e.g., microscopic surface debris, fill tube or stalk) can increase the in-flight shell thickness (i.e., decompress the shell) during the acceleration phase. Signatures from self-emission X rays versus the ablation front are investigated to diagnose the cryogenic layer, similar to what was done on warm LDD implosions of gas-filled, plastic shell targets [D.T. Michel et al., Phys. Rev. E 95, 051202(R) (2017)]. The feasibility of extending a diagnostic technique to obtain the in-flight shell thickness measurements of an LDD ICF DT cryogenic implosion from the spatial distribution of the X-ray emission will be presented for a range of adiabat, between 2 and 5. The shell trajectories are inferred by comparing the hydrodynamic profiles of the target with self-emission profiles obtained from a radiative transport post-processor, including the instrument response function of the X-ray imager.   

Live

The scaling of hot-electron preheat with capsule size or laser energy has been studied in warm polar-direct-drive implosions at the National Ignition Facility (NIF) and OMEGA. The experiments were designed to produce hydrodynamically equivalent implosion conditions despite differences of a factor of 3.4 in capsule diameter and 40 in laser energy (2.3 mm, 720 kJ on the NIF; 0.69 mm, 18 kJ on OMEGA). Hard x-ray emission from Ge-doped layers was used to infer the hot-electron energy deposited in the unablated shell. Although the hot-electron mechanism is different at each scale---two-plasmon decay on OMEGA and stimulated Raman scattering on the NIF---both experiments demonstrate 0.2{\%} of laser energy deposited as hot-electron preheat in the inner 80{\%} of unablated shell at a hard-sphere intensity of 1.2 \texttimes 10$^{\mathrm{15}}$ W/cm$^{\mathrm{2}}$ despite more hot-electron generation on the NIF. This result is partially attributed to electron stopping in the thicker shell on the NIF. Implications for scaling of direct-drive cryogenic implosion performance on OMEGA to NIF scales 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

Understanding of hydrodynamic scaling is necessary to assess the relative performance of OMEGA cryogenic implosions. We present the results of 1-D radiation-hydrodynamic code \textit{LILAC} simulations, performed to study the hydrodynamic scaling of OMEGA cryogenic implosions in the absence of alpha heating. Scaled implosions were simulated only for the deceleration phase to maintain identical energy coupling and to have an ensemble of simulations. The no-alpha yield scaling with size was found to be dependent on implosion time, defined as $R$/$V_{\mathrm{i}}$. We show that this is a direct consequence of the physics of electron--ion temperature equilibration, which does not hydro scale. The no-alpha scaling exponent for yield varies between 4 for very fast implosions ($V_{\mathrm{i}}$ \textgreater 500 km/s) and 4.3 for slower implosions ($V_{\mathrm{i}}$ \textless 350 km/s) Additionally, the 2-D radiation hydrodynamic code \textit{DEC2D} was used to study hydrodynamic scaling for nonuniform implosions for shot 90288. We show that, as a consequence of non-scaling physics of thermal conduction, implosions degraded by mid-modes (\textunderscore $=$ 12) show a worse scaling with size than when degraded by a low mode (\textunderscore $=$ 2,4). This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

Recent progress in direct-drive inertial confinement fusion has considerably improved the prospects for achieving thermonuclear ignition with megajoule-class lasers. When hydrodynamically scaled to laser energies on the National Ignition Facility (NIF) and symmetric illumination, recent implosions on OMEGA are expected to produce about 500 kJ of fusion yield and 75{\%} of the Lawson triple product required for ignition. Those implosions have benefited from an increase in implosion velocity obtained through larger-diameter targets and thinner ice layers. A statistical approach [V. Gopalaswamy \textit{et al.}, Nature \textbf{565}, 581 (2019)] used in designing OMEGA targets has demonstrated a considerable predictive capability, thereby enabling the design of targets with improved performance, leading to tripling the fusion yield and increasing the areal density by over 60{\%}. Systematic experiments such as scans of laser smoothing, laser beam radii, and DT fill age are used to identify the mechanisms of performance degradation and implosion optimization. Ongoing improvements in laser performance and target quality are expected to further augment implosion performance toward the goal of demonstrating core conditions that scale to ignition at NIF energies. Based on these results, a path to demonstrating hydro-equivalent ignition on OMEGA is presented This work was supported by the DOE-NNSA under Award Number DE-NA0003856   

Live

An overview of the first subscale implosions of cryogenically layered deuterium--tritium targets on OMEGA in 40-beam polar-direct-drive (PDD) and 60-beam symmetric direct-drive (SDD) configurations will be presented. Those implosions use a hydro-scaled version of the best-performing full-scale SDD implosion with a smaller target, small-spot phase plates, and about half of the laser energy. The goal is to assess the effect on implosion performance from the PDD geometry. Initial results indicate that PDD neutron yields and $\rho $R's are 45{\%} and 70{\%}, respectively, of that of companion SDD implosions. The partition of the ring energy is varied while keeping the total laser energy constant, significantly affecting the symmetry of the imploding shell and the shape of the hot spot. The statistical approach [Gopalaswamy et al., Nature 565, 581 (2019)] will be applied to optimize PDD performance by varying parameters such as DT ice thickness, target size, and ring energy partition.   

Live

Energy-coupling experiments relevant to laser-direct-drive (LDD) ignition-target designs are being conducted on the National Ignition Facility (NIF) using a spherical, solid-plastic target with a 2.1 mm diam. One hundred eighty-four NIF laser beams having total energy of 0.5 MJ irradiated the target in a Polar-Direct-Drive (PDD) geometry with a peak intensity of 8x10$^{\mathrm{14}}$ W/cm$^{\mathrm{2}}$. The trajectory of the spherically-converging shock wave was recorded using gated, x-ray backlighting at 8.4-keV. Solid spheres offer the advantage for quantifying energy coupling without the challenges from hydrodynamic instabilities of thin shell implosions or kinetic effects in exploding pushers. Initial shock-trajectory measurements will be presented and compared with 2-D \textit{DRACO} radiation-hydrodynamics simulations using CBET and nonlocal heat-transport models. The overarching goal is to test the scaling arguments of PDD implosions from the 20-kJ OMEGA (configured for PDD) to the 2.1 MJ NIF.   

Live

Laser--plasma instabilities (LPI's) 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, polar-direct-drive experiments have been performed at the National Ignition Facility to study the hot-electron energy deposition in an unablated shell. The experiments employed mass-equivalent plastic targets with Ge-doped layers to measure the radial energy deposition profile of hot electrons in the unablated capsule. Hot-electron properties and energy deposition have been inferred through comparisons of hard x-ray spectra to simulations of electron transport and x-ray generation. Recent experiments used thin silicon layers in the outer portion of the ablators, designed to pass through the quarter-critical region during the period of hot-electron generation in order to suppress LPI. Analysis indicates a reduction in hot-electron generation by a factor of $\sim $2, showing promise as a preheat-mitigation strategy that can expand the ignition-design space to higher intensity.   

Live

In laser-driven cryogenic spherical implosions, the density of the imploding DT shell during deceleration determines its dynamic pressure and consequently the hot-spot stagnation pressure. Hydrodynamic instability growth during the acceleration phase will decompress the shell, thereby compromising performance. Relaxed density profiles which result from decompressed shells will increase early hot-spot x-ray emission, thus providing an experimental signature for DT cryogenic implosions. We present the use of time-resolved images to characterize the onset of hot-spot emission and infer in-flight decompression at the start of deceleration in direct-drive DT cryogenic implosions. The hot-spot emission is observed to begin at a larger shell radius as compared to a 1-D radiation-hydrodynamic implosion model. A 3-D radiation-hydrodynamic code that includes a model for laser-speckle induced perturbations agrees with observation when imprint growth is severe. However, discrepancies of the emission onset between measurement and models persist even when the imprint is predicted to be minor and its inclusion in the model does not account for the observation. An inverse dependence of the emission discrepancy with implosion stability suggests the presence of additional hydrodynamic perturbations in the direct-drive DT cryogenic implosions.   

Live

The application of laser direct drive (LDD) research to ignition on the OMEGA laser depends on the degree to which implosions can be scaled in size (and energy) and use different beam geometries. In this talk we present an analysis of cryogenic experiments at two hydrodynamic scales ($S \quad =$ 0.8 and 1.0) [Ref. \footnote{ V. Gopalaswamy \textit{et al.}, Nature \textbf{565}, 581 (2019).}] and find that yield increases as $S^{\mathrm{4}}$ or faster after correcting for shot-to-shot sources of asymmetry (e.g., the laser pointing). These data also indicate a potential benefit in areal density (in excess of expectations) as targets become larger/thicker. This behavior could be due to changes in preheat or instability/mix (e.g., the capsule stalk) and will be discussed. We are also planning experiments that will quantify performance versus $\eta \quad = \quad R_{\mathrm{b}}$/$R_{\mathrm{t}}$, which is the ratio of the laser beam to target radius. The goal is to change the amplitude of the applied mode 10, which has been found to correlate with yield as $\eta^{\mathrm{2}}$ to $\eta^{\mathrm{3}}$ (Ref. \footnote{ A. Lees, ``Understanding the Fusion Yield and All of Its Dependencies Using Statistical Modeling of Experimental Data,'' to be presented at the 62nd Annual Meeting of the American Physical Society Division of Plasmas Physics, Memphis, TN, 9--13 November 2020.}). Lastly, we will connect this work to studies comparing spherical direct drive and polar direct drive,\footnote{ P. B. Radha \textit{et al.}, ``Polar-Drive Cryogenic Implosions on the OMEGA Laser,'' to be presented at the 62nd Annual Meeting of the American Physical Society Division of Plasmas Physics, Memphis, TN, 9--13 November 2020.}$^{\mathrm{,}}$\footnote{ W. Theobald \textit{et al.}, ``OMEGA Cryogenic Target Implosions in Polar-Direct-Drive Beam Geometry,'' to be presented at the 62nd Annual Meeting of the American Physical Society Division of Plasmas Physics, Memphis, TN, 9--13 November 2020.} and consider implications to LDD research on OMEGA and the National Ignition Facility.   

Live

The first set of cryogenic PDD implosions on the OMEGA laser are described. The goal of these implosions is to identify and validate techniques to achieve comparable performance to the long-standing spherical direct-drive cryogenic campaign on OMEGA. Trends in the shape of the imploding shell were studied in the first set of PDD implosions by varying beam energies for specific OMEGA beams. Hot-spot images measured using x rays indicate the expected change in shape from prolate to oblate as the energy of the beams closer to the equator is decreased relative to that of the beams near the poles. The reduction in yield in these implosions is comparable to those in previous room-temperature implosions. Experimental observations will be compared with simulations. The path forward is described.