Session UO4: Direct, Indirect and Polar Drive

Polar-Drive Implosions on OMEGA and the NIF

Symmetry and implosion velocity are two important aspects of polar-drive (PD) implosions that determine target performance. OMEGA and NIF PD implosions driven at ignition-relevant intensities are discussed. Nonuniformities from simulated x-ray images of the backlit imploding shell on OMEGA and of target self-emission are compared to observations. The trajectory from the self-emission region in x-ray images is also compared to simulations. The observations are generally consistent with simulations, but some differences are seen. Possible reasons for these differences are discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Initial Polar-Drive Implosions on the NIF

The polar drive (PD) concept\footnote{S. Skupsky\textit{ et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} for inertial confinement fusion is the only near-term alternative to indirect-drive ignition on the National Ignition Facility (NIF).\footnote{S. W. Haan\textit{ et al.}, Phys. Plasmas \textbf{18}, 051001 (2011).} It requires that the NIF beams be pointed toward the equator of a direct-drive capsule and that direct-drive--specific beam smoothing be installed. The first ignition-relevant PD implosions have been performed on the NIF. While the direct-drive specific beam smoothing is not yet available, these experiments test many aspects of implosion physics at ignition-relevant energies and scale lengths. This talk reports on results from these initial experiments including initial estimates of the effects of laser--plasma instabilities and the levels of hot-electron generation caused by the two-plasmon$-$decay instability. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Optimization of Azimuthal Uniformity in NIF Polar-Drive Implosions

The primary method for optimizing polar-drive experiments on the National Ignition Facility (NIF) is beam repointing in the polar direction, leading to designs that are uniform in two-dimensional, azimuthally symmetric hydrodynamic simulations. However, in some cases, azimuthal variations in the deposited energy can affect the implosion uniformity and may be observable in self-emission images. Azimuthal uniformity has been investigated using the hydrodynamics code \textit{SAGE},\footnote{R. S. Craxton and R. L. McCrory, J. Appl. Phys. \textbf{56}, 108 (1984).} which includes three-dimensional ray tracing. Optimal azimuthal adjustments to the beam pointings have been developed for the ongoing LLE polar-drive campaign\footnote{P. B. Radha\textit{ et al.}, Phys. Plasmas \textbf{20}, 056306 (2013).} on the NIF. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Three-Dimensional Modeling of the X-Ray Self-Emission Images on NIF Polar-Drive Implosions

Polar-drive experiments are being performed on the National Ignition Facility (NIF) with indirect-drive phase plates that produce beam radii smaller than the target radius at best focus.\footnote{P. B. Radha\textit{ et al.}, Phys. Plasmas \textbf{20}, 056306 (2013). } These smaller laser spots create shell nonuniformities around the equatorial region. These nonuniformities have been modeled with the hydrodynamics code \textit{SAGE},\footnote{R. S. Craxton and R. L. McCrory, J. Appl. Phys. \textbf{56}, 108 (1984).} which uses 3-D ray tracing to calculate the on-target laser intensity and estimate the azimuthal variations in ablation pressure. A new radiation transport postprocessor has been developed to use this data to calculate x-ray self-emission and the formation of x-ray images at the diagnostic plane of a framing camera. A comparison of measured, time-resolved self-emission images with images calculated from various simulations will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Evaluation of the Effects of Cross-Beam Energy Transfer in NIF Polar-Drive Exploding-Pusher Experiments

Polar-drive (PD)\footnote{A. M. Cok, R. S. Craxton, and P. W. McKenty, Phys. Plasmas \textbf{15}, 082705 (2008).} target implosions have been designed and fielded for neutron diagnostic development on the National Ignition Facility (NIF). Experimental results evaluating the overall hydrodynamic assembly have previously indicated a significant discrepancy with \textit{DRACO} predictions of the in-flight shell evolution. New physics models, addressing nonlocal electron thermal transport and cross-beam energy transfer within the incoming laser light, have been implemented into \textit{DRACO}. Results detailing comparisons of experiments with simulations using these models will be presented that indicate significantly better agreement and may provide insight into the application of these models in other inertial confinement fusion experiments. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Determination of Size and Shape of Imploding Polar-Driven Targets on OMEGA from X-Ray Images

This talk will describe the method for determining the size and shape of the imploding polar-driven targets on OMEGA from framed x-ray images of the plasma self-emission or of backlighter emission absorbed by the plasma. Accurate low-mode fits to the images are accomplished in the presence of both neutron-induced noise and inherent framing-camera noise. The same fitting procedure is used on \textit{DRACO} 2-D hydrodynamic simulations allowing for comparisons that take into account the spatial and temporal resolution of the experimentally determined measurements. Radiographs obtained on OMEGA with high-speed framing cameras have obtained the evolution of the low Legendre modes with 30-ps frame-to-frame intervals and with accuracies of $\sim $1{\%} of the radius. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Effect of Nonlocal Thermal Electron Transport on the Symmetry of Polar-Drive Experiments

A nonlocal, multigroup diffusion model for thermal electron transport\footnote{G. P. Schurtz, Ph. D. Nicola\"{\i}, and M. Busquet, Phys. Plasmas \textbf{7}, 4238 (2000).\par } has been added to the 2-D hydrodynamic code \textit{DRACO}. This model has been applied to simulations of polar-drive (PD) experiments on the OMEGA Laser System and the National Ignition Facility. When compared with the simulation with flux-limited diffusion transport, the nonlocal transport under the same laser illumination pattern increases the drive at the equator, resulting in an increase of the amplitude of modes two to six at end of target acceleration. The increased drive is caused by the larger heat flux at the equator than near the pole, which results from the coronal temperature being driven purposely high to compensate for the oblique illumination when using the flux-limiter model. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Optimization of the NIF Polar-Drive Ignition Point Design

Polar drive (PD)\footnote{S. Skupsky\textit{ et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} allows one to conduct direct-drive--ignition experiments at the National Ignition Facility while the facility is configured for x-ray drive. A PD-ignition design has been developed\footnote{T. J. B. Collins\textit{ et al.}, Phys. Plasmas \textbf{19}, 056308 (2012).} achieving high gain in simulations including single- and multibeam nonuniformities, and ice and outer-surface roughness. This design was optimized with \textit{Telios} to reduce the in-flight aspect ratio (IFAR) and implosion speed, increasing target stability while maintaining moderately high thermonuclear gains.\footnote{T. J. B. Collins, J. A. Marozas, and P. W. McKenty, Bull. Am. Phys. Soc. \textbf{57}, 155 (2012).} With the recent advent of new numerical models treating the effects of nonlocal thermal transport and cross-beam energy transfer, the design has undergone a re-evaluation. Results describing the effects of these processes on the drive and implosion uniformity of the design and the overall target gain will be described. Optimization of both polar and azimuthal beam pointing angles has also been investigated using the optimizer \textit{Telios}. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Integrated Two-Dimensional \textit{DRACO} Simulations of Cryogenic DT Target Performance on OMEGA

Integrated simulations of cryogenic deuterium--tritium (DT) target implosions on OMEGA have been performed using the radiation--hydrodynamic code \textit{DRACO}. Taking into account the known nonuniformities of target and laser irradiation, 2-D simulations examine the target performance of a variety of ignition-relevant implosions. The effects of cross-beam energy transfer and nonlocal heat transport are mimicked by a time-dependent flux limiter. \textit{DRACO} simulations show good agreement with experiments in $\rho R$, neutron yield, $T_{\mathrm{i}}$, neutron rate, and x-ray images for the mid-adiabat $\left( {\alpha \approx 4} \right)$ implosions. For low-adiabat $\left( {\alpha \approx 2} \right)$ and high in-flight aspect ratio $\left( {\mbox{IFAR}>24} \right)$ implosions, the integrated simulations with the known nonuniformity sources cannot fully explain the reduction in target performance. Examinations of other possible nonuniformity sources and the thermal conductivity model will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Direct-Drive--Ignition Designs with Moderate-\textit{Z} Ablators

Mitigation of laser--plasma and hydrodynamic instabilities is crucial for the ultimate goal of ignition in inertial confinement fusion. Moderate-$Z$ (MZ) materials (6 \textless\ $Z$ \textless\ 10) are expected to reduce both energy loss and hot-electron preheat due to the laser--plasma interaction. High-gain ignition designs for the National Ignition Facility (NIF) with MZ ablators are described and compared with a pure-plastic design. The NIF beam quads are split to irradiate the target with smaller laser focal spots during the main drive to reduce the losses caused by cross-beam energy transfer. Two-dimensional hydrodynamic simulations assess the robustness of these designs to the NIF specifications for target and laser nonuniformities including beam geometry, laser imprint, and ice and outer-surface roughness. Results indicate that MZ-ablator designs can achieve ignition for direct-drive implosions on the NIF. This material is based upon work supported by the Department of Energy National Nuclear Security Administration DE-NA0001944 and the Office of Science under DE-FC02-04ER54789.   

Room-temperature, ignition-scale hohlraum experiments on NIF

We have fielded six shots (symmetry capsules and convergent ablators) to develop a room-temperature (``warm'') ignition-scale platform. These have lower cost than cryogenic (\textless~30 K) shots, and allow higher-Z hohlraum and capsule fill gases. Compared to the cryo He hohlraum fill, the warm neopentane fill (C$_{5}$H$_{12})$ produces comparable x-ray drive, but requires less cross-beam energy transfer to achieve a round implosion ``hot spot.'' The higher Z results in a hotter plasma, which appears to reduce Raman scattering from the inner beams. Warm shots also have more outer-beam Brillouin scattering. In-flight measurements of the shell show a positive P$_{4}$ Legendre mode (diamond shape) in both warm and cryo shots, consistent with predictions from the radiation-hydrodynamics code Hydra. The code also predicts a negative P$_{4}$ (square) hot spot shape for both warm and cryo shots, but only warm shots typically exhibit this. Improved Hydra modeling is being applied to the warm shots, including a self-consistent, inline package for energy transfer and backscatter. The warm capsule fill has been nominal or deuterated (C$_{3}$D$_{8})$ propane, giving \textgreater\ 2x10$^{11}$ neutrons. The hot spot is cooler in warm than in cryo shots (D-He$^{3}$ fill) due to increased radiation from hot-spot C.   

Investigation of Electric and Self-Generated Magnetic Fields in Implosion Experiments on OMEGA

Electric and self-generated magnetic fields in direct-drive implosion experiments on the OMEGA laser\footnote{T. R. Boehly\textit{ et al.}, Opt. Commun. \textbf{133}, 495 (1997).} were investigated using proton radiography. The experiments use plastic-shell targets with various surface defects (glue spot, wire, and stalk mount) to seed perturbations and generate localized electromagnetic fields at the ablation surface and in the plasma corona surrounding the targets. Proton radiographs show features from these perturbations and quasi-spherical multiple shell structures around the capsules at earlier times of implosions (up to $\sim $700 ps for a 1-ns laser pulse) indicating the development of the fields. Two-dimensional magnetohydrodynamic simulations of these experiments predict the growth of magnetic fields up to several MG. The simulated distributions of electromagnetic fields were used to produce proton images, which show good agreement with experimental radiographs. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Layered DT Direct-Drive Implosion Performance Using Neutron Spectroscopy on OMEGA

The performance of recent cryogenic DT implosions on OMEGA vary significantly with changes in the adiabat and implosion velocity. At lower adiabats and high implosion velocities, the areal densities ($\rho R$'s) decrease to less than a third of 1-D predicted values. This observation may explain hydro instabilities that could potentially break up the shell in flight. A neutron time-of-flight detector and a magnetic recoil spectrometer are used to infer a fuel $\rho R$ by measuring the primary neutrons that elastically scatter off the dense deuterium and tritium. An additional nuclear diagnostic with different lines of sight will be used to correlate potential variations from the cold fuel shell instability. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Simulations of the late-phase of NIF capsule implosions using a LES code

Detailed simulations of Inertial Confinement Fusion (ICF) implosion experiments attempt to provide a complete description of the imploding capsule by using measured capsule geometry and perturbations and a 3D model of the radiation source. These simulations can match many experimental observables but the yield remains several times higher than measured. Speculation about greater-than-predicted mixing and the possibility of turbulence have motivated the use of the Miranda code to closely compare with the existing HYDRA simulations. Miranda is a Eulerian radiation-hydrodynamics code with high-order accuracy, physical viscosity and diffusion, and Large Eddy Simulation (LES) modeling of sub-grid scale turbulent dissipation. In this work, HYDRA is used to simulate the capsule of specific NIF experiments in 3D up until the shock has nearly reached the center of the capsule. The subsequent evolution is carried out in both codes. Without viscosity, both codes show very similar results for the assembled fuel with significant low-mode asymmetries and fine-scale, high-velocity flows in the hot spot center. With viscosity, however, Miranda simulations show a significant dissipation of hot spot turbulent kinetic energy that is absent in the inviscid HYDRA simulations.