Session BO5: Direct Drive and Shock Ignition

Defect Implosion Experiments (DIME) at OMEGA

The Los Alamos DIME campaign involves perturbed spherical implosions, driven by 60 OMEGA beams with uniform, symmetrical illumination. D-T-filled CH-shell targets with equatorial-plane defects are designed to produce a non-spherical neutron burn region. The objectives of the DIME series are to observe the non-spherical burn with the neutron imaging system (NIS) and to simulate the physics of the neutron and x-ray production. We have demonstrated adequate neutron yield for NIS imaging with targets of diameter 860 $\mu $m. All targets are filled with 5 atm of DT. We used two separate shell thicknesses: 8 $\mu $m and 15 $\mu $m, thus testing both exploding pusher and ablative designs. Defect channel depth ranges from 0 -- 8 $\mu $m. Width is 20 -- 40 $\mu $m. Perfect targets have no defect. Numerical simulations predict enhanced x-ray emission, that is suggested by experiment. Results from a recent DIME campaign will be discussed.   

Dynamic defects for diagnosing ICF burn degradation mechanisms

We have analyzed the effects of using a short pulse ion beam to provide a precise dynamic defect with which to perturb burn in a NIF ignition or sub-ignition capsule. A short pulse of carbon ions can be produced using the ARC laser at NIF by focusing it onto a thin curved diamond target outside the hohlraum. Using the Target Normal Sheath Acceleration (TNSA) mechanism, a 100 MeV-class, 100J-regime carbon beam can be produced and targeted to intercept a 100x100 micron$^{2}$ patch on the ignition capsule surface during its implosion. By applying this dynamic energy deposition or ``defect'' relatively late in the implosion sequence, a separation of perturbed implosion shock effects from material mix effects (at the defect location) can be achieved. This provides a tunable platform for investigating and validating ICF yield degradation effects from changes in material morphology. Simulation results showing yield degradation from various perturbation fluxes and injection times will be shown.   

Exploring the effects of defects on DT burn, the DIME experiment and measuring capsule zero-order hydrodynamics using Polar direct drive

Small capsule perturbations may impact our ability to achieve high yields on NIF. Diagnosing the hydrodynamic development and the effect of defects on burn will be difficult. Los Alamos is developing a program to better understand the hydrodynamics of defects and how they influence burn. Our first effort to study the effects of defects was on Omega. Both thin-shelled (exploding pusher) and thick-shelled capsules were shot and the results published [1]. In this work we add experimental shots done recently on Omega. These shots were to complete the study of how the width and depth of the defect affects DT yield. Our AMR code is used to predict the yield. Comparisons between capsule and experimental yields will be given. Experiments are also being designed for Polar direct drive. Our first experiments are being designed to understand the zero-order hydrodynamics with Polar direct drive. Capsules about a millimeter in radius are being designed with one to two dopants in the CH shell for radiograph and MMI usage. Also, to minimize the effect of mix on the radius versus time trajectory, some capsules will replace the DT with Xe.\\[0pt] [1] Magelssen G. R. et al., to be published in the 2009 IFSA proceedings.   

Preparing for Polar Drive at the National Ignition Facility

Polar drive (PD)\footnote{ S. Skupsky \textit{et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} will make it possible to conduct direct-drive-ignition experiments at the National Ignition Facility (NIF) while the facility is configured for x-ray drive. Recent experiments on both OMEGA and the NIF have successfully employed PD. \textit{DRACO} simulations of these experiments reproduce the important features of the time-resolved x{\-}ray emission and, in the case of the NIF experiments, the overall neutron yield. Drawing from the success of these experiments, a PD-ignition design that employs a wetted-foam ablator (for higher laser-energy coupling) and a single decaying-shock picket-pulse shape for adiabat shaping will be shown. This design has a 2-D predicted gain of 17. In addition, the application of PD to our latest ignition-scale design, employing a solid-CH ablator and three relaxation pickets to facilitate experimental shock timing, will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302, DE-FC02-04ER54789, and DE-FG02-05ER54839.   

Evaluation of the First Polar-Drive, DT-Gas--Filled Target Implosions on the NIF

Polar-drive (PD) target implosions using DT fuel have been designed and fielded for neutron diagnostic development on the NIF. The experiments use thin, room-temperature glass shells filled with low pressures (10 atm) of DT. Initial target implosions on the NIF are expected to produce DT yields in the range of a few 10$^{14}$~neutrons. The predicted yields are consistent with earlier data (10$^{14}$ neutrons at 30~kJ) and recent PD-scoping experiments performed on OMEGA. The experiments used existing x-ray-drive phase plates with judicious repointing and defocusing to drive the implosions as uniformly as possible. These implosions are modeled with three separate hydrodynamics codes: \textit{LILAC}, to optimize the 1-D design; \textit{SAGE}, to optimize the pointing uniformity; and \textit{DRACO}, to predict the yield from 2-D implosion simulations. Experimental results evaluating the yield performance and the overall hydrodynamic assembly of the implosions (as recorded on x-ray diagnostics) will be compared to simulations results and post-processed Spect3D analysis of the implosion. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

NIF-Relevant, Polar-Drive Illumination Tests on OMEGA

The OMEGA 60-beam Laser System has been used to test illumination schemes relevant to polar-drive experiments that are proposed for the National Ignition Facility (NIF). A set of 40 OMEGA beams are similarly arranged to the 48 quads of the NIF, providing the basis for the tests. This 40-beam set is re-aimed to more uniformly illuminate the target and drive a symmetric implosion. In these tests six different beam-pointing schemes were used to implode a series of D$_{2}$-filled spherical CH shells. The uniformity of the resulting compressed shell layers was diagnosed by x-ray radiography taken in two nearly orthogonal backlighter directions. Target performance was evaluated by a suite of neutron-yield diagnostics. Optimized implosions on OMEGA are achieved by the choice of beam pointing. Two-dimensional \textit{DRACO} simulations show nearly the same size and shape of the compressed shell layer as that measured. These experiments increase confidence in our ability to optimize the choice and predict the outcome of polar-drive experiments on the NIF. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302, DE-FC02-04ER54789, and DE-FG02-05ER54839.   

High-Gain Shock Ignition on the National Ignition Facility

Shock ignition offers the possibility for a near-term test of high-gain ICF on the NIF at less than 1MJ drive energy and with day-1 laser hardware. We will summarize the status of target performance simulations, delineate the critical issues and describe the R{\&}D program to be performed in order to test the potential of a shock-ignited target on NIF. In shock ignition, compressed fuel is separately ignited by a late-time laser-driven shock and, because capsule implosion velocities are significantly lower than those required for conventional hotpot ignition, simulations indicate that fusion energy gains of 60 may be achievable at laser energies around 0.5MJ. Like fast ignition, shock ignition offers high gain but requires only a single laser with less demanding timing and focusing requirements. Conventional symmetry and stability constraints apply, thus a key immediate step towards attempting shock ignition on NIF is to demonstrate adequacy of low-mode uniformity and shock symmetry under polar drive   

A 96/96-Beam Polar-Drive Configuration for Shock Ignition on the NIF

A polar-drive configuration for implementing shock ignition\footnote{R. Betti\textit{ et al}., Phys. Rev. Lett. \textbf{98}, 155001 (2007).}$^{,}$\footnote{L. J. Perkins\textit{ et al}., Phys. Rev. Lett. \textbf{103}, 045004 (2009).} on the NIF is proposed in which 96 beams delivering a ``compression'' pulse are focused on the initial target and 96~beams delivering a short-pulse, ``shock'' pulse are focused at a later time on the compressed target surface. Since the NIF requires each quad to have a single pulse shape, 24 quads are used to deliver each pulse with all beams given horizontal repointings (half to the left and half to the right), in addition to the vertical repointings needed for polar drive, resulting in close to 48-quad symmetry for both compression and main pulses. The 2-D hydrodynamics code \textit{SAGE} has been used to optimize the beam pointing and focusing parameters for a proposed initial experiment in which a surrogate plastic-shell target is irradiated with 96 beams to explore the uniformity that can be achieved with the compression pulse. The design uses the phase plates currently installed on the NIF. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

A Plastic-Ablator Shock-Ignition Design for the NIF

Shock ignition\footnote{ R. Betti \textit{et al}., Phys. Rev. Lett. \textbf{98}, 155001 (2007).} allows for the possibility of achieving high gain on a single laser system with less input laser energy than is required for standard hot-spot ignition. In this paper, a plastic-ablator shock-ignition design for the National Ignition Facility (NIF) is investigated. Results from one- and two-dimensional simulations will be presented that study the robustness of this target caused by various quantities including ice roughness, beam geometry, laser power balance, laser imprint, and hot-electron energy deposition. Of late, particular attention has been focused on two-plasmon decay (TPD), which can accelerate hot electrons from the corona into the shell, thereby raising the target adiabat. Experiments indicate that target designs that employ plastic ablators have a higher intensity threshold for TPD than either all-DT or wetted-foam designs. Such plastic-ablator shock-ignition designs can, therefore, avoid preheat issues during the main drive and even use the production of hot electrons during the spike pulse to improve the energy coupling to the target late in the implosion. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Assessing and improving the robustness of shock ignition targets

Shock ignition is a promising approach to inertial confinement fusion. Like fast ignition, it separates the stages of compression and ignition, but does not require ultraintense lasers. In addition, it may allow for relatively simple spherical targets. Here, we report studies concerning a simple all D-T target and a target with CH ablator, which could be tested at the National Ignition facility. They achieve (1D) gain in the range 60-100 at laser energy 0.5 -- 1~MJ. These targets are driven by shaped laser pulses preceded by an an adiabat-shaping picket, which (according to our 2D simulations) is required to reduce both RMI and RTI growth at the ablation front. We have performed a wide 1D scan of performance sensitivity to changes in pulse shaping and target parameters (including initial temperature). By 2D simulations we have then studied how irradiation nonuniformities associated to the beam configuration, as well as to beam imbalance and mispointing and to target mispositioning degrade target performance. We discuss how robustness to such errors can be improved by optimizing beam configuration and target design.   

Shock-Ignition Studies on OMEGA

Recent theoretical work\footnote{R. Betti et al., Phys. Rev. Lett. \textbf{98}, 155001 (2007)} has shown that the gain in an inertial confinement fusion (ICF) experiment can be significantly increased through separation of the compression and ignition stage by launching a strong, spherically convergent shock at the end of a compression pulse. This scheme, referred to as \textit{shock ignition}, reduces the energy required for ignition compared to ``conventional'' ICF or fast ignition. Through potentially relaxed requirements for the ignitor-shock spherical symmetry, it can be carried out in a polar-drive configuration and is therefore applicable to the National Ignition Facility. The results of a series of spherical and planar-target experiments on OMEGA to study the shock-ignition technique and to infer the shock strength, hot-electron generation, and light reflectivity at the high intensities relevant to shock ignition will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302 and DE-FC02-04ER54789.   

X-ray Spectroscopy of ICF Shock-Ignition Implosions at OMEGA

OMEGA direct-drive shock-ignition implosions of thick-wall CH spherical shell targets filled with deuterium and a tracer amount of argon were performed. The argon line spectrum is primarily emitted at the collapse of the implosion thus providing a spectroscopic signature of the state of the implosion core. The spectra analysis also yields information about the state of the compressed shell since the argon emission from the core is significantly attenuated by the compressed shell confining the core. The observed spectra include line transitions in H-, He- and Li-like argon ions thus covering a broad photon energy range from 3.0 keV to 4.3 keV. A detailed atomic physics and radiation transport model has been developed to interpret and analyze the recorded data. The method was first tested with synthetic spectra computed by post-processing LILAC 1D hydrodynamic simulations, and then applied to the diagnosis of OMEGA shock-ignition implosions.   

One-Dimensional Hydrodynamic Theory of Shock Ignition

This work investigates the theoretical foundations of shock-ignition hydrodynamics. A simple analytical model is used to determine the maximum hot-spot pressure theoretically achievable for an inertial confinement fusion implosion with a given input energy. The analysis is carried out for a simple slab model of a planar foil (the shell) compressing a low-density plasma (the hot spot). In such a model, the compression is driven by reflected shocks and the decompression is produced by rarefaction waves. It is shown that capsule implosions may approach the theoretical maximum by applying a series of weak shocks to prevent the generation of rarefaction waves in the shell caused by multiple shell-shock interactions during the deceleration phase. These results are then applied to understand and optimize hot-spot performance in shock-ignition implosions. Various capsule dimensions, implosions velocities, and shock strengths are studied in maximizing the peak hot-spot pressure. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302 and DE-FC02-04ER54789.   

Preliminary experiment at LULI toward shock ignition feasibility

Shock ignition is a novel scheme to assemble and ignite thermonuclear fuel. In this scheme, the assembled fuel is separately ignited by a strong, spherical shock driven by the high intensity spike at the end of the laser pulse. In this context, we have performed an experiment on LULI2000 laser facility using a simpler geometry to investigate the possibility of generating high shock pressure in large plasma. This experiment required two beams: the first one (I$\sim $ 5x10$^{13}$ W/cm$^{2}$ at 2w) to launch a shock on a planar target and consequently a long plasma on the front side, the second one (I$\sim $10$^{15}$ W/cm$^{2}$ at 2w) for the spike. In this presentation, we report the first results concerning: (i) the measurement of the laser backscattered energy via stimulated Brillouin and Raman ; (ii) the characterization of the shocks (velocity and temperature).   

Comparison of Stopping Power Models in Shock Ignition Target Simulations

The calculation of alpha particle energy deposition is very important for the prediction of ignition and fusion burn in inertial confinement fusion targets. ICF ignition targets experience a large range of temperatures, densities and particle velocities. There are many stopping power models that cover various fractions of this phase space, but none cover it entirely. Our confidence in the accuracy of these stopping power models is related to the consistency of the plasma conditions assumed in developing the model and the actual conditions realized in the simulation. We present calculations comparing several stopping power models and their effect on shock ignition targets. We also show that the prediction of ignition is strongly dependent on the method in which one extrapolates a model beyond strictly consistent conditions.