Session FO3: Compression and Burn

Z-pinch Driven Fusion Capsules using the Hybrid Hohlraum Concept

A new z-pinch approach to generate thermal x-rays suitable for driving inertial fusion capsules is presented. This hybrid hohlraum concept uses aspects of the two z pinch approaches to inertial fusion that are presently being studied. Similar to the dynamic hohlraum approach [S. A. Slutz et al., Phys. Plasmas, 10, 1875], a tungsten z pinch is imploded onto a multi-component convertor to efficiently obtain high radiation temperatures and an appropriate temporal pulse shape. The multi-component convertor is designed to generate primary and secondary vacuum hohlraums which are separated by baffles positioned to obtain adequate radiation symmetry around the capsule in a manner similar to the double ended hohlraum [R. A. Vesey et al., Phys. Plasmas, 10, 1854]. Numerical simulations indicate that high convergence capsule implosions driven by a hybrid hohlraum on ZR could yield deuterium/tritium densities in excess of 300 g/cc. Progress toward the design a hybrid hohlraum to drive inertial fusion capsules to ignition and high yield using future pulsed power machines is presented.   

High-Z Coatings Produce Significant Improvements in the Fusion Burn of Laser Driven ICF Implosions

Recent experiments on the Omega Laser Facility have demonstrated that the absolute number of neutrons and the yield-over-clean parameters show significant increases for implosion targets that have ablators with thin ($\sim $ 200-400 Ang.) high-Z layers on the outer surface. In the experiment, 850 micron diameter, 20 micron thick, CD shells where imploded on a low adiabat. The laser pulses where shaped such that a low foot, first compressed the target and a subsequent one nanosecond high intensity top-hat pulse accelerated and imploded the target. As was demonstrated in earlier NRL experiments,\footnote{S.P. Obenschain et al., Phys. Plasmas 9, 2234 (2002).} the reduction of laser imprint may be responsible for improving the performance of these implosions. Results from the latest implosion experiments, computer simulations, and their analysis will be reported.   

Experimental study of fill-tube hydrodynamic effects on implosion

Planned cryogenic ignition experiments at the National Ignition Facility (NIF) are expected to use a fill tube to introduce liquid DT into the capsule prior to solid layer formation. This fill tube is expected to form a hydrodynamic jet during the deceleration phase of the implosion. Numerical simulations indicate that a 10um tube with a 3um hole has an acceptable impact on the implosion. However, the hydrodynamic effects of the fill tube have not been explored by experiments. We have begun the first indirect-drive experiments to explore the hydrodynamic effects of fill tubes on implosion performance. In these experiments, we have concentrated on developing diagnostic techniques by replacing the fill tube with a bump on the outer shell surface. This bump is hydrodynamically similar to a fill tube but much easier to fabricate and simulate. We present experimental data obtained from temporally resolved, high spatial resolution x-ray emission imaging of Ti-doped shell material that has been swept into the core at times near peak compression. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W- 7405-Eng-48.   

Simulations of Omega Fill Tube Experiments

Current plans are to use fill tubes to inject liquid DT into the interior of inertial confinement fusion (ICF) capsules in future experiments on the National Ignition Facility (NIF) laser. The fill tube is a perturbation on the surface of the capsule and hydrodynamic instabilities will cause this perturbation to grow during an implosion. Simulations show that the fill tube leads to a jet of shell material that might push far enough into the fuel to significantly reduce the yield of a NIF implosion. Experiments to investigate the growth of perturbations due to fill tubes (and due to bumps with a mass similar to a fill tube) have been carried out on the Omega laser. The goal of these experiments is to validate simulations at Omega energy scales and thus increase confidence in the use of simulations in planning for NIF experiments. The capsules used in these experiments have a small amount of titanium placed in the inner layers of the plastic shell to diagnose the growth of the jet. This paper presents the results of 2D simulations of x-ray emission from Omega capsule implosions with a bump or a fill tube. The emission from the jet can be seen crossing the capsule in roughly 300 ps in both the experiments and the simulations. The dependence of the x-ray images (experimental and simulated) on the initial bump size will be discussed. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.   

The effects of fill tubes and capsule perturbations on NIF ignition implosions.

The notion of using a fill tube to charge an ignition capsule in-situ with DT fuel is very attractive because it eliminates the need for cryogenic transport of the target from the filling station to the target chamber, and in principle is one way of allowing any material to be considered as an ablator. A nominal configuration we are studying is a $\sim $ 1mm radius Cu doped Be capsule with a $\sim $ 10 $\mu $m diameter glass fill tube driven at $\sim $ 300 eV. To explore the effect of the tube on capsule performance we use the radiation hydrocode HYDRA in 2D. For the capsule above, the perturbation is small and HYDRA predicts very close to 1D clean yield. In this talk we will consider how this result is affected by the choice of capsule and tube material, the specifics of how the tube is attached to the capsule, and by interaction with perturbations from surface roughness. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.   

Target Performance of Direct-Drive, D$_{2}$-, D$^{3}$He- and DT-Filled, Plastic-Shell Implosions on OMEGA

The effects of reduced laser irradiation nonuniformities in the $\ell <$ 80 range on the target performance of high-adiabat, D$_{2}$-, D$^{3}$He-, and DT-filled, plastic-shell implosions on the 60-beam, 30-kJ, 351-nm OMEGA Laser System were investigated. Improved performance was observed for targets that are less susceptible to high $\ell $-mode laser imprint. Simulations from the 2-D hydrodynamics code \textit{DRACO}, initialized with the calculated on-target laser irradiation nonuniformities caused by single beam far-field, beam-to-beam energy imbalance, beam mispointing, and target offset, will be compared with the measured results. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Direct-Drive, Low-Adiabat ICF Implosions

A series of experiments has been started to study and optimize low-adiabat (\textit{$\alpha $}), direct-drive implosions of CH targets. \textit{$\alpha $} is defined as the shell pressure divided by the Fermi pressure, and low adiabat is defined as \textit{$\alpha $} $\le $ 4. The OMEGA laser system was used to irradiate solid CH targets with shell thicknesses up to 35 \textit{$\mu $}m and CH foam targets. The CH foam targets are mass and density equivalent to cryogenic D$_{2}$ targets with a 5-\textit{$\mu $}m CH layer on top of a 0.18-g/cc, 90-\textit{$\mu $}m-thick, foam layer. Ablation-interface perturbations were changed by varying the smoothness of the laser irradiation with SSD. X-ray diagnostic data will be presented showing the measured shell trajectory and core formation. Nuclear diagnostic data will be presented to show the core conditions and fusion performance. Selected target implosions are simulated with both 1-D and 2-D hydrodynamic simulations. The results from the simulations are compared to the experimental data in order to assess what physical processes limit the performance of low-adiabat implosions. This information is then used to optimize the laser pulse shape and target design. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Two-Dimensional Simulations of Low-Adiabat Plastic Shell Implosions on OMEGA

D$_{2}$- and DT-filled plastic shells with thicknesses varying between 27 and 35 \textit{$\mu $}m and pressures varying between 3 and 15 atm have been imploded on OMEGA using pulse shapes that set the shell on a low adiabat. These implosions are simulated with the 2-D hydrocode \textit{DRACO}, with detailed calculations of the on-target nonuniformity because of beam-beam imbalance and single-beam nonuniformity as input. Target performance has been measured experimentally through neutron yields, neutron production rates, and ion temperatures. Simulation results will be compared with these observables. The modes that influence target performance will be identified. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Recent Cryogenic Implosion Results on OMEGA

Direct-drive implosions using cryogenic fuel are being used to validate the performance of ignition-scaled targets on the OMEGA laser. This effort includes the validation of adiabat shaping using picket pulses with and without smoothing by spectral dispersion and the demonstration of high fuel areal density. These experiments are currently being performed with pure deuterium fuel using high-convergence, low-adiabat drive pulses. Work is under way to prepare the cryogenic target filling system to accept tritium with the expectation of filling, layering, and imploding the first DT cryogenic capsules before the end of 2005. This talk will present the latest results from the current D$_{2}$ implosion experiments and provide an update on the status of the DT target readiness. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Studies of shock convergence in ICF implosions using nuclear burn history measurements

Ignition of ICF capsules on the NIF will be critically dependent on the quality of hot-spot heating due to converging shock waves. On OMEGA, the convergence of strong spherical shocks heats the inner gas sufficiently to induce nuclear fusion. Measurements of the time history of this nuclear burn show that the burn induced by shock convergence occurs several hundred picoseconds before the deceleration-phase compression burn. The nuclear burn history measurements of two distinct nuclear reactions will be compared to a Guderley self-similar imploding shock analysis, as well as to 1-D hydrodynamic simulations. This work was supported in part by LLE, LLNL, the U.S. DoE, the Univ. of Rochester, and the N.Y.State Energy Research and Development Authority.   

Measured nuclear burn region sizes and symmetries in direct-drive ICF implosions vs. capsule and drive conditions

Proton emission imaging is used to measure the size and shape of the nuclear burn region in D$^{3}$He-filled capsules imploded at the OMEGA laser facility. Systematic differences in burn-region size and symmetry are found for different capsule shells, fill pressures, and drive conditions. For symmetric implosions, measurements for capsules with thin glass shells agree with 1-D simulations while measurements for capsules with thick plastic shells do not. Larger-than-predicted burn regions for plastic shells may be due to effects of mix and/or preheat. Experiments with asymmetric laser drive or asymmetric capsule shells are being performed to determine whether asymmetric capsules might be used to counteract the effects of asymmetric drive either in polar direct drive or in indirect-drive. This work was supported in part by UR-LLE, LLNL, the U.S. DoE, and the N.Y.State Energy Research and Development Authority.   

Gain Curves for Inertial Fusion Targets

The gain curve for an inertial fusion target of a given class -- that is, fusion energy gain as a function of driver energy -- is a common central design need. However, the formal process of obtaining such data is a complex undertaking. In particular, as driver energy changes, target characteristics must change to optimize performance subject to constraints such as ignitability and stability. Accordingly, every point on a gain curve is a different optimized target design. In this paper we present a methodology for determining gain curves for a high-gain, laser direct drive reactor target. Formally, there are seven independent variables that must be defined to delineate a target design: driver energy on target, laser wavelength (blue, green, etc), laser power over the peak portion of the drive, shell outer radius, ablator thickness, fuel thickness and the in-flight adiabat. Given typical rad-hydro calculations take minutes per point through ignition and burn, a prohibitive amount of computational time would be expended to map this space even in 1-D. Accordingly, we have developed a fully dynamic 0-D model based on six coupled ODEs that describe energy, momentum and mass balances across the hotspot/cold fuel system. We obtain fast ($\sim $seconds) accounting of the processes from time zero through stagnation, ignition and burn, including full thermonuclear energy production under burn disassembly. Good agreement with 1-D simulations is obtained in gain-curve space for integral quantities such as gain, yield and stability margins.   

Effects of Perturbed Picket Pulses in Adiabat-Shaped Direct-Drive Implosion Experiments

The leading ``picket'' component of the laser drive pulse in implosion experiments launches a shock wave that tailors the entropy profile of the target shell for greater hydrodynamic stability when accelerated by the main pulse component. We determine the required picket timing and energy tolerances for individual beams of the laser system from one-dimensional simulations. Multidimensional simulations refine these specifications by considering the target's tolerance to shock wave nonuniformities in setting up a sufficiently uniform shell for the main pulse. The timing specification for the individual beams is significantly less stringent than for the picket of the total pulse because of the averaging effect on the random mistiming of the individual beams that contribute to the irradiance at any one point on the target surface. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Stability and Performance of a Direct-Drive, 1-MJ, Wetted-Foam Target Design

Wetted-foam, high-gain, direct-drive targets designs have been proposed for use on the National Ignition Facility by, among others, S. Skupsky \textit{et al}.\footnote{ S. Skupsky \textit{et al}., in \textit{Inertial Fusion Sciences and Applications 2001}, edited by K. Tanaka \textit{et al}. (Elsevier, Paris, 2002), p. 240.} These designs take advantage of the increased absorption provided by the higher-atomic-number elements in the mixture of ``wetted'' foam and deuterium--tritium (DT), which allows greater coupling of the laser to the target. One of these designs has been scaled and retuned with one-dimensional simulations to 1 MJ. We will show a stability analysis of this design performed using two-dimensional simulations. The sources of nonuniformity taken into account from the laser include power imbalance between beams and imprint of single-beam nonuniformities on the target. Target nonuniformities modeled include surface finish and inner-surface DT ice roughness. The relative impacts of these four sources of instability and their effects on target performance will be described, as well as scaling of target performance. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Role of Hydrogen Fractionation in ICF Ignition Target Designs

The need of cryogenic hydrogenic fuels in inertial confinement fusion (ICF) ignition targets has long been established. Efficient implosion of such targets has mandated keeping the adiabat of the main fuel layer at low levels to ensure drive energies are kept at reasonable minima. In fact, it has been shown by many authors that the minimum drive energy of an ICF implosion scales roughly as the square of the fuel adiabat. The use of cryogenic fuels helps meet this requirement nicely and has therefore become the standard in most ICF ignition designs. To date, most theoretical ICF ignition target designs have assumed a homogenous layer of deuterium--tritium (DT) fuel kept roughly at or just below the triple point. Such assumptions have lead to several promising ICF target designs that have numerically demonstrated ignition and burn under a variety of illumination schemes. However, recent work done at the Laboratory for Laser Energetics has indicated the possibility that, as cryogenic fuel layers are formed inside an ICF capsule, isotopic dissociation of the tritium (T), deuterium (D), and DT can take place leading to a ``fractionation'' of the final ice layer. This paper will numerically investigate the effect that various scenarios of fractionation have on hot-spot formation, ignition, and burn of ICF ignition target designs. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under the Cooperative Agreement No. DE-FC52-92SF19460.   

First Intended Experiment for Impact Fusion Ignition

Sufficient suppression of the Rayleigh-Taylor (RT) instability not only increases compressed density, but it may also revive an old ignition idea: High velocity implosion with 1000 km/s may configure a hot-spark without a surrounding cold main fuel and thereby ignite at a very low laser energy of 30-100 kJ. A major criticism of no pathway towards high gain may be solved by the impact fusion ignition (IFI) configuration [M. Murakami, NIM-A 05]. In this scheme, a main fuel is first imploded, whereas the ignition is made by impact collision of the second partial shell with high velocity of 1000 km/s. The first intended experiment using a RT suppressed target has demonstrated the velocity of 600 km/s. We plan to employ several RT suppression schemes in attempts to reach higher velocities using the HIPER and NIKE lasers.