Session GO2: ICF I: Compression and Burn

Design of a diagnostic technique for timing the 4$^{th}$ shock on NIF

Ignition capsule implosions planned for the National Ignition Facility (NIF) require a pulse shape with a carefully designed series of steps, which launch a series of shocks through the ablator and DT ice shell. The relative timing of these shocks is critical for maintaining the DT fuel on a low adiabat. The current NIF specification requires that the timing of the first three shocks be determined to an accuracy of +/- 50ps and the 4$^{th}$ shock be tuned to an accuracy of +/- 100ps. To meet these requirements, experiments are being planned to measure the shock timing on NIF. A reliable technique (VISAR) has been demonstrated [1] for timing the first three shocks. At the hohlraum conditions present during transit of the 4$^{th}$ shock, however, the required VISAR window is expected to go opaque. A new technique using the Streaked Optical Pyrometer (SOP) to measure shock breakout through a diagnostic ``inner'' shell is proposed. The accuracy of this technique is presented. \newline [1] D. H. Munro \textit{et al}., \textit{Phys. Plasmas} \textbf{8}(5), 1552 (2001).   

Shock Timing Plan for the National Ignition Campaign

We report progress on the design of the shock timing tuning procedure for the 2010 ignition campaign at the National Ignition Facility. Our keyhole target design provides adequate drive surrogacy for us to time the first three shocks empirically. The major risk to our plan is hard x-ray preheat, which can cause the diagnostic window to become opaque.   

Direct- and Indirect-Drive Shock-Timing Experiments on the OMEGA Laser

The National Ignition Campaign (NIC) requires diagnostics to measure the timing of multiple shocks in targets that closely resemble ignition capsules. One such target is a sphere with an embedded cone that contains liquid deuterium. It is applicable to both direct and indirect drive. We report on OMEGA experiments that study the effect of x-ray emission on the windows used in these cryogenic targets using open geometry and on direct-drive spherical shock-timing experiments that replicate ignition targets. These experiments indicate that the techniques planned for future NIC experiments can be performed under the conditions anticipated for ignition targets. This work was supported by U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Spherical, Converging-Shock Breakout Measurements on OMEGA

Converging-shock breakout measurements were performed on the OMEGA Laser System using 860-\textit{$\mu $}m-diam spherical targets with 400-\textit{$\mu $}m-diam diagnostic entrance holes. The diagnostic holes were used to allow ASBO and SOP diagnostics to collect shock-breakout signals from the back surface of the spherical shell. The diagnostic entrance holes were shielded with gold cones to protect the target interior from laser light. The experiments were performed with 20-, 27-, and 33-\textit{$\mu $}m-thick shells and with 1-ns square and shaped laser pulses at intensities of $\sim $10$^{15}$ W/cm$^{2}$ (similar to OMEGA spherical implosion experiments). Experimental results will be presented and compared to predictions of various theoretical models including models with constant and time-varied flux limiters and with nonlocal electron transport. This work was supported by U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Optimized Polar-Direct-Drive Experiments on OMEGA

Polar-direct-drive experiments on OMEGA are being performed with 40 beams arranged similarly to the 48-beam, indirect-drive configuration on the NIF. The beams are re-aimed toward the target equator to compensate for the nonuniform illumination. Additionally, a ``Saturn-like,'' toroidally shaped ring placed around the target equator is used to refract light toward the equator, further enhancing target-implosion symmetry. The latest experiments have succeeded in minimizing the lowest $\ell $-mode distortions of the implosions ($\ell $ = 1 to 6), and resulted in fusion yields approaching that achieved with 60-beam symmetrically illuminated targets. The symmetry of the implosions is diagnosed with framed x-ray backlighting using additional beams of OMEGA. The implosions have been optimized by systematically varying the beam pointing and, in the case of Saturn targets, by varying the ring diameter. The beneficial effects of 1-THz SSD available on OMEGA are investigated by comparing target performance with and without the high-frequency component of SSD. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Dynamics of the x-ray-emitting region of ICF capsules used for diagnosing radiation-drive asymmetry in ignition hohlraums

The shape of the x-ray self-emission image of an imploded ICF capsule has been used for many years to diagnose hohlraum radiation-drive asymmetry, because such asymmetry is manifested in a corresponding easily seen distortion of the x-ray-emitting region of the capsule at the time of peak x-ray brightness $t_{max,x}$. The shape of the capsule's x-ray-emitting region may be highly dynamic near $t_{max,x}$, making it desirable to record a time sequence of images. The temporal behavior of this shape can be quite different depending on whether the capsule shell is relatively thick or thin. For thin shells and constant drive asymmetry, the distortion of the shape increases monotonically near $t_{max,x}$. For thick shells, the distortion reaches a peak near $t_{max,x}$ and then decreases. In addition, for constant drive asymmetry, the shape of the x-ray-emitting region can undergo a phase reversal, leading to a non-intuitive and non-unique relationship between the sign of the drive asymmetry and the image shape.   

Inferring Areal Density in OMEGA-DT Cryogenic Implosions

Neutron and charged-particle observables have been traditionally used to infer areal density (\textit{$\rho $R}) in ICF implosions. Inferring moderate \textit{$\rho $R} ($\le $200 mg/cm$^{2})$ in OMEGA cryogenic deuterium--tritium (DT) targets, however, is challenging for several reasons. The elastically scattered deuteron peak is distorted due to significant energy loss in the cryogenic shell requiring a model-dependent interpretation. Limited detector sensitivity requires much higher tertiary neutron yields to diagnose \textit{$\rho $R} than expected in these implosions. Down-scattered neutrons and neutrons from the deuteron break-up reaction are among other observables that depend on \textit{$\rho $R}, although also with significant model dependence. Other options include the addition of $^{3}$He or H dopant gases that would produce energetic protons that could be used to diagnose \textit{$\rho $R}. A study of all observables from cryogenic-DT targets will be presented with the view of consistently describing the compressed core. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Diagnosing cryogenic D$_{2}$ and DT implosions at OMEGA using charged-particle spectrometry

Charged-particle spectrometry is a well established technique for determining areal density (\textit{$\rho $R}) in cryogenic D2 implosions at OMEGA. From the energy-down shift of the secondary D$^{3}$He-proton spectrum, measured in several different directions, \textit{$\rho $R} and \textit{$\rho $R} asymmetries have been successfully determined. Inferring \textit{$\rho $R} and \textit{$\rho $R} asymmetries in cryogenic DT implosions is more challenging as it requires new spectrometry techniques to be developed. Currently, a novel neutron spectrometer for down-scattered neutron measurements is being developed at OMEGA and the NIF. In this paper, we describe the complementary charged-particle-spectrometry techniques available and under development at OMEGA for simultaneous measurements, in several different directions, of knock-on deuterons, knock-on tritons, (n,2n)-protons, and primary D3He protons produced in cryogenic DT implosions. It will be shown that \textit{$\rho $R} and \textit{$\rho $R} asymmetries in moderate \textit{$\rho $R }($<$200 mg/cm$^{2})$ implosions can be determined from the set of measured charged-particle spectra. This work was supported in part by UR-LLE, LLNL, the U.S. DoE, and the N.Y. State Energy Research and Development Authority. This work was supported in part by LLE, LLNL, the U.S. DoE, and the N.Y.State Energy Research and Development Authority.   

Using target shimming to compensate for asymmetric drive in ICF implosions

Proton emission imaging has been used to measure the size and shape of the nuclear burn region in D$^{3}$He-filled capsules imploded with direct drive at the OMEGA laser facility. We have shown that intentional P2 asymmetry in the thickness of a plastic capsule shell, in combination with symmetric laser illumination, results in a P2 asymmetry in the burn region. We have also shown that intentional P2 asymmetry in the laser illumination of a symmetric capsule results in a P2 asymmetry in the burn region together with diminished burn yield. The measured relationship between shell-asymmetry amplitude and burn-asymmetry amplitude has been combined with the measured relationship between drive-asymmetry amplitude and burn-asymmetry amplitude, together with simple models, to prediction how the thickness of a capsule shell could be varied to compensate for asymmetric drive and produce symmetric implosions. We will discuss application of this prediction to both ``Polar Direct Drive'' and indirect drive without ``shine shields'' at the NIF, each of which could produce prolate implosions without target shimming. \textit{This work was supported in part by LLE, LLNL, the U.S. DoE, and the N.Y.State Energy Research and Development Authority.}   

Measurement of the Neutron Energy Spectrum in T-T Inertial Confinement Fusion

Neutron energy spectra from the T-T fusion reaction were measured by imploding plastic capsules filled with high-purity tritium gas (99.76{\%}). The experiments were performed on the 60-beam, 30-kJ OMEGA Laser Facility at the University of Rochester's Laboratory for Laser Energetics. The neutron spectra were measured using an absolutely calibrated neutron time-of-flight detector located 12.4 m from the target. The signals from the detector were recorded using a 1-GHz, 5-GS/s Tektronix TDS-684 oscilloscope. The observed fusion neutrons show a continuous energy distribution with a maximum energy of 9 MeV. The measured neutron spectrum is compared with hydrocode predictions and other experimental data. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under the Cooperative Agreement No. DE-FC52-92SF19460.   

Imprint Simulations of 1.5-MJ NIF Implosions Using a Refractive 3-D Laser Ray Trace with an Analytic SSD Model

Direct-drive implosions for the 1.5-MJ National Ignition Facility are simulated to study the effects of perturbation growth using a refractive 3-D ray trace in the multidimensional hydrodynamics code \textit{DRACO}. An analytic formulation of 2-D SSD has been coupled with the refractive 3-D ray trace to provide a continuous temporal description of the modal amplitudes incident on target. This analytic SSD model, together with an inverse projection method of ray-initialization and adaptive integrators, minimize laser deposition noise reducing it to levels well below imprint efficiencies. Previous simulations have ignored effects of refractive smoothing because of approximations in the laser deposition models and issues with noise. These simulations will provide the first look at the effect of imprint on an imploding target with 2-D SSD smoothing using a refractive 3-D ray trace. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Be capsule implosions driven by dynamic hohlraum x-rays

Be is a promising ablator material for ICF capsule implosions. Ignition-size 2-mm-diameter capsules filled with 20 atm D$_{2}$ + 0.085 atm Ar absorbing up to 80 kJ were driven with a dynamic hohlraum (peak Tr $>$ 200 eV). The capsule wall was $\sim $20 microns CH overcoated with 33-55 microns Be. The thin-wall capsules produced implosion cores with T$_{e} \quad \sim $ 1 keV, $\rho \quad \sim $ 1 g cm$^{-3}$, convergence $\sim $ 7.5, neutron yield up to 3 x10$^{11}$, and pole/equator symmetry ratio of $\sim $ 1. The thickest wall capsules implode into multiple hot spots. These first-ever indirect drive Be implosions provide a foundation for examining issues such as the fill tube effects, sensitivity to columnar growth in sputtered Be fabrication, and of Cu dopant effects.   

The Effect of High-z Impurities on Implosions and Burn in SiO$_{2}$ Shells.

Impurities in imploding capsules affect the equilibration among the three temperatures: ion, electron and radiation temperatures. The changes in the ion temperature and the transport properties then affect the burn of DT within an imploding shell. We have started an initiative to explore the physics of ignition and burn in high Z capsules through both theory and experiments now possible using innovative fabrication techniques. The goal of this work is to create a viable high Z shell ignition and burn program for NIF. In this work, we have fielded thin l mm diameter glass shells overcoated with plastic and filled with varying amounts of xenon and krypton gas to study the progression from non- equilibrium to equilibrium burn as the dopant gas concentration is increased. While a normal glass capsule with a plastic overcoat loses energy to radiation during compression, the high Z shell confines the radiation even as equilibrium burn is approached. The shells also used some fill of $^{3}$He to measure the proton spectrum from the D$^{3}$He reaction to measure the target temperature and $\rho$R. We will discuss the results of these first experiments and show the variation of the yield with doping concentration.   

Spectroscopic Measurements of ICF Capsules Doped with Hi-Z Impurities.

Energy exchange between ions, electrons, and radiation plays an important role in determining the temperature of the ions in an ICF capsule. Because the simulation codes that are used to design and predict ICF implosion experiments often have difficulty predicting the ion temperature correctly, we are embarking on a series of experiments investigating this physics using ICF capsules that have been doped with varying amounts of Hi Z gases. In this work, we have fielded thin l mm diameter glass shells filled with 6.7 atmospheres of D$_{2}$ and varying amounts of xenon and krypton gas to study the progression from non-equilibrium to equilibrium burn as the dopant gas concentration is increased. The shells also contained 3.3 atmospheres of 3He, which is used to measure the proton spectrum from the D3He reaction to measure the target temperature and $\rho $R. Measurements of the neutron and proton yield were obtained, along with time-resolved x-ray images of the capsules and x-ray spectroscopy of the emission from the plasma. In this talk, we will discuss the results of these first experiments, focusing on the time-resolved x-ray spectroscopy.