Session CO1 - Compression and Burn.
ORAL session, Monday afternoon, November 15
Room 201/202, SCC

[CO1.001] Dynamic O-D Models for the Compression, Ignition and Burn of ICF Targets

Observations of rad-hydro-burn simulations of high-gain direct-drive and NIF ignition targets show that the common wisdom of isobaric (constant pressure) conditions around stagnation is incorrect. In particular, at ignition: (a) hotspot and cold fuel are not in pressure equilibrium, (b) fusion alpha energy produced up to this time can exceed the PdV work from the stagnating shell, (c) the cold fuel partitions into a stagnated tamp mass and a still-ingoing unstagnated portion that contains a substantial fraction of the fuel mass [Betti et al, Phys Plas. 9, 2277 (2002)], and (d) the hotspot criterion for ignition, rho-R*T, is not fixed but depends on the tamp conditions. We infer that hotspot formation, ignition and propagating burn is a dynamic process that necessitates a time-dependent description and, accordingly, have formulated 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 (\sim10s) accounting of the processes through stagnation, ignition and burn, including full thermonuclear energy production under burn disassembly. Good agreement with 1-D simulations is obtained for integral quantities such as gain, yield and ignition margins.

[CO1.002] Sensitivity of NIF ignition capsules to hot electron preheat

In indirect drive inertial confinement fusion, a fraction of the laser’s energy may be transferred to super thermal electrons through stimulated Raman scattering. These hot electrons can irreversibly deposit energy in the cryogenic DT fuel, raising the isentrope of the fuel (also known as “preheat”). We explore the sensitivity of potential NIF ignition capsules to hot electron preheat and discuss how a given amount of hot electron production raises the minimum energy of ignition and, consequently, affects the margin. 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.

[CO1.003] High-Areal-Density Cryogenic D_2 Implosions on OMEGA

Direct-drive implosions of cryogenic D_2-filled capsules are being performed on the OMEGA laser. The targets and drive pulses are energy scaled from the baseline direct-drive ignition design developed for the NIF. The cold-fuel areal density (\rhoR) achieved during the burn phase is an important measure of target performance: adequate fuel \rhoR is one of the prerequisites for ``hot-spot'' ignition. On OMEGA, \rhoR's are inferred from the energy loss of D^3He protons. Low-\alpha implosions have produced \rhoR's in excess of 100 mg/cm^2, while high-\alpha experiments have produced yields in excess of 2 \times 10^11. Higher \rhoR's are observed in capsules with smoother inner ice surfaces, and, in general, the inferred \rhoR's agree with 1-D predictions. This talk will summarize the results of several-dozen implosions. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.

[CO1.004] Effects of the Low-\ell-Mode Irradiation Nonuniformities on the Performance of Direct-Drive Spherical Implosions

The low-\ell-mode laser irradiation nonuniformity was recently reduced on the 60 beam OMEGA laser system with a new phase-plate design along with improvements in beam pointing, beam balance, and target positioning accuracy. The target performance is quantified by the ratio of the measured primary neutron yield to the 1-D predicted yield (YOC). The YOC's for high-adiabat (\alpha \sim 5, where \alpha is defined as the ratio of the shell pressure to the Fermi-degenerate pressure), D_2-filled-plastic-shell implosions with predicted convergence ratios ranging from 15 to 40 will be compared with the calculated improvements in the on-target laser irradiation nonuniformity levels. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.

[CO1.005] Effects of Dopant Radiative Cooling in Indirect-Drive Capsule Implosion Experiments

We present results from simulations performed to investigate the effects of dopant radiative cooling in inertial confinement fusion indirect-drive capsule implosion experiments. Using a 1-D radiation-hydrodynamics code that includes inline collisional-radiative modeling (HELIOS-CR), we compute in detail the non-LTE atomic kinetics and spectral characteristics for the Ar-doped DD fuel. We examine the sensitivity of the fuel temperature and neutron yield to the Ar dopant concentration. Simulation results with 0.1% Ar are compared with electron temperature and density distributions inferred from data obtained in OMEGA indirect-drive experiments. The incident radiation drive on the capsule is computed with a 3-D view factor code (VISRAD) using the laser beam pointings and powers from the OMEGA experiments. We also examine the sensitivity of the calculated compressed core electron temperatures and neutron yields to the radiation drive on the capsule, and to the radiation and atomic modeling in the simulations.

[CO1.006] Direct drive double shell target implosion hydrodynamics on OMEGA using offset pointing

Imploding direct-drive double shell targets may provide an alternative, non-cryogenic path to ignition on the National Ignition Facility (NIF) and the Laser Megajoule (LMJ). Experiments are being pursued at OMEGA to understand the hydrodynamics of these implosions and the possibility of scaling to NIF designs. We have used 40 beams from the OMEGA laser to directly drive the capsules, and we have used the remaining 20 beams to backlight the imploding shells from two different directions at multiple times. Because the number of beams was fewer than the 60 normally used to drive a capsule symmetrically at OMEGA, we use offset pointing to produce relatively uniform drive. Offset pointing may also be used at other lasers not normally designed for uniformly driven direct drive targets, such as the NIF an LMJ. We will review the recent experiments to measure the zero-order hydrodynamics of the targets, and the symmetry of the drive. We use two-view x-ray radiography of the capsules. Experiments were pursued using direct drive in which the M-band effect, experienced in the indirect drive experiments, could be eliminated or controlled. We will review the method used to radiograph the targets and the techniques used to extract useful information to compare with calculations. The effect of imperfections in the target construction will be shown to be minimal during the initial stage of the implosion. (LA-UR-04-4939) This work was performed at Los Alamos National Lab under the auspices of the US DOE under contract No. W-7405-ENG-36

[CO1.007] Integrated 2D simulations of dynamic hohlraum driven inertial fusion capsule implosions

Simulations have been useful for improving the design of dynamic hohlraums for the purpose of imploding inertial fusion capsules [S. A. Slutz, J.E. Bailey, G.A. Chandler et al Phys. Plas. 10, 1875, 2003]. These design changes, which have resulted in capsule implosions with hot dense cores [J.E. Bailey, G.A.Chandler, S.A. Slutz et al PRL 92, 085002-1, 2004] and the production of thermonuclear neutrons [C.L. Ruiz, G. Cooper, S.A. Slutz, et al. PRL, 93, 015001-1], were based primarily on a series of 1D numerical simulations, which treated the dynamic hohlraum and the capsule implosion separately. This was appropriate for thin walled capsules, but is not suitable for higher performance capsules that are ablatively driven, since there can be a significant interaction between the z-pinch driven shock in the convertor material and the ablative blow off from the capsule. 2D simulations will be presented which include the implosion of wire arrays onto foam convertors, the capsule imbedded in the foam, and the absorption of radiation into the electrodes. These simulations are in good agreement with most of the quantities that have been measured experimentally. Simulations indicate that appropriately shaping the convertors, should improved radiation symmetry and produce larger neutron yields.

[CO1.008] Measurements of hot spot energy in capsule implosions driven by dynamic hohlraum x-rays

The hot spot energy in ICF implosions can be estimated from measurements of the temperature, density, and volume provided by time- and space-resolved Ar tracer spectroscopy. The method is applied to experiments using 2-mm-diameter 40-80-micron-thick CH wall capsules filled with 10-25 atm D_2 + 0.085 atm Ar. The capsules implode when they absorb 20-40 kJ of x-rays in a dynamic hohlraum configuration. Preliminary analysis using a uniform core approximation indicates that the hot spot energy is 300-800 J in these relatively thin wall capsule experiments. Refinement to include spatial core gradients, simulation comparisons, and the possibility of increasing the efficiency will be discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under contract No. DE-AC04-94AL85000.

[CO1.009] Measurements of Time Evolution of Ion Temperature of D^3He Implosions on OMEGA

We report here the first measurements of time evolution of ion temperature T_i(t) during the shock-burn and compression-burn phases of direct-drive D^3He implosions on the 60-beam, UV OMEGA laser system. The experiments were accomplished using a proton temporal diagnostic (PTD) to measure the D^3He-reaction rate, and a neutron temporal diagnostic (NTD) to measure the DD-reaction rate. By taking the ratio of the measured burn histories, and initially assuming n_i and T_i profiles upon which we iterate, a final T_i(t) can be inferred for the entire burn. The results from these measurements are compared to numerical simulations.

This work was performed in part at the LLE NLUF, and supported in part by the U.S. Department of Energy (DE-FG03-03SF22691, DE-FG03-03NA00058, and Cooperative Agreement DE-FC52-92SF19460), LLE (412160-001G), and LLNL (B504974).

[CO1.010] Inference of Imprint at Onset of Deceleration Phase Using Shock-Burn Measurements

A vital concern for ICF is the premature quenching of the nuclear burn due to mixing of cold shell material into the hot spot. This mixing is caused largely by Rayleigh-Taylor growth during the deceleration phase of modulations imprinted into the shell at earlier times. Spectral, spatial, and temporal measurements of nuclear products emitted during the shock-burn phase of the implosion, which occurs just before the onset of the deceleration phase, can put useful constraints on the amplitude of imprinted areal-density variations at this time. Another indicator of shell integrity is the shell areal density at shock-burn time, which we can also infer from the nuclear measurements. Results from recent experiments will be presented.

This work was supported in part by the U.S. Department of Energy (DE-FG03-03SF22691, DE-FG03-03NA00058, and Cooperative Agreement DE-FC52-92SF19460), LLE (412160-001G), and LLNL (B504974).

[CO1.011] Studying the Burn Region in ICF Implosions with Proton Emission Imaging

Proton core imaging spectroscopy (PCIS) is being used to study the effects of fuel shell mix in implosions of D^3He-filled capsules at OMEGA. Penumbral imaging of fusion products is used to obtain the size and shape of the nuclear burn region. Experimental differences in the D^3He burn region will be explored for varying laser conditions and for different shell types (2 to 3 mm glass and 17, 20, 24 and 27 mm CH). Secondary D^3He proton production profiles from a D_2-filled-capsule implosion will be examined to explore the feasibility of imaging cryogenic implosions. Measured burn profiles will be compared to predictions of clean 1-D simulations and to x-ray images.

This work was supported in part by the U.S. Department of Energy (DE-FG03-03SF22691, DE-FG03-03NA00058, and Cooperative Agreement DE-FC52-92SF19460), LLE (412160-001G), and LLNL (B504974).

[CO1.012] Relationship of Asymmetries in Fusion Burn and Areal Density to Asymmetries in Laser Drive for ICF Implosions at OMEGA

The physical characteristics of asymmetric ICF implosions, and their relationship to laser drive asymmetry, are being studied on OMEGA using capsules with D3He fill and diagnostics that detect D3He protons. The spatial distribution of D3He burn is measured with three orthogonally oriented proton imaging cameras, while the angular distribution of areal density is studied with multiple proton spectrometers. Results from experiments involving imposed drive asymmetry will be discussed, along with comparisons of data with x-ray images and with 2-D simulations.

This work was supported in part by the U.S. Department of Energy (DE-FG03-03SF22691, DE-FG03-03NA00058, and Cooperative Agreement DE-FC52-92SF19460), LLE (412160-001G), and LLNL (B504974).

[CO1.013] Shock-Timing Experiments in Planar Cryogenic Deuterium Targets

Direct-drive ICF target designs use multiple shocks to stabilize and condition the imploding shell. The magnitude and timing of these shocks are critical to optimization of those designs. We present results from planar cryogenic D_2 experiments that use two 100-ps pulses to produce two shocks at various conditions. The velocity profiles of these shocks (from VISAR) and self-emission are used to investigate the coupling of multiple beams to the targets and to demonstrate the ability of hydrodynamic codes to model multiple shocks in cryogenic D_2. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.

[CO1.014] The Effect of Incidence Angle on Laser-Driven Shocks

The proper timing of shocks is essential for inertial confinement fusion ignition targets and to the development of polar direct drive. The effect of the angle of incidence on energy coupling and the production of laser-driven shocks are measured in planar target experiments. Two 100-ps pulses produce a pair of shocks with various strengths and relative delays. Velocity profiles (from VISAR) and self-emission from these shocks are compared to one-dimensional hydrodynamic simulations. A simple refraction model adequately treats the coupling of high-angle beams to the target. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460 and Lawrence Livermore National Laboratory under contract No.W-7405-Eng-48.

[CO1.015] Streaked Optical Pyrometer for Shock Wave and EOS Studies

The optical self-emission from laser-driven shock waves provides important information about the strength of a shock and the EOS of the shocked material. A streak camera was used to temporally and spatially resolve emission from shocks in transparent targets. A NIST traceable lamp was used to calibrate the device to provide brightness temperatures. Various shock-timing and EOS experiments demonstrate the relationship between shock velocity (strength) and optical emission. The coalescence of two shocks and their arrival at the rear surface are observed, providing valuable shock-timing data. Temperature measurements in \alpha-quartz samples are used to evaluate various EOS models. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.

Part C of program listing