Session GO4: Compression and Burn

First streaked radiography experiments of indirect drive ICF capsule implosions on the National Ignition Facility

1-dimensional (slit imaging) time resolved radiography of capsule implosions in ignition hohlraums on the National Ignition Facility (NIF) is used to measure the time history of implosion velocities, ablator shell thickness and remaining ablator mass in the last 5 ns before peak implosion time [1]. While first experiments on the NIF performed with gated imagers recorded these quantities at four adjustable times, streaked radiography [2] adds the tremendous benefit of recording the full implosion evolution through capsule stagnation and explosion phase. First streaked radiography experiments of Si doped indirect drive ignition capsule surrogates with an initial radius of 1.1 mm successfully measured implosion performance with required accuracies at radii in the 0.9 to 0.2 mm range. These experiments were performed in Au and Au/DU gas filled ignition hohlraums driven by laser pulses with a peak power in the 330-420 TW range and total laser energy up to 1.8 MJ. Data quality and inferred statistical uncertainties in implosion velocity, remaining mass and capsule thickness will be discussed. [1] O.L. Landen et al, Phys. Plasmas 18, 051002 (2011). [2] D.G. Hicks et al, Phys. Plasmas 17, 102703 (2010).   

Post Shot Simulations of NIF Convergent Ablator Experiments

Post shot simulations of NIF convergent ablator experiments will be described. The experiments use a streaked radiograph of a backlit capsule implosion to measure the trajectory, velocity, remaining mass, and ablator rhoR and are an important component of the U. S. National Ignition Campaign. The integrated (capsule-in-hohlraum) post shot simulations use measured target parameters, measured laser input powers, measured time-resolved backscatter, and calculated cross-beam power transfer. The integrated calculations are post-processed to provide simulations of the key diagnostics, including: 1) Dante measurements of the hohlraum x-ray flux and spectrum; 2) streaked radiographs of the imploding ablator shell; 3) wedge range filter measurements of D-He3 proton output spectra; and 4) GXD images of the imploded core. The simulated diagnostics are compared to the experimental measurements to provide an assessment of the accuracy of the design code, to enhance understanding of the experiments, and to assist in choosing parameters for subsequent steps in the path towards optimal ignition capsule tuning.   

Analysis of NIC Cryogenic Layered Experiments via Integrated Hohlraum plus Capsule Post-Shot Simulations

We previously reported a simulation-based semi-empirical model of the NIC layered implosions [1]. In this model input parameters (chiefly laser power) are adjusted so that the simulated capsule's shock timing, time-dependent shell velocity, and imploded core shape closely match experimental measurements. We have applied this model to a growing database of layered implosions. We find that simulated ion temperatures agree well with experiments that are not heavily mixed. The model was able to predict the increase in neutron down-scattered ratio (DSR) that was seen experimentally when the laser peak power was lowered from 420 to 320 TW and extended in time to minimize shell re-expansion (so-called ``no-coasting'' pulse). The DSR correlates with calculated fuel adiabat and suggests that the improved fuel adiabat was the primary reason for the DSR improvement.\\[0pt] [1] Jones, et al., Phys of Plasma, 19, 056315 (2012)   

Arithmetic reconstruction tomography of self-emission x-ray images of the imploded core plasmas at National Ignition Facility

To achieve density and temperature required for the thermonuclear ignition, capsules of inertial confinement fusion targets have to be compressed with keeping good spherical symmetry. Due to axisymmetric geometry of the targets (a spherical capsule located in the center of a cylindrical hohlraum), non-uniformity of the x-ray drive is usually dominated by axisymmetric modes. However, x-ray core images observed on the hohlraum axis often show azimuthal perturbation caused by the diagnostic holes, the fuel filling tube, or limited number of the beam-spots on the hohlraum wall. To reduce the non-uniformities and improve the volumetric compression, it is crucial to quantify deformation of the core-fuel boundary in a three-dimensional manner. We performed an arithmetic reconstruction tomography (ART) of the x-ray images obtained from two orthogonal directions and demonstrated capability to track deformation of the gas-fuel boundary including azimuthal perturbation.   

Alpha heating and implosion performance in cryogenic layered NIF implosions

To achieve fusion ignition, implosions on the NIF must first demonstrate significant heating of the hot spot by alpha particle deposition. Improvements in capsule performance, as measured by ITFX, lead to increased alpha heating. Using a large database of two-dimensional simulations, we show that low-performing capsules fall into two limiting categories: low velocity and low areal density. As the capsule performance is improved on approach to the heating and ignition regimes, all implosions approach similar behavior at a given ITFX. We will describe the experimental observables used to diagnose alpha heating and its effects, and we will discuss these effects in the context of current cryogenic layered implosions on NIF. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Modeling of the neutron spectrum from NIF surrogate CH-shell DT implosions

Surrogate implosions play an essential role in the National Ignition Campaign for isolating aspects of complex physics associated with fully integrated ignition experiments at the National Ignition Facility (NIF). One such surrogate, currently planned on the NIF, will use indirectly driven, DT-gas filled, CH-shell implosions. To measure the neutron spectrum from these implosions the Magnetic Recoil Spectrometer and the neutron-Time-of-Flight detectors will be used. As there are currently no techniques to separate the down-scattered neutron (DSn) components from DT and CH, this talk examines the simulated neutron spectrum and evaluates the possibility of breaking this DT/CH degeneracy using the details of DSn spectral shape. This work was supported in part by LLE, the NLUF, the FSC, the US DOE, LLNL, and GA.   

Charged-particle measurements of $\rho $R symmetry at shock-bang time in NIF implosions

V. GLEBOV, P. RADHA, T. SANGSTER, LLE, R. OLSON, R. LEEPER, SNL, J. KLINE, G. KYRALA, D. WILSON, LANL, J. KILKENNY, A. NIKROO, GA. -- The Wedge Range Filter (WRF) proton spectrometers were developed for OMEGA and transferred to the NIF as National Ignition Campaign (NIC) diagnostics. In tuning campaign implosions containing D and $^{3}$He gas, the WRFs are used to measure the spectrum of protons from D-$^{3}$He reactions. From the measured energy downshift of the D$^{3}$He protons, the total $\rho $R is inferred through the plasma stopping power. Data from WRFs fielded simultaneously on the pole and equator indicate low-mode polar $\rho $R asymmetries at shock flash. Significant swings in $\rho $R P2/P0 are also observed over the ignition campaign data set, attributed to low-mode x-ray drive inhomogeneity. This work was supported in part by the U.S. DOE, LLNL and LLE.   

Anomalous shock-yields in direct- and indirect-drive D$^{3}$He exploding pushers

Anomalous reduction of the fusion yield relative to expected values from hydrodynamic scaling was first observed in D$^{3}$He gas-filled ablatively-driven capsule implosions. . Whether a similar reduction exists in D$^{3}$He gas-filled exploding pushers is still being debated. Recent direct- and indirect-drive exploding pushers filled with mixtures of D$^{3}$He fuel at OMEGA have demonstrated anomalous reduction of the DD-neutron shock yield in both platforms. A reduction of $\sim $50{\%} is determined for 50:50 mixtures of D$^{3}$He fuel, when compared to the expected yield scaled from implosions filled with a hydroequivalent pressure of D$_{2}$ fuel. The methodology and results of these experiments will be presented, and possible explanations for the anomalous yield reduction will be discussed and compared to LASNEX simulations. This work was supported in part by the U.S. DOE, LLNL and LLE.   

Design of a hohlraum-driven exploding pusher capsule experiment for NIF neutron diagnostic calibration

Neutron diagnostics are a critical component of the National Ignition Facility and measure key parameters of capsule performance such as neutron yield, ion temperature, neutron bang time, and down scattered ratio. Therefore, accurate calibration of these diagnostics is essential. Such calibration requires high DT yield (greater than 1.e14) with low $\rho $R as well as a symmetric implosion -- an implosion with small non-radial velocities remaining in the fuel during the burn phase. In order to meet these requirements, we have used the radiation-hydrodynamics code HYDRA to design an indirect-drive exploding pusher capsule experiment, driven with a 2.5 ns pulse in a vacuum hohlraum. Features of this design will be presented as well as its feasibility for symmetry control.   

NIF Ignition Targets with Uniformly Cu Doped Berylllium Capsules

The low opacity of beryllium leads to a high, radiation driven, mass ablation rate in the shell of an ICF capsule. Exploitation of this advantage leads to thicker shells using less copper dopant than previously designed. But a relatively harder drive spectrum causes x-ray preheat to the inner beryllium surface that may negate this advantage. Both the mass ablation rate and ablation pressure decrease as dopant is added to protect against this preheat. In a standard design with stepped dopant levels, the ablation front reaches the highest doped layer during the main laser pulse, reducing the drive pressure. An optimal design will ablate as much mass with as little dopant as possible, to drive as heavy a fuel and ablator payload as possible to high velocity. We have explored capsule designs with uniformly doped shells using 0 to 1\% copper. As the dopant concentration is lowered the initial shell thickness increases from about 150 to 330$\mu$m. An integrated simulation using a 0.25\% cu doped shell, a standard U/Au hohlraum, 1.46 MJ laser energy, and a peak laser power of 417 TW calculates to ignite.   

NIF Ignition Targets with Graded Copper-doped Beryllium Capsules

Current NIF plastic capsules are under-performing, and alternate ablators are being investigated. Beryllium presents an attractive option, since it has lower opacity and therefore higher ablation rate, pressure, and velocity [1,2]. We recently designed 300-TW, 345-TW and 420-TW NIF laser pulses for a current Be ignition capsule design in the standard 5.75 mm hohlraum; and investigated in integrated rad-hydro simulations sensitivity of main implosion characteristics to variations in the hohlraum fill gas density, laser beams pointing, and cross-beam energy transfer. An important conclusion is that the ablator shell in the current design is too thin, and thus Be ablation potential is not fully utilized. In addition, the capsule performance degradation due to ablator preheat is significant. Preliminary simulations indicate that the Be thickness can be increased 2-3 times, resulting, for the same radiation drive, in a significant increase in the fuel velocity and/or mass of unablated Be plus fuel. This can be used in a number of ways to mitigate mix, reduce preheat, and otherwise enhance the capsule performance. Herein, we present detailed results of this study.\\[4pt] [1] D. C. Wilson et al., Phys. Plasmas {\bf 5}, 1953 (1998).\\[0pt] [2] R. E. Olson et al., Phys. Plasmas {\bf 18}, 032706 (2011).   

Dopant Distribution in NIF Beryllium Ablator Capsules

Good implosion performance requires capsule ablator material with spherically uniform \hbox{x-ray} opacity, which is controlled by one of several dopants (Cu, Si, Al, etc.) in the Be shell. During production, the dopant concentration is radially stepped. However, the various Be-dopant interactions result in vastly different dopant distribution patterns, some quite inhomogeneous. We have characterized these structures and established the phenomenological basis and the magnitudes of the inhomogeneity both in spatial length scales and in atomic percent. We will discuss the case of inhomogeneous Cu diffusion in detail, followed by discussions of other dopants and the estimate of the impact of these structures on target implosion.   

High-Density Carbon (HDC) Ablator for NIC Ignition Capsules

HDC ablators show high performance based on simulations, despite the fact that the shorter pulses for HDC capsules result in higher M-band radiation compared to that for plastic capsules. HDC capsules have good 1-D performance because HDC has relatively high density (3.5 g/cc), which results in a thinner ablator that absorbs more radiation. HDC ablators have good 2-D performance because the ablator surface is more than an order-of-magnitude smoother than Be or plastic ablators. Refreeze of the ablator near the fuel region can be avoided by appropriate dopant placement. Here we present two HDC ignition designs doped with W and Si. For the design with maximum W concentration of 1.0 at{\%} (and respectively with maximum Si concentration of 2.0 at{\%}): peak velocity = 0.395 (0.397) mm/ns, mass weighted fuel entropy = 0.463 (0.469) kJ/mg/eV, peak core hydrodynamic stagnation pressure = 690 (780) Gbar, and yield = 17.3 (20.2) MJ. 2-D simulations show that yield is close to 80{\%} YoC even with 2.5x of nominal surface roughness on all surfaces. The clean fuel fraction is about 75{\%} at peak velocity. Doping HDC with the required concentration of W and Si is in progress. A first undoped HDC Symcap is scheduled to be fielded later this year.   

High-Density Carbon Ablators for Inertial Confinement Fusion

A series of experiments on the Omega laser have been preformed to measure high-density carbon (HDC) ablator performance for indirect drive inertial confinement fusion (ICF). The Omega laser was used to generate shaped laser pulses with varying powers during the first nanosecond of the drive to investigate drive pressures between 1.7 Mb and 7.5 Mb. The total neutron yield, ion temperature, neutron bang time and x-ray bang time were measured and compared to simulations. Experiments using HDC ablators are planned for the National Ignition Facility and will be discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by LDRD-11-ERD-075.   

Effects on Implosion Characteristics of High-Z Dopant Profiles in ICF Ignition Capsule Ablators

For ignition target design (ITD) of indirect drive ICF [J. Lindl, PoP 2, 3933(1995)], high-Z dopants in capsule ablators were used to prevent preheat of DTadjacentablators by Au M-band flux in laser-driven gold Hohlraums, therefore to restrain the growth of high-mode hydro-instabilities and to improve the targetrobustness.Based on NIC's Rev. 5 ITD[S. W. Haan et al., PoP 18, 051001(2011)], we investigated the effect of thickness and dopant concentration of doped layers on implosion characteristics, including the Atwood number (AWN) of fuel-ablator interface, the density gradient scale length (DGSL) of ablation front and the implosion velocity (VIM); all three variables decrease with increment of dopant dosage, and increase with dopant concentration while keeping dosage constant. Since a smaller AWN, a larger DGSL, and a faster VIM always characterize a more robust ITD, one should make tradeoff among them by adjusting the dopant profiles in ablators.A Gaussian spectrum (GS) was used to imitate the Au M-band flux [Y. S. Li et al., PoP 18, 022701(2011)], and the impact of GScenter on implosion characteristics of Rev. 5 ITD was studied while moving the GScenter towards higher energy, the ablatorpreheat got severe, AWN got larger, DGSL got larger, and VIM got faster.