Session PO6: Warm Dense Matter; NIF Target Design

Matter in Extreme Conditions Instrument with LCLS for the study of High Energy Density Physics

Understanding the fundamental physics determining the equation of state and electronic transport properties of matter in extreme conditions (MEC) is of paramount importance to the high energy density community. With the very recent advent of a high peak brightness x-ray free electron laser source, the LCLS, we are constructing the MEC instrument that will cover a wide range of the extreme conditions phase space. The research areas that this instrument will address include warm dense matter, equation of state, hot dense matter, behavior of high-pressure materials, and phenomena of solid materials under extreme conditions. Here, we present the details of the MEC instrument.   

Measurement of ionization in shock-compressed deuterium using x-ray Thomson scattering

There is currently significant interest in the behavior of dense hydrogen under shock conditions, with applications ranging from planetary science to inertial confinement fusion. Here, we present the first x-ray Thomson scattering measurements on warm, dense deuterium in the collective regime. The experiment, performed on LLNL's Janus laser, used one 2 ns beam to drive a shock at a nominal pressure of 0.5 MBar into a deuterium target held at liquid conditions (19 K). A second 2 ns pulse pumped the Si Ly-$\alpha $ x-ray probe at 2 keV. Scattered x-rays were collected at 45 degrees in the forward direction and spectrally dispersed with a HOPG crystal spectrometer. A plasmon was detected, providing a direct measure of electron density. Simultaneous velocity interferometry was performed to determine pressure, allowing ionization state to be inferred. These results are compared to \textit{ab initio} and hydrodynamic simulations.   

Measuring the Ion Temperature in X-ray Thomson Scattering Experiments of Warm Dense Matter (WDM)

X-ray Thomson scattering offers an unprecedented method for measuring the properties of dense plasmas, including the electron temperature, the ionization state, and the free electron density. More difficult to extract from the measurements is the ion temperature. To date, the ion temperature has been extracted from experimental data using the diffractive portion of the scattering signal, which is a measure of the ion static structure S(k); thus, an accurate measurement requires an accurate model for S(k). Here, we compare models for S(k) in WDM and find that their differences are significant. Because of the paucity of WDM data, we use very accurate liquid metal data as a proxy. No current model reproduces all of the liquid metal data, although some are better for certain metals than others. We also introduce a new model that employs an effective screening length, obtained from the finite-temperature Lindhard response, in a modified hypernetted chain approach. This approach is superior to the previous approaches and we expect it will be even better for WDM.   

X-ray Thomson Scattering from Spherically Imploded ICF Ablators

Time-resolved X-ray Thomson scattering measurements from spherically imploded inertial fusion capsules-type targets have been obtained for the first time at the Omega OMEGA laser facility to characterize the in-flight properties of ICF ablators. In these experiments, the non-collective, or microscopic particle behavior, of imploding CH and Be shells, was probed using a 9 keV Zn He-alpha x-ray source at scattering angles of 113$^{\circ}$ and 135$^{\circ}$. for two drive pulse shapes.As an example, the analysis of In-flight scattering measurements from one set of directly-driven compressed 8600 $\mu $m-diameter, 40-$\mu $m thick Be shells taken (4.2 ns after the start of the compression beamswhen compressed a factor of $\approx $ 4.83x) yielded electron densities of $\sim $ 1.2$\pm $0.23x10$^{24}$cm$^{-3}$, temperatures of $\sim $13$\pm $32 eV, and an ionization state of Be(+2), with uncertainties in the temperature and density of about 40{\%} and 20{\%}. These conditions resulting in an inferred adiabat (ratio of plasma pressure to Fermi degenerate pressure) of 1.797 +0.3/-.5 with an error of about 30{\%}. The high signal-to-noise and high signal-to-background ratio of data obtained in these experiments provides a platform for studying the adiabat of other indirect-drive ICF ablators such as CH and High Density Carbon (HDC) ablators and demonstrates the viability of using this diagnostic to study the in-flight properties adiabat of implosion targets at the National Ignition Facility (NIF).   

X-ray Thomson scattering of isochorically proton heated Boron Nitride

We have measured for the first time the temperature of proton heated Boron Nitride using X-ray Thomson scattering. The experiment has been performed on the 300J, 10 ps Titan laser at Lawrence Livermore National Laboratory. The ultra-intense laser beam was split into two beams. 30\% of the energy was directed onto a 10$\mu$m Aluminum foil to generate a proton beam, and the remaining 70\% was focused onto a 10$\mu$m iron foil to generate a k-alpha backlighter at 6.4 keV. The proton beam isochorically heats a Boron Nitride foil, creating a solid density plasma with a temperature between 10-20 eV. X-rays are scattered from the heated target onto a curved HOPG crystal. X ray Thomson scattering in the collective regime provides an accurate measurement of the temperature from the ratio of up- vs. down-shifted plasmon signals. Temperature has been measured as a function of time (from 200 to 400 ps after the proton irradiation) and proton flux (by changing the intensity of the laser on the proton target).   

Laser Design for Next Generation Compton Scattering Source at LLNL

We describe laser systems designed for the next generation Mono-Energetic Gamma-Ray (MEGa-ray) Compton scattering light source at LLNL. An 80 fs Yb:doped fiber oscillator seeds a photogun drive laser (PDL) and a high energy interaction system laser (ILS). Utilizing chirped pulse amplification (CPA) in fiber, the PDL will generate 80 $\mu $J, spatially and temporally shaped pulses at 263 nm at 120 Hz precisely synchronized to the linac RF. The PDL system employs large mode photonic bandgap fibers and large area multi-layer dielectric gratings to generate over 1mJ per pulse with high recompression fidelity prior to frequency quadrupling. The high energy, 120 W ILS utilizes (CPA) in Nd:YAG to amplify a sub-nanometer bandwidth 20 $\mu $J pulses from a fiber system to 1 J. A novel pulse stretcher provides a dispersion of over 7000 ps/nm to expand a several picosecond wide seed pulse to 6 ns. After amplification, the pulse is recompressed to 10 ps with a hyper-dispersive pulse compressor. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Design Calculations for NIF Convergent Ablator Experiments

Design calculations for NIF convergent ablator experiments will be described. The convergent ablator experiments measure the implosion trajectory, velocity, and ablation rate of an x-ray driven capsule and are a important component of the U. S. National Ignition Campaign at NIF. The design calculations are post-processed to provide simulations of the key diagnostics -- 1) Dante measurements of 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 measurements of the imploded core. The simulated diagnostics will be compared to the experimental measurements to provide an assessment of the accuracy of the design code predictions of hohlraum radiation temperature, capsule ablation rate, implosion velocity, shock flash areal density, and x-ray bang time. Post-shot versions of the design calculations are used to enhance the understanding of the experimental measurements and will assist in choosing parameters for subsequent shots and the path towards optimal ignition capsule tuning. *SNL, LLNL, and LANL are operated under US DOE contracts DE-AC04-94AL85000. DE-AC52-07NA27344, and DE-AC04-94AL85000.   

NIF Convergent Ablator Performance Measurements

Assessing the ablator performance around the time of peak implosion velocity is an important step towards achieving ignition. Using x-ray radiography the time-resolved velocity, mass, and areal density of the remaining unablated shell were measured near peak velocity on NIF implosions. With the simple rocket model this allowed the ablation pressure and mass ablation rate to be estimated. Spectrally-resolved D-3He proton measurements were used to determine the combined unablated and ablated areal density. Together these information-rich diagnostics provide tight constraints on the conditions in the ablator at its maximum kinetic energy.   

Modeling capsule performance on the National Ignition Facility

Experiments on the National Ignition Facility (NIF) in September-December, 2009, and resuming in 2010, employed gas-filled symmetry capsules (SymCaps). These capsules were hydrodynamic surrogates of the ignition design, which has a layer of DT ice. While the main purpose of these capsules was to facilitate tuning of implosion symmetry, they also provide a significant set of capsule performance data. We examined a subset of experiments which gave nearly round core images and found systematic deviations of measurements from nominal modeling. Exploration of known uncertainties suggested than mix arising from isolated defects on the capsule outer surface best matched performance systematics. Upcoming experiments have been designed to test this hypothesis.   

Assessing NIF Ignition Capsule Performance Sensitivity to Hot Electrons

We report on recent work to characterize the effects of hot electrons on the performance of the ignition capsule point design on the National Ignition Facility. We study effects of hot electrons as a function of their time of deposition, their quantity and their temperature. We also explore the sensitivity to where they are deposited in relation to the gold hohlraum and Helium fill gas. Capsule performance impact is assessed based on fuel adiabat maximum and implosion velocity.   

Layered capsule implosions on the National Ignition Facility

Recent experiments on the National Ignition Facility have provided the hohlraum target and laser drive conditions suitable for compressing inertial confinement fusion capsules with cryogenic fuel layers. As the next step towards ignition, these experiments employ capsules with layered TH ice and trace amounts of D that generate $\sim $ 10$^{14-15}$ 14 MeV (primary) neutrons. The modest neutron yield allows fielding a suite of x-ray and neutron diagnostics to study the assembly of thermonuclear fuel. In particular, x-ray imaging and radiography as well as neutron yield Y, and the fuel down scattered fraction dsf, are developed to measure dense fuel layers with rR $\ga$ 1 surrounding a hot spot of multi-keV temperatures. The comparison with a large 2D simulation data base will be used to investigate implosion performance. In particular, a good predictor of performance has been found to be ITFX=Ydsf$^{2}$. The goal of the experiments is to optimize performance by tuning implosion velocity, entropy, shape and mix before deployment of DT fuel for achieving ignition and fusion burn.   

Measurements of THD Implosions for the Prediction of Ignition Implosion Performance

The National Ignition Campaign (NIC) will use non-igniting ``THD'' capsules to study and optimize the hydrodynamic assembly of the fuel without burn. These capsules are designed to simultaneously reduce DT neutron yield and to maintain hydrodynamic similarity with the DT ignition capsule. We will discuss nominal THD performance and the experimentally observable ignition threshold factor (ITFX) for both CH and Be targets. We will also cover simulated xray and nuclear diagnostic signatures produced by the suite of THD diagnostics. We will finally explore enhancements of the ITFX metric via combinations of xray and nuclear measurements.   

Modeling polar-direct-drive implosions at the NIF using HYDRA

2D, HYDRA [M.~M.~Marinak et al., Phys. Plasmas \textbf{3}, 2070 (1996)] simulations of polar-direct-drive (PDD) implosions of NIF symmetry capsules are presented. Direct-drive implosions of symmetry capsules at the National Ignition Facility (NIF), may provide a path to validating inertial-confinement-fusion (ICF) modeling capability in the presence of high mode number features. One particular complication facing PDD validation experiments, is the development of hydrodynamic instabilities, driven unstable by spatial inhomogeneities in laser power deposition across the surface of the target, that dominate the effects of imposed features (e.g. non-symmetric laser drive, surface irregularities etc.). In order to make experimentally relevant predictions of PDD implosion characteristics, the laser ray source is modeled using a beam (cone) geometry which matches the laser pointing at the NIF. The ray power comprising of each beam (cone) is distributed over a super-Gaussian ellipse in the focal plane, with exponent chosen to match empirical phase plate data for the intensity distribution. Here, we study the effects of spatial inhomogeneity in the laser drive resulting from laser pointing and ray noise on implosion symmetry, and instability growth.   

Design of an Experiment to Test Three Color Tuning on NIF

In the 2009 NIC Campaign, we demonstrated symmetry tuning in NIF targets using crossbeam transfer [1,2]. By making small ($\sim$ 1.5-8.5 A) changes to the wavelength of the outer cone relative to the inner cone, energy is transferred from outer to inner cone and changes the symmetry of the imploded capsule from oblate to round or prolate. We measured backscattered light on one of the inner cone beams (30 degree) and found that the backscatter did not change significantly as power was transferred to the inner cone. Subsequent analysis of the hot electron measurement suggests that the backscatter did increase on the 23.5 degree cone, which is not directly measured. Adding a third wavelength to the 23.5 degree cone will allow us to control the crossbeam transfer to the 23.5 and 30 degree cones independently. This talk will discuss the design of an experiment to test the energetics and symmetry implications of a third color. \\[4pt] [1] S. H. Glenzer, et. Al., Science, 327, 1228 (2010).\\[0pt] [2] P. Michel, et. Al, Phys of Plasmas, 17, 056305, (2010).   

Application of a third laser wavelength option to optimize mega-joule laser hohlraum coupling on the National Ignition Facility

A third laser wavelength option is currently under development on the NIF. The third oscillator will complement the existing two that seed the inner (at 23.5 and 30 degree from hohlraum axis) and outer (at 44.5 and 50 degree) cones of beams. By shifting the wavelength of the oscillators we have previously demonstrated control of energy transfer between the inner and outer cones of beams. The new third oscillator will be added to manipulate the power of the 23.5 degree cone of beams with the goal to redistribute the energy among the inner (23.5 and 30 degree) beams to reduce Stimulated Raman Scattering losses and achieve high coupling for hohlraum experiments that are driven with more than 1 MJ of energy. In this talk, we will focus on the physics of the energy transfer with three wavelengths, and show how the intrinsic nature of the plasma flow in a typical hohlraum target allows to exchange energy between the 23.5 and 30 degree cones with minimal impact on the outer cones - hence preserving radiation symmetry with the benefit of controlling the hohlraum internal laser beam intensities.