Session CO8: Compression and Burn I

NIF capsule modeling

Implosions have been carried out at the National Ignition Facility (NIF) of three capsule types, symmetry capsules (SymCaps), convergent ablation capsules. and THD capsules. These capsules are surrogates of the ignition capsule design, each optimized to measure specific implosion characteristics. In addition, shock timing data was obtained with specially-designed targets. An abundance of capsule performance data has been obtained with x-ray and nuclear diagnostics, including implosion velocity, remaining ablator mass, times of peak x-ray and neutron emission, core image size, core symmetry, neutron yield, and x-ray spectra. We have attempted to match this data set with simulations, adjusting the radiation drive, capsule surface roughness, and physics uncertainties. Comparison of simulated and measured performance parameters will be shown.   

Ablation Rate and Implosion Velocity in ICF Capsules at the NIF

For ICF ignition at the National Ignition Facility (NIF), it is thought that the spherically imploding capsule must be tuned so that 90-96{\%} +/- 1{\%} of the original ablator (the percentage of ablated mass depends upon ablator type and capsule design details) is removed at the time when the rocket payload (the DT fuel plus remaining ablator mass) reaches a peak implosion velocity of about 370 um/ns. An assessment of the accuracy of the integrated (hohlraum with capsule) design code predictions of capsule ablation rate together with hohlraum radiation temperature, implosion velocity, ablator remaining mass, shock flash areal density, shock flash time, and x-ray bang time is provided by comparing the simulated diagnostic outputs from the integrated calculations to the measurements made in convergent ablation experiments at the NIF.   

High-accuracy measurements of yield and areal density ($\rho $R) using the Magnetic Recoil Spectrometer (MRS) at the National Ignition Facility (NIF)

Proper assembly of capsule mass, as manifested through the evolution of \textit{$\rho $R}, is essential for achieving hot-spot ignition planned at the NIF. Experimental information about \textit{$\rho $R} and\textit{$\rho $R} asymmetries, ion temperature ($T_{i})$ and yield ($Y_{n})$ are therefore critical for understanding how the assembly of the fuel occurs. To obtain this information, a neutron spectrometer, called the MRS, was commissioned on the NIF in the fall of 2010 for measurements of the absolute neutron spectrum. From measured down-scattered ratios (\textit{dsr}) in the range 2.4-4.9{\%}, \textit{$\rho $}\textit{Rs} in the range 600-1300 mg/cm$^{2}$ have been inferred. The accuracy of these measurements was as good as 6{\%}, a value that will improve as $Y_{n}$ and \textit{dsr} increase. From measurements of the primary neutron spectrum, $Y_{n}$ has also been determined to an accuracy of 5{\%}. This work was supported in part by the U.S. DOE, LLNL and LLE.   

First ion temperature measurements with the Magnetic Recoil Spectrometer (MRS) at the National Ignition Facility (NIF)

Measuring the plasma ion temperature ($T_{i})$ is essential for understanding the performance of capsule implosions at the National Ignition Facility (NIF). Recent improvements in our understanding of the instrument response of the MRS, in operation on the NIF since the fall of 2010, allow for accurate determination of $T_{i}$ and possibly implosion-drift velocity from the measured MRS spectrum. In this presentation, we discuss the accuracy of the measured $T_{i}$, compare the results to data obtained from other neutron diagnostics, and look at what additional information can be determined from the MRS data, given the improved understanding of the instrument response. A potentially important observable in the neutron spectrum is the peak energy shift, which may provide a measurement of the collective implosion-drift velocity. This work was supported in part by the U.S. DOE, LLNL and LLE.   

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

V. GLEBOV, P. RADHA, T. SANGSTER, \textit{LLE}, R. OLSON, R. LEEPER, SNL, J. KLINE, G. KYRALA, D. WILSON, \textit{LANL,} J. KILKENNY, A. NIKROO, GA, C. SANGSTER, \textit{LLE} The Wedge Range Filter (WRF) proton spectrometer was developed for OMEGA and transferred to the NIF as a National Ignition Campaign (NIC) diagnostic. In tuning campaign implosions containing D and $^{3}$He gas, the WRFs measure the spectrum of protons from D-$^{3}$He reactions. The energy downshift of the 14.7-MeV proton is directly related to total $\rho $R through the plasma stopping power. WRFs fielded simultaneously on the pole and equator measure low-mode polar $\rho $R asymmetries due to drive inhomogeneity. We find no correlation between shock $\rho $R symmetry and x-ray self-emission symmetry near peak compression for low polar modes. Adjacent WRFs are sensitive to high-mode asymmetries due to hydro instabilities; these have not been observed. This work was supported in part by the U.S. DOE, LLNL and LLE.   

Measurements of shock-bang and compression-bang time in NIF implosions using the particle Time-of-Flight (pTOF) diagnostic

The nuclear shock-bang and compression-bang times provide unique information about the shock physics in, and dynamics of, ICF implosions at the National Ignition Facility (NIF). The shock-bang time may address questions about fuel preheat, whereas compression-bang time is an indicator of energy coupling to the capsule. The particle Time-of-Flight diagnostic (pTOF), recently implemented on the NIF, measures the bang times using shock-generated D$^{3}$He protons and compression-generated DD-neutrons in D$^{3}$He gas-filled tuning shots at the NIF. In combination with a Wedge Range-Filter (WRF) proton spectrometer fielded at the pTOF, time-resolved $\rho $R measurements are also conducted. The diagnostic also measures the bang time in cryogenic DT and DT-Exploding Pusher implosion. The pTOF results from the tuning campaigns in 2011 will be presented, and the implications of the data will be discussed. This work was supported in part by the U.S. DOE, LLNL and LLE.   

Diagnosing ablator-fuel mix in NIF implosions using charged-particle spectrometry

Achieving areal-density and temperature conditions necessary for hot-spot ignition at the National Ignition Facility (NIF) requires careful control of four key implosion parameters, which characterize the expected performance. These are implosion velocity, fuel adiabat, hot-spot shape and ablator-fuel mix. It has been shown that these four parameters can be combined into an ignition threshold factor (ITF) that is a good predictor of implosion yield. In this talk, we present a new technique for diagnosing the extent of ablator-fuel mix. This technique, which utilizes compact Wedge-Range-Filter (WRF) spectrometers routinely used on NIF, relies on spectral measurements of elastically-scattered deuterons from the fuel, which should not have escaped the implosion without the presence of mix,. This work was supported in part by the U.S. DOE, LLNL and LLE.   

Measurements of the ablator-ion energy-loss channel in direct-drive implosions on OMEGA

Measurements of ablator-ion spectra produced in direct-drive experiments on the OMEGA laser facility are presented. These ablator-ions are accelerated by the presence of hot electrons generated by laser-plasma interactions. Extensive measurements have been made with two magnet-based charged-particle spectrometers and more recently, with a Thomson Parabola Ion Energy Analyzer. The maximum ion energy and total energy carried by the ions depend strongly on the laser intensity and that as much as 3-4{\%} of the incident laser energy is lost to these ions. The ablator-ion energy spectra from warm (CH) and cryogenic (D$_{2}$/DT) targets have been used to infer the temperature of the hot electrons, and the results are in good agreement with hard x-ray inferred temperatures. Using the ablator-ion method combined with modeling of the initial electron energy distribution function, the level of preheat has been estimated. This work was supported in part by DOE, LLE and LLNL.   

Capsule design for charged-particle stopping power measurements using simultaneous gamma-ray, particle, and x-ray observations of implosions

We plan to measure the stopping power $\Delta E$/$\rho \Delta x$ of nonthermal charged particles in ICF plasmas, using an imploded capsule containing DT$^{3}$He. We will measure (1) ablator areal density $\rho \Delta $\textit{x via} $^{12}$C(n,n'$\gamma )$ gamma-ray detection, using the Gamma Reaction History diagnostic, where the gamma rays are generated by 14.1-MeV DT neutrons; and (2) proton energy downshift $\Delta E$ \textit{via} spectrometry of 14.7-MeV D$^{3}$He protons, through collaboration with the MIT PSFC group. To measure $\Delta E$/$\rho \Delta x$ with a given accuracy imposes requirements on the accuracy of the separate measurements of $\Delta E$ and $\rho \Delta x$, and in turn on the yields of DT and D$^{3}$He reactions and $\rho \Delta x$ of the capsule. Other requirements include optimizing the shell to have shallow gradients of temperature $T_{e}$ and density $n_{e}$ so that most of the particle slowing occurs at well defined conditions. Further major necessities are: the ability to diagnose $T_{e}$ and $n_{e}$ in the shell \textit{via} x-ray spectra; and minimizing shell perturbation growth and anisotropy of particle emission.   

Analysis of the First NIF Neutron Images

Neutron imaging at the National Igntion Facility obtained its first images from both directly laser driven and X-radiation driven implosions. A directly driven DT filled glass microballoon gave an oblate image (P2/P0=-45{\%}) whose size (P0=70$\mu $m) fit within the X-ray images. Simulations using the polar direct drive laser pointing give a round image of P0 $\sim $95$\mu $m. However as the electron flux limiter is reduced from 0.06 to 0.03 the image becomes oblate. The observed asymmetry can be reproduced by transferring $\sim $10{\%} of the energy from the outer laser beams to the inner. Radiation driven implosions of ignition capsules with 20{\%}D, and 50{\%}D produced $\sim $ 30$\mu $m radius oblate images in 12-15 MeV neutrons. Images in 10-12 MeV neutrons, which have experienced one scattering in the fuel and number $\sim $ 4{\%} of the primaries, showed larger images ($\sim $44-56 $\mu $m). Image sizes indicate the compression of the fuel and are consistent with observed 10-12/13-15MeV yield ratios. Work funded by the USDOE at LANL, LLNL, NSTEC and LLE.   

Comparison of downscattered neutron measurements using gated images and time-of-flight detectors at the NIF

Data collected by the NIF neutron imaging system includes two energy gated neutron images and spatially averaged time-of-flight (nToF) data from the same neutron fluence used by the imaging system. Images are produced 28 m from the target, at time windows corresponding to neutron kinetic energies of 10 to 12 MeV and 13 to 17 MeV. Time-of-flight data are produced in a thin scintillating paddle detector located 27.26 m from the target. The preliminary corrected downscattered ratios produced by spatially averaging subregions of the image data from three cryogenic, equimolar DT implosions during June 2011 are: 2.5 $\pm $ 0.9{\%}, 2.5 $\pm $ 0.9 {\%}, and 2.9 $\pm $ 1.1 {\%}. The corresponding downscattered ratios produced by the nToF detector are 3.7 $\pm $ 0.9 {\%}, 3.8 $\pm $ 0.9 {\%}, and 5.6 $\pm $ 1.3 {\%}. We will present an overview of the measurement methodologies and analysis studies of the apparent systematic difference.   

Observation of bright spots on x-ray image of imploded core with combination of a spectrally resolved and a temporally gated x-ray imager

To measure drive asymmetry, we use surrogate capsules made of plastic resin. However, small bright spots are often observed in the x-ray images of the imploded core. It is believed that those spots are caused by hydrodynamic instabilities initiated by initial perturbations on the capsule. To investigate mechanisms of the spot formation, we observed the core emission with combination of a spectrally resolved imager and a temporally resolved x-ray imager. Comparing those images, we found the spots observed before maximum compression are dominated by germanium line emission. In contrast, spots emerge after maximum compression is dominated by Bremsstrahlung below 9 keV. The observed difference in spectral contents is suggesting different mechanisms of the spot formation. The experimental images and mechanisms of the bright spot formation will be presented.   

Analysis of Diagnostic X-Ray Spectra of Implosions at the NIF

Ge dopant is added to CH ablators of NIF implosions to absorb x-ray preheat. Ge K-shell spectral line emission provides a core/shell-mix diagnostic.\footnote{S. P. Regan, this conference.} K$_{\alpha }$ fluorescence from shell dopant is directly related to the K-edge absorption by the dopant, giving a measure of shell compression. A model of the He-like line and satellite emission is fit to measured spectra to assay the mix mass. Constraints from 1-D simulations are considered. Ge line spectra are fit well by the model at single temperature-density points, giving mix masses that are typically below the 100-ng allowance for hot-spot mix in ignition implosions on the NIF.\footnote{S. W. Haan\textit{ et al.}, Phys. Plasmas \textbf{18}, 051001 (2011).} This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Ignition Capsules with Aerogel-Supported DT Fuel for the National Ignition Facility (NIF) and for Reactor Applications

For high rep-rate reactor applications, capsules with aerogel-supported liquid DT fuel (``foam-filled'') can have much reduced fill time compared to $\beta $-layering. The liquid DT vapor pressure is lowered once liquid DT is imbedded in a foam matrix, and the gas density is consequently closer to the desired density. We present NIF-scale foam-filled capsules in both 1-D and 2-D simulations. For foam density at 0.02 g/cm$^{3}$, there is a 9{\%} degradation in the clean 1-D yield if we include 2-D roughness up to a Legendre mode number of 60. This degradation in yield can be partially recovered if the capsule aspect ratio is increased. Optimal configuration is obtained when aspect ratio is increased until the clean fuel fraction is about 70 -- 75{\%} at peak velocity. Herrmann scan (in ablator and fuel thickness parameter space) will be presented. We will also present a statistical assessment of the capsule reliability to all expected manufacturing and physics uncertainties between capsules with clean DT fuel and with liquid DT in a foam. The performance of larger foam-filled capsules will also be presented.