Session NO5: Hohlraum Physics

Drive and Symmetry Experiments in Ignition-Scale Hohlraums

The goal of the symmetry capsule (``symcap'') experimental campaign on the National Ignition Facility (NIF) is to achieve a symmetric implosion at a radiation temperature suitable for ignition. In this talk, we describe the design of the symcap experiments, including performance predictions from the radiation-hydrodynamics code H\textsc{ydra}. Performance predictions are based on levels of Raman backscatter (SRS) and cross-beam transfer extrapolated from 2009 ignition-scale hohlraum experimental results. Simulated diagnostic results include x-ray drive (radiant intensity) and spectrum, laser-entrance-hole (LEH) x-ray images, imploded core self-emission brightness and emission time, imploded core shape and size, and D-D neutron yield. Recent experimental results are compared to predictions and to 2009 ignition-scale hohlraum data.   

Implosion symmetry tuning with megajoule laser pulses on the National Ignition Facility

A key element for indirect drive inertial confinement fusion is tuning the implosion symmetry. Symmetric implosions maximize the transfer of kinetic energy to the hot spot. One technique to measure the drive symmetry is the symcap. A symcap is a surrogate capsule that replaces the DT fuel layer by an equivalent mass of ablator material to mimic the hydrodynamic behavior of the capsule. The symcaps are filled with gas that provides an x-ray self-emission flash upon stagnation and is used to diagnose the radiation drive based on the shape of the emission. Simulations indicate that the shape of the emission flash correlates well with an ignition capsule's core shape. Using this data, the radiation drive in the hohlraum can be tuned to achieve symmetric implosions. The current symmetry campaign sets the initial hohlraum conditions to provide symmetric implosions for the ignition campaign. Experimental results will be presented for symmetry tuning with laser energies up to 1.3 MJ. Work for DOE by LANL (DE-AC52-06NA25396 and by LLNL (DE-AC52-07NA27344).   

Origins of the high flux hohlraum model

We review how the ``high flux model'' (HFM) helped clarify the performance of the Autumn 09 National Ignition Campaign (NIC) gas filled/capsule imploding hohlraum energetics campaign. This campaign showed good laser-hohlraum coupling, reasonably high drive, and implosion symmetry control via cross beam transfer. Mysteries that remained included the level and spectrum of the Stimulated Raman light, the tendency towards pancaked implosions, and drive that exceeded (standard model) predictions early in the campaign, and lagged those predictions late in the campaign. The HFM uses a detailed configuration accounting (DCA) atomic physics and a generous flux limiter (f=0.2) both of which contribute to predicting a hohlraum plasma that is cooler than the standard, XSN average atom, f=0.05 model. This cooler plasma proved to be key in solving all of those mysteries. Despite past successes of the HFM in correctly modeling Omega Laser Au sphere data and NIC empty hohlraum drive, the model lacked some credibility for this energetics campaign, because it predicted too much hohlraum drive. Its credibility was then boosted by a re-evaluation of the initially reported SRS levels.   

Time- and spatially-resolved characterization of halfraum radiation temperature using a VISAR interferometer measurement of quartz shock velocity at the National Ignition Facility

A VISAR diagnostic has recently been commissioned at the National Ignition Facility (NIF). Experiments will be conducted using a 500 micron quartz window with a 70 micron aluminum ablator. This package is located at the back plane of a 5 mm diameter halfraum driven by 80 beams from the NIF laser delivering a total of 240 kJ. The VISAR records the speed of the shock resulting from the 9 ns laser pulse at it traverses the quartz window. The spatial dimension of the VISAR field of view will capture the radial uniformity of the drive pressure from the halfraum. 2-D integrated simulations have been run predicting the shock speed and pressure uniformity, and results will be compared. Because the ablation pressure that drives the shock has a power law dependence on the drive temperature, there should be a similar power law scaling between the measured shock velocity and the drive temperature. This scaling will be examined with comparisons to the radiation drive temperature in the simulations, as well as with comparisons to the NIF DANTE measurement of power from the halfraum laser entrance hole.   

Thomson scattering measurements at the hohlraum laser entrance hole

Thomson scattering measurements of the plasma flow velocity and electron temperature at the laser entrance hole (LEH) of a gas-filled hohlraum have been performed for multiple LEH diameters. These measurements are compared to hydrodynamic simulations to assess calculations of crossed-beam energy transfer; a laser-plasma interaction process that is being applied on the National Ignition Facility to transfer large amounts of laser energy between beam cones. This process is used for tuning the capsule implosion symmetry and predictions rely on our detailed understanding of electron temperatures and plasma flow velocity in the LEH region. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

The effect of the initial hohlraum fill gas density on laser propagation in NIF hohlraums

Stimulated Raman (SRS) and Brillouin backscatter (SBS) from gas-filled hohlraums are measured from two quads of laser beams at the National Ignition facility. The time history and the spatial distribution in the near field are measured for the outer beams at 50 and the inner beams at 30 to the hohlraum axis. The time-dependent SRS spectrum, the radiation flux, and the symmetry of the imploded capsule support the use in Lasnex and Hydra of weakly flux-limited electron heat transport and atomic physics models that convert a large fraction of the laser energy absorbed in high-Z material to x-rays. The initial density of the gas filling the hohlraum has been shown to affect the amount of SBS but has little effect on the amount of SRS. We have shown that the scaling of SBS can be understood given the plasma parameters from these radiation-hydrodynamic simulations. Motivated by that success, we explored more generally how the initial fill density affects the plasma properties at peak power when the laser plasma instabilities are most potent.   

Tuning experiments for the first 2 ns of the NIF ignition pulse

To achieve ignition it is necessary that the capsule fuel remain in a low adiabat during compression. The fuel entropy at ignition time is mainly determined by the level of the first shock and the amount of early-time hot electron preheat. Drive asymmetry over the first 2 ns of the laser pulse leads to asymmetry in velocity and timing of the first shock, giving rise to additional source of entropy above the ideal case. The re-emit technique currently underway, is designed to tune the first 2 ns of the NIF laser pulse and to give a first assessment of preheat caused by hot electrons produced during window burn-through. We have performed several 3D rad-hydro simulations in support of upcoming experiments. Simulations are used to explore the use of various diagnostics and their implications for our tuning plan. Simulations have also been used to establish the strategy for adjusting the symmetry cone-fraction as the level of the first picket is varied. We will present the tuning procedure and compare with experimental results.   

The HYDRA DCA Atomic Kinetics Package

HYDRA is a multi-physics, 2D/3D radiation hydrodynamics design code that is routinely used to simulate inertial confinement fusion (ICF) experiments. HYDRA's in-line NLTE atomic kinetics capabilities include an average-atom treatment (XSN) and a newer super-configuration based treatment that incorporates the kinetics and screened hydrogenic models from CRETIN (DCA). We will discuss the HYDRA-DCA package and highlight recent updates: physics improvements in the kinetics package/models, HYDRA support for NLTE equations of state, and support for mixed-cell advection. HYDRA-DCA has been validated against tabulated opacities (LTE) and compared to XSN (NLTE) for ICF-relevant materials. \\ Results from HYDRA simulations of recent National Ignition Facility Hohlraum experiments (DCA modeling of gold) as well as capsule design calculations (DCA modeling of germanium dopant) will be presented. In making comparisons between NLTE models and against experiment, we are able to assess the physics and modeling parameters that have the largest impact on our integrated simulation results.   

Shock-Timing Measurements in ICF Targets Filled with Cryogenic Deuterium

High-performance ICF target designs use multiple shock waves to condition the target before it is imploded. Accurate timing of these shocks is critical to target performance. A series of experiments at the Omega Laser Facility have used velocity interferometry to measure shocks in directly driven cryogenic targets. Multiple spherical shocks were observed propagating in liquid deuterium. The measured shock velocities and shock-coalescence times are compared to hydrodynamic simulations. We discuss similar experiments to be performed at the National Ignition Facility. These will be used to fine-tune laser pulse shapes to achieve the specifications and performance needed for ignition targets. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Initial NIF Shock Timing Experiments: Comparison with Simulation

Initial experiments are underway to demonstrate the techniques required to tune the shock timing of capsule implosions on the National Ignition Facility (NIF). These experiments use a modified cryogenic hohlraum geometry designed to precisely match the performance of ignition hohlraums. The targets employ a re-entrant Au cone to provide optical access to the shocks as they propagate in the liquid deuterium-filled capsule interior. The strength and timing of the shocks is diagnosed with VISAR (Velocity Interferometer System for Any Reflector) and DANTE. The results of these measurements will be used to set the precision pulse shape for ignition capsule implosions to follow. Experimental results and comparisons with numerical simulation are presented.   

First early time symmetry and hot electron measurements for indirect drive ignition implosions on the National Ignition Facility

In ignition experiments on the National Ignition Facility (NIF), the symmetry of the hohlraum radiation drive for the first 2 ns is tuned using the re-emit technique [1]. At the same time, in order to maintain the capsule fuel on a low adiabat for successful ignition, the level of early $>$ 170 keV hot electrons generated in the hohlraum that reach the DT fuel [2] has to be $<$ few Joules. The generated hot electrons are inferred from the hohlraum hard x-ray (20-500 keV) spectra measured with the FFLEX diagnostic [3]. We report on the first re-emit symmetry experiments performed on NIF in full ignition scale hohlraums. We also infer the level of hot electrons intercepted by the capsule from $>$30 keV x-ray imaging of the re-emit sphere and hohlraum bremsstrahlung onto image plates whose data is normalized to the FFLEX spectra. [1] E.L. Dewald, \textit{et. al.,} Rev. Sci. Instrum. 79, 10E903 (2008). [2] E.L. Dewald \textit{et. al}., \textit{J. Phys.:Proceedings IFSA 2009}.   

Measurement of hard x-ray bremsstrahlung emission from indirectly driven capsules at peak of the laser drive

There is currently significant interest in understanding the fraction of hot electrons impinging on the capsule in indirectly driven implosions on the NIF. They may degrade implosion performance by causing pre-heat and potentially sending an additional shock wave into the capsule at the rise of the main pulse. In order to quantify their impact during the rise and the peak of the laser drive, we are fielding a high aspect ratio pinhole imager with 400 $\mu$m resolution, 2x magnification viewing through a Laser Entrance Hole and equipped with both a gated x-ray detector and absolutely calibrated imaging plate. By differentially filtering we will record images of the bremsstrahlung emission from both the capsule and hohlraum in the energy range between 50 - 100 keV, and also normalize the results by the spatially integrating absolutely calibrated time-resolved hard x-ray-spectrometer FFLEX.   

Hot electron measurements on NIF: energetics and preheat

On NIF, the time-integrated FFLEX broadband spectrometer measures Bremsstrahlung hard X-rays emitted in the hohlraum wall by energetic electrons created inside the hohlraum. The hot electron spectrum inferred from FFLEX measurements shows 2 distinct components for most gasfilled hohlraums. A low temperature part (T$_{hot} \sim $ 20 keV) can be related to the total amount of stimulated Raman backscatter (SRS) and this allows us to reconcile measurements of the energy balance of gas filled hohlraum when the frequency of beam cones was changed. The higher energy component (T$_{hot} \sim $ 40-50 keV) will be discussed in terms of preheat. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Gas-filled Rugby hohlraum energetics and implosions experiments on OMEGA

Recent experiments [1,2] have validated the x-ray drive enhancement provided by \textit{rugby-shaped hohlraums} over cylinders in the indirect drive (ID) approach to inertial confinement fusion (ICF). This class of hohlraum is the baseline design for the Laser M\'{e}gajoule program, is also applicable to the National Ignition Facility and could therefore benefit ID Inertial Fusion Energy studies. We have carried out a serie of energetics and implosions experiments with OMEGA ``scale 1'' rugby hohlraums [1,2]. For empty hohlraums these experiments provide complementary measurements of backscattered light along 42\r{ } cone, as well as detailed drive history. In the case of gas-filled rugby hohlraums we have also study implosion performance (symmetry, yield, bangtime, hotspot spectra...) using a high contrast shaped pulse leading to a different implosion regime and for a range of capsule convergence ratios. These results will be compared with FCI2 hydrocodes calculations and future experimental campaigns will be suggested. \\[4pt] [1] F. Philippe \textit{et al}., Phys. Rev. Lett. \textbf{104}, 035004 (2010). \\[0pt] [2] H. Robey \textit{et al., }Phys. Plasnas \textbf{17}, 056313 (2010).   

Proton Radiography of Spontaneous Fields, Plasma Flows and Dynamics in X-Ray Driven Inertial-Confinement Fusion Implosions

Backlighting of x-ray-driven implosions in empty hohlraums with mono-energetic protons on the OMEGA laser facility has allowed a number of important phenomena to be observed. Several critical parameters were determined, including plasma flow, three types of spontaneous electric fields and megaGauss magnetic fields. These results provide insight into important issues in indirect-drive ICF. Even though the cavity is effectively a Faraday cage, the strong, local fields inside the hohlraum can affect laser-plasma instabilities, electron distributions and implosion symmetry. They are of fundamental scientific importance for a range of new experiments at the frontiers of high-energy-density physics. Future experiments designed to characterize the field formation and evolution in low-$Z$ gas fill hohlraums will be discussed.