Session CO7: CBET and Hohlraum Drive

The Quartraum: A platform for investigation of cross-beam energy transfer

Cross-beam energy transfer [1] is a methodology used at the National Ignition Facility (NIF) to control implosion symmetry, by transferring energy between the inner and outer cones of laser beams. This process is time-dependent, and does not transfer power in a spatially uniform manner. A platform to investigate the spatial non-uniformity of laser beams after having undergone cross-beam energy transfer is under development. This target consists of the slice of the hohlraum near the laser entrance hole (LEH), where transfer occurs, which is roughly one-fourth the length of a nominal hohlraum (quartraum). Beams are incident on this LEH from only one side of the quartraum. The outer beams hit the quartraum cylindrical walls, and the inner beams hit the far endcap of the cylinder. This far endcap is either a 100{\%} LEH window that the inner beams burn through (and then strike a witness plate), or it is a thin wall that is imaged. Stretch goals for this platform are detection of specularly reflected outer beams, or of Brillouin Enhanced Four-Wave Mixing. \\[4pt] [1] P. Michel \textit{et al.}, Phys. Plasmas \textbf{16}, 042702 (2009), and references therein.   

Mitigation of Cross-Beam Energy Transfer in Direct-Drive Plasmas

Cross-beam energy transfer (CBET) during OMEGA low-adiabat cryogenic experiments reduces the hydrodynamic efficiency by 35{\%}, which lowers the calculated one-dimensional (1-D) yield by nearly an order of magnitude. Reducing the diameter of the laser beams after a sufficient conduction zone has been generated (two-state zooming), is predicted to mitigate CBET while maintaining low-mode uniformity. A radially varying phase plate is proposed to implement two-state zooming on OMEGA. Hydrodynamic simulations, using the calculated laser spots produced by the proposed zooming scheme on OMEGA, show that implementing zooming will provide a more hydrodynamically stable implosion, allowing the in-flight aspect ratio to be reduced from 30 to 22. Alternate zooming schemes that improve the power spectrum by controlling the correlation between multiple sub-apertures will be discussed. Demonstrating zooming on OMEGA would validate a viable direct-drive CBET mitigation scheme and establish a hydrodynamically equivalent implosion pathway to direct-drive ignition. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

A Laser--Plasma Interaction Model for Cross-Beam Energy Transfer

Interaction between multiple crossing laser beams is a common feature in direct-drive inertial confinement fusion (ICF) plasmas. Hydrodynamic simulations of direct-drive ICF target evolution\footnote{ I. V. Igumenshchev\textit{ et al.}, Phys. Plasmas \textbf{19}, 056314 (2012).} have shown that cross-beam energy transfer (CBET) has a strong effect on the coupling of laser energy to the target. A laser--plasma interaction (LPI) approach for CBET has been developed based on the model\footnote{ A. V. Maximov\textit{ et al.}, Phys. Plasmas \textbf{11}, 2994 (2004).} of nonparaxial propagation of multiple laser beams coupled to the plasma response. The effects of laser incoherence have been considered, leading to an interaction that occurs mostly in high-intensity laser speckles. Crossing laser beams can drive common ion-acoustic waves and scatter off them, increasing the level of light scattering. Implementation of the LPI CBET model in hydrodynamic modeling is discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Comparison of 2-D \textit{DRACO} Cross-Beam Energy Transfer Simulations with OMEGA and NIF Experiments

Cross-beam energy transfer (CBET) causes pump and probe laser beams to exchange energy via stimulated Brillouin scattering,\footnote{ C. J. Randall, J. R. Albritton, and J. J. Thomson, Phys. Fluids \textbf{24}, 1474 (1981).} which increases the scattered light and redistributes absorbed laser energy. A new CBET model has been incorporated into the 2-D hydrodynamics code \textit{DRACO}.\footnote{ P. B. Radha \textit{et al.}, Phys. Plasmas \textbf{12}, 056307 (2005).} Simulations using both \textit{LILAC} and \textit{DRACO} agree well with OMEGA measurements when CBET is included in tandem with a nonlocal electron transport model. The polar-angle--dependent scattered-light measurement for OMEGA polar-drive (PD) experiments was reproduced using the CBET model in \textit{DRACO}. There is an observable difference in the morphology of the imploding target on the first National Ignition Facility (NIF) PD shots. \textit{DRACO} with the CBET model will be used to simulate the early NIF implosions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Near-field analysis of stimulated Brillouin scattering in the presence of strong cross-beam energy transfer on the National Ignition Facility

Laser beam amplification in plasmas via cross-beam energy transfer (CBET) can lead to increased levels of backscatter due to increased laser intensities. Besides a reduction of laser energy coupling to the target, this can create a risk of optics damage in the case of stimulated Brillouin scattering (SBS). Because the SBS wavelength is very close to that of the incident laser, it can back-trace the path of the laser inside the targets, and can return a well-collimated beam of light, which increases the fluence on the optics compared to stimulated Raman scattering (SRS). In this presentation, we present a near-field analysis of the SBS light for a shot where CBET amplification of some beams by a factor $\sim$4-5 led to optics damage on six transport mirrors. Experimental results are compared to three-dimensional simulations, which highlight the role of speckles in seeding and collectively amplifying the SBS light. In particular, the study reveals that the SBS light largely retains the polarization of the incident laser beam, has a nearly plane wave front and can be even more collimated than the incident laser beam; the latter feature is detrimental for optics damage as it further increases the fluence, but could also be a useful feature for a Brillouin backward laser amplifier.   

Hohlraum Design for a High Foot, High Adiabat Implosion on NIF

We recently began a campaign on NIF to test capsule performance for a higher adiabat design. The higher adiabat is achieved by using a higher power in the foot of the laser pulse, which produces a stronger first shock. The higher power in the foot presents some challenges and opportunities for the hohlraum design. In particular, the higher foot causes more capsule ablation early in the pulse. This increases the margin against hydrocoupling, which occurs when pressure waves launched from the laser-heated hohlraum gas impact the capsule, and allows the use of a higher hohlraum fill density compared to the low foot design. Higher fill density results in less wall motion during the pulse. The higher foot also increases risk for hot electrons generated by the 2wp instability in the window. This talk will summarize hohlraum performance for the high foot design.   

Three dimensional simulations of NIF ViewFactor hohlraums

The ViewFactor target is a modified version of a National Ignition Facility (NIF) ignition hohlraum that allows extensive x-ray imaging of the hohlraum wall. Thus, data from ViewFactor experiments are uniquely suited for studying the physics of the hohlraum wall/laser beam interaction. This study reports on 3D simulations of NIF ViewFactor experiments using the target design code \textsc{hydra}. Simulated x-ray images show azimuthal variations in the x-ray brightness of the hohlraum wall; however, these variations are larger in magnitude in the data. This is possibly due to the 3D simulations not correctly reproducing the motion of the hohlraum wall in the laser spots. A detailed in-line cross-beam power transfer model that includes the change in laser beam intensity due to transfer is tested and compared with the data.   

Experiments Constraining Indirect Drive Hohlraum Performance on the National Ignition Facility

``Viewfactor'' experiments on the National Ignition Facility are used to characterize the x-ray drive seen by an indirect drive ICF capsule. The experiments employ a truncated hohlraum affording views of the soft x-ray emission striking the capsule as well as the emission exiting the laser entrance holes. The experiments show that the drive measured from the capsule view is 15 to 20{\%} lower than that inferred from calculations. Additionally, they show the entrance hole does not close as much as the simulation would predict by the end of the pulse. The results are consistent with simulations of capsule implosion experiments that require multipliers less than unity on the backscatter-corrected laser power into the hohlraum. The results also explain the discrepancy between the observed entrance hole flux and that extracted from the reduced-drive simulations. Finally, both time-integrated and time-resolved x-ray images from the experiments demonstrate the motion of the wall plasma directly heated by the outer-cone beams is more extensive than predicted. These results provide much greater constraints on a predictive model of indirect drive ICF hohlraum performance. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.   

The gold bubble feature seen in NIF ignition hohlraums and its 8-fold symmetry

At the National Ignition Facility (NIF), a fuel capsule is imploded by an x-ray drive created by 192 laser beams heating a gold hohlraum. The beams enter in openings at the ends of the hohlraum in four cone angles, the outer cones (50$^{\mathrm{o}}$ and 44.5$^{\mathrm{o}})$ and the inner cones (30$^{\mathrm{o}}$ and 23.5$^{\mathrm{o}})$. The region where the outer cones hit the hohlraum wall ablates wall material radially into the hohlraum, producing a gold ``bubble''. The region where the inner beams hit the hohlraum wall is prevented from ablating by material blowing off from the capsule. Recent ``ViewFactor'' experiments have used a truncated hohlraum (one side cut off at 40{\%} of standard length) to study hohlraum performance. Hard x-ray (\textgreater 3 keV) images taken from this open end show that the gold ``bubble'' has an 8 fold symmetry corresponding to the 50$^{\mathrm{o}}$ beam geometry. This different behavior of the 50$^{\mathrm{o\thinspace }}$and 44.5$^{\mathrm{o}}$ beams is actually visible in the hard x-ray images of the standard hohlraum targets. These latter images are used to study variations in the bubble as a function of laser pulse shape, gas fill, hohlraum length, and energy transfer.   

Determining the hohlraum radiation temperature and M-band fraction by a shock wave technique

Experiments have been conducted with two materials Al and Ti as shock wave witness plates to demonstrate the proposal of determining the hohlraum peak temperature and M-band fraction. The peak radiation temperature T$_{\mathrm{R}}$ and M-band fraction f$_{\mathrm{m}}$ are simultaneously determined by using the observed shock velocities in Al and Ti. This is the first experimental determination of T$_{\mathrm{R}}$ and f$_{\mathrm{m}}$ by using the shock wave technique. For the Au hohlraum used in the experiments, T$_{\mathrm{R}}$ is about 160 eV and f$_{\mathrm{m}}$ is between 4.3-6.3{\%} under 1ns laser pulse of 2 k, and T$_{\mathrm{R}}$ is about 202 eV and f$_{\mathrm{m}}$ is about 9{\%} under 1ns laser pulse of 6 kJ. The results from this technique are complementary to those of the broadband soft x-ray spectrometer.   

Enhanced NLTE Atomic Kinetics Modeling Capabilities in HYDRA

In radiation hydrodynamics modeling of ICF targets, an NLTE treatment of atomic kinetics is necessary for modeling high-Z hohlraum wall materials, high-Z dopants mixed in the central gas hotspot, and is potentially needed for accurate modeling of outer layers of the capsule ablator. Over the past several years, the NLTE DCA atomic physics capabilities in the 3D ICF radiation hydrodynamics code HYDRA have been significantly enhanced. The underlying atomic models have been improved, additional kinetics options including the ability to run steady-state NLTE have been added, and the computational costs have been significantly reduced using OpenMP threading. We will discuss how these code improvements will enable higher fidelity simulations of ICF hohlraum energetics, laser irradiated sphere experiments, and ICF capsule implosions.   

Hohlraum design for doped-HDC ablator ignition capsule

Ignition designs using W-doped high-density-carbon (HDC) ablators have performed well in 1D- and 2D-studies [1]. A major advantage of HDC over CH ablators is that the surface roughness is 10x-smoother. However, higher pressure ($\ga$ 6.5 MB) is required to stay above the melt curve, to avoid seeding non-uniformities at the fuel-ablator interface. We have used the design code HYDRA to obtain the laser pulse that minimizes the low-mode radiation asymmetry. The HDC design requires a smaller inner-to-outer wavelength separation to achieve symmetry during the peak of the laser pulse and a much smaller inner cone fraction at early times [2] than the CH target. This design has recently been fielded at NIF using undoped-HDC capsules with significant success (largest neutron yields to date). In this paper we will present a low- (4-shock) and high- (3-shock) adiabat designs and compare them with the HDC experimental database.\\[4pt] [1] D. Ho et al, BAPS.2012.DPP.GO4.13\\[0pt] [2] J. Milovich et al, BAPS 2011 DPP.CO6.00003   

Current Filaments, Plasma Flow and their Spontaneous Fields in Laser-Holhraum Interactions

We present the first time-gated, side-on proton radiography data that reveals the structure and dynamics of current filaments, plasma flow, and their spontaneous electromagnetic fields occurring at the hohlraum laser-entrance holes. Plasma instabilities are shown to play a critical role in such dynamic structures. In the early phase, these instabilities show up as collisionless Weibel-induced current filaments resulting from expansion of low-density plasma into vacuum, and in the later phase as resistive MHD modes resulting from the adiabatic expansion of on-axis, stagnated wall plasma blowoff. Time-resolved observations of electromagnetic fields associated with these plasma instabilities have been made. The experiments demonstrate the dominance of magnetic fields over electric fields, consistent with self-emissions of charged fusion products observed from experiments at the NIF and OMEGA. This work was supported in part by the U.S. DOE, LLNL and LLE.   

Hohlraum energetics study on Shenguang-III prototype laser facility

Comprehensive and accurate characterization of hohlraum drive need to use a variety of methods resolving different photon range and multiple viewing area. In recent years, hohlraum physics have been studied extensively on Shenguang-III prototype. These experiments employed mainly Au hohlraum with or without a capsule, heated by smoothing beams where scattering loss is less than 10{\%}. With compact flat-response X-ray detector array and 14-channel soft X-ray spectrometer, the radiation flux is measured through the laser entrance hole(LEH) or diagnostic hole(DH) at different photon range and multiple line of sight. The difference in radiation between laser spot and re-emitting wall, the time history of capsule absorbing and emitting flux, is quantitatively studied to interpret flux onto the capsule. The radiation driven shock propagating in Al and Ti sample placed over a hole in the hohlraum wall, which is more representative of the drive inside the hohlraum, also provide a unique information of radiation. In order to better improve our physics model, the motion of laser ablated bubble and radiation ablated blow-off plasma is directly measured, and their effects on laser absorption and X-ray escaping LEH as well as flux diagnostics are evaluated.   

KULL Simulations of OMEGA Radiation Flow Experiments

The problem of radiation flow in a right circular cylinder is of interest for the verification and validation of radiation codes since the flow is analytically analogous to diffusive free molecular flow in a similar geometry.\footnote{E. Garelis and T.E. Wainwright. Phys. Fluids. \textbf{16}, 4 (1973)} Experiments were conducted on the OMEGA laser utilizing a low-density heated-cylindrical-wall target. The targets consisted of a 1.6 mm diameter gold hohlraum containing an on-axis 700 $\mu $m diameter SiO$_{2}$ cylinder inside an 80 $\mu$m thick Ta$_{2}$O$_{5}$ aerogel tube. The FY13 targets also feature ``light-pipe'' diagnostics to measure the progression of the radiation front inside the foam. Simulations were run with the KULL multi-physics code, employing a new laser ray-tracing package. Comparisons of synthetic diagnostics derived from code results to x-ray measurements of drive temperature and heat front propagation provide a methodology to constrain simulation models.