Session UO7: Hohlraum and X-Ray Cavity Physics

New Advanced Hohlraums Utilizing Unique Geometries and Foam Components

To date the indirect drive experiments on the NIF have principally utilized cylindrical gas-filled hohlraums that have been subject to a number of challenges, including generating inner cone SRS backscatter (up to 18 percent of total laser energy), producing hot electrons, and requiring cross beam energy transfer to inner beams to obtain adequate drive symmetry. Proposed new hohlraum concepts address the challenges facing standard cylindrical gas-filled hohlraums by having the beams traverse shorter, hotter plasmas to reduce backscatter, shielding the capsule from direct illumination from the laser spots, or avoiding cross-beam transfer altogether by not allowing crossing of the beams. These concepts also utilize high-Z and mid-Z foams to increase stability of the wall/fill interface, increase x-ray conversion efficiency, reduce backscatter, reduce symmetry swings, and allow smaller, more efficient, laser entrance holes.   

Radiation Hydrodynamics Modeling of Hohlraum Energetics

Attempts to model the energetics in NIF Hohlraums have been made with varying degrees of success, with discrepancies of 0-25\% being reported for the X-ray flux (10-25\% for the NIC ignition platform [1] hohlraums). To better understand the cause(s) of these discrepancies, the effects of uncertainties in modeling thermal conduction, laser-plasma interactions, atomic mixing at interfaces, and NLTE kinetics of the high-Z wall plasma must be quantified. In this work we begin by focusing on the NLTE kinetics component. We detail a simulation framework for developing an integrated HYDRA [2] hohlraum model with predefined tolerances for energetics errors due to numerical discretization errors or statistical fluctuations. Within this framework we obtain a model for a converged 1D spherical hohlraum which is then extended to 2D. The new model is used to reexamine physics sensitivities and improve estimates of the energetics discrepancy.\\[4pt] [1] Lindl et al, Physics of Plasmas 21, 020501 (2014); Maclaren et al, PRL 112, 105003 (2014).\\[0pt] [2] M. Marinak et al., Phys. Plasmas 8, 2275 (2001).   

In Pursuit of a More Ideal Hohlraum

Current hohlraum designs have a number of issues which are detrimental to achieving ignition; including LPI, CBET, hot electrons, non-ideal spectral emission(gold M-Band) and wall motion leading to implosions with large symmetry swings. We are undertaking a campaign on the NIF to address many of these issues through the use of thin wall liners. We will present a comparison between three experiments, a gold hohlraum, a copper-lined hohlraum and a zinc oxide foam-lined hohlraum and discuss our future experimental plans which will utilize very low density foam liners, $\sim$ 10 mg/cc, and low gas fill densities, \textless 0.6 mg/cc. This combination is predicted in simulations to greatly reduce the expansion of the gold wall leading to reduced symmetry swings, result in large reductions in LPI(SBS, SRS and 2Wpe) and eliminate gold m-band emission. The removal of the gold m-band spectra reduces the ablator-fuel instability growth and allows the use of undoped or less doped capsules which in turn reduces the ablation front growth factors leading to a more stable implosion.   

Re-examining our inhibitions: A speculative re-analysis of data from gold spheres illuminated by the URLLE Omega laser

A 2006 campaign, that illuminated 1 mm diameter gold spheres using the Omega laser at LLE, required the simulations to use a ``liberal'' flux limiter of f$=$0.15 (or equivalently a non-local model) in order to match the high levels of measured x-ray emission. In 2013, Thomson Scattering (TS) diagnosed the plasma conditions in the Au sphere's laser heated corona at various radial positions as a function of time. The simulation model using non-local transport compared well for some of the TS data (for ZTe) but not for all of it. Meanwhile, using this model for hohlraums, led to discrepancies with data (such as drive) when applied to some hohlraums, though less-so for others. As a result, hohlraum models with a more restrictive flux limiter, including a ``two-stream-instability (TSI)'' flux limit model (which, when operative, is effectively f$=$0.015) are being considered. Here we invoke the possibility that the same ion acoustic turbulence (an outgrowth of the TSI), which acts like an effective scatterer to inhibit electron transport, can, by the same token, also increase absorption. This increase in absorption, applied (speculatively) close by the critical surface, can begin to match the Au sphere x-ray emission, as well as a preponderance of the ZTe data.   

The Evolution of the Gold Bubble in NIF Ignition Gas-Filled Hohlraums

At the National Ignition Facility (NIF), the energy from 192 laser beams is converted to an x-ray drive in a gas-filled gold hohlraum. The x-ray drive heats and implodes a fuel capsule. The ViewFactor platform uses a truncated hohlraum to measure the x-ray drive from the capsule point-of-view. This platform also affords excellent diagnostic views of the hohlraum interior, in particular, of the region in which the outer beams deposit their energy (the ``gold bubble'') Time-resolved and time-integrated images in the hard x-ray range (\textgreater 3 keV) reveal an 8-fold symmetry in the gold bubble. The Au plasma in the bubble from the eight 50 degree quads expands faster than that from the interleaved 44.5 degree quads. The variation in this structure with laser intensity, with pulse shape and cross beam energy transfer, and comparison to models, will be discussed.   

Hydrodynamic modeling of gold bubble expansion in gas-filled hohlraums

Experimental campaigns have been conducted using gas-filled hohlraums on the LIL facility, which was a prototype of one quadruplet of the french laser megajoule (LMJ), to study the gold bubble expansion seen by the outer beams in ignition hohlraum. This setup, with 6ns laser pulse duration and a maximum energy of 15kJ, allowed us to study the gold bubble expansion that occurs in ignition hohlraum and to test our hydro-radiative simulations by variations in thermal conduction and atomic physics models. In this talk we will present and discuss hydrodynamic calculations together with experimental results of the campaign. Under this configuration, with the temperature gradient obtained in the simulation, we will examine and discuss the possibility of an anomalous process involving ion-acoustic turbulence excited by the heat-flux driven return current instability. This process could lead to an increased laser light absorption in gold as well as thermal transport inhibition. Comparisons between experimental measurements and simulations will be discussed to support our interpretation.   

Laser spot movement inside spherical hohlraums and hohlraum energetics

According to the ignition experiments performed at the NIF, the radiation asymmetry is a serious problem to be solved in indirect drive ICF. Lan \textit{et al.} proposed an octahedral spherical hohlraum in order to obtain good radiation symmetry. However, one potential problem of the spherical hohlraum is that the laser beams are close to the hohlraum wall. The wall blow-off may cause the LEH to close faster and result in strong laser absorption in the LEH region. Aimed at alleviating the problem, Lan and Zheng proposed a novel octahedral hohlraum with cylindrical LEHs. In this work, we report the experimental observation of laser spot movements inside the spherical hohlraums with plane LEHs and cylindrical LEHs on the SGIII-prototype laser facility. The experimental results indicate that the cylindrical LEH could dramatically improve the laser propagation inside spherical hohlraum. We also completed the hohlraum energetics experiment on the SGIII-prototype laser facilty. We obtained good reproducible shock velocities in Al and Ti. For the hohlraum used in the experiment, the hohlraum radiation temperature is about 200 eV according to the FXRD's results with the driven laser energy o 5.5 kJ.   

Exploring symmetry in near-vacuum hohlraums

Recent experiments with near-vacuum hohlraums, which utilize a minimal but non-zero helium fill, have demonstrated performance improvements relative to conventional gas-filled (0.96 -- 1.6 mg/cc helium) hohlraums: minimal backscatter, reduced capsule drive degradation, and minimal suprathermal electron generation. Because this is a low laser-plasma interaction platform, implosion symmetry is controlled via pulse-shaping adjustments to laser power balance. Extending this platform to high-yield designs with high-density carbon capsules requires achieving adequate symmetry control throughout the pulse. In simulations, laser propagation is degraded suddenly by hohlraum wall expansion interacting with ablated capsule material. Nominal radiation-hydrodynamics simulations have not yet proven predictive on symmetry of the final hotspot, and experiments show more prolate symmetry than preshot calculations. Recent efforts have focused on understanding the discrepancy between simulated and measured symmetry and on alternate designs for symmetry control through varying cone fraction, trade-offs between laser power and energy, and modifications to case-to-capsule ratio.   

Study on Octahedral Spherical Hohlraum

In this talk, we report our recent study on octahedral spherical hohlraum which has six laser entrance holes (LEHs). First, our study shows that the octahedral hohlraums have robust high symmetry during the capsule implosion at hohlraum-to- capsule radius ratio larger than 3.7 and have potential superiority on low backscatter without supplementary technology. Second, we study the laser arrangement and constraints of the octahedral hohlraums and give their laser arrangement design for ignition facility. Third, we propose a novel octahedral hohlraum with LEH shields and cylindrical LEHs, in order to increase the laser coupling efficiency and improve the capsule symmetry and to mitigate the influence of the wall blowoff on laser transport. Fourth, we study the sensitivity of capsule symmetry inside the octahedral hohlraums to laser power balance, pointing accuracy, deviations from the optimal position and target fabrication accuracy, and compare the results with that of tradiational cylinders and rugby hohlraums. Finally, we present our recent experimental studies on the octahedral hohlraums on SGIII prototype laser facility.   

Reproducibility of NIF hohlraum measurements

The strategy of experimentally ``tuning'' the implosion in a NIF hohlraum ignition target towards increasing hot-spot pressure, areal density of compressed fuel, and neutron yield relies on a level of experimental reproducibility. We examine the reproducibility of experimental measurements for a collection of 15 identical NIF hohlraum experiments. The measurements include incident laser power, backscattered optical power, x-ray measurements, hot-electron fraction and energy, and target characteristics. We use exact statistics to set 1-sigma confidence levels on the variations in each of the measurements. Of particular interest is the backscatter and laser-induced hot-spot locations on the hohlraum wall. Hohlraum implosion designs typically include variability specifications [S. W. Haan, etal, Phys. Plasmas 18, 051001 (2011)]. We describe our findings and compare with the specifications.   

Early hot electrons generation and beaming in ICF gas filled hohlraums at the National Ignition Facility*

In laser driven hohlraum capsule implosions on the National Ignition Facility, supra-thermal hot electrons generated by laser plasma instabilities can preheat the capsule. Time resolved hot electron Bremsstrahlung spectra combined with 30 keV x-ray imaging uncover for the first time the directionality of hot electrons onto a high-Z surrogate capsule located at the hohlraum center. In the most extreme case, we observed a collimated beaming of hot electrons onto the capsule poles, reaching 50x higher localized energy deposition than for isotropic electrons. A collective SRS model where all laser beams in a cone drive a common plasma wave provides a physical interpretation for the observed beaming. Imaging data are used to distinguish between this mechanism and 2$\omega_{\mathrm{p}}$ instability [2]. The amount of hot electrons generated can be controlled by the laser pulse shape and hohlraum plasma conditions. [1] E.L. Dewald, \textit{et. al.,} \textit{Rev. Sci. Instrum.} \textbf{81}, 10D938 (2010). [2] S. Regan et al, \textit{Phys. Plasmas} \textbf{17, }020703 (2010). *This work performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.   

Near-vacuum hohlraums with uranium walls for driving high density carbon ablators

We present experimental results for unlined uranium near-vacuum hohlraums on the National Ignition Facility. X-ray wall losses are lower in uranium than in gold at radiation temperatures near 300 eV. In addition, the intensity of x-rays with energy $h\nu > 1.8$ keV is lower for uranium hohlraums. The softer uranium spectrum allows the use of ablators with lower levels of dopants that reduce rocket efficiency and increase the risk of polluting the hot-spot with emissive material. Experiments in the ViewFactor platform\footnote{S. A. MacLaren et al., Phys. Rev. Letters 112, 105003 (2014).} measured 5\% higher total x-ray intensity and 30\% lower intensity of $h\nu > 1.8$ keV for uranium relative to gold. Back-lit implosions using undoped high-density carbon (HDC, or diamond) capsules achieved a velocity of $400 \pm 20$ km/s compared to $360 \pm 20$ km/s for gold. These results have led the NIF HDC campaign to baseline uranium hohlraums for upcoming experiments.   

Compressive asymmetry evaluation for M-Band Radiation generated from the interaction of high energy laser and the hohlraum

In indirect drive inertial confinement fusion, intense laser interacts with high-Z materials in the hohlraum and X-rays are generated to heat and drive the centrally located capsule. Most of these X-rays emitted from the wall of hohlraum are soft x-rays, but also a comparable fraction of them are high-energy X-rays (mainly from M band of wall material, \textgreater 2keV for Au), which may lead to preheat and compressive asymmetry on the capsule, and affect final ignition result. Therefore, such preheat and compressive asymmetry needs to be characterized and evaluated, to enable it restrained or controlled. In this paper, by using one-dimensional multi-group radiation hydrodynamic codes and view-factor based radiation transport codes, we evaluate the compressive asymmetry on the centrally located capsule for various fractions of M-band X-rays. The result shows that: 1) The M-band X-rays may lead to significant compressive asymmetry when the thermal flux is symmetric,2) More fractions of M-band X-rays tends to result in more compressing asymmetry, and 3) 15{\%} of M-band X-rays may result in 50{\%} compressive asymmetry. Base on the above analysis, such significant compressive asymmetry due to M-band radiation may decrease the compressibility of the fuel or the capsule performance. Therefore, it motivates us to validate and measure such quantity of compressive asymmetry occurred on the capsule in recent experiments.   

Gas-filled hohlraum study on Shenguang-III prototype

Experimental studies on gas-filled hohlraum were performed extensively in recent years on Shenguang-III prototype laser facility. These experiments employed Au hohlraums within C5H12 gas fill heated by smoothing beams. In the first round of experiments, although the low-Z gas fill impeded the blow-off plasma ablated from hohlraum wall, the x-ray flux from the LEH decreased dramatically compared with the vacuum hohlraum. Further analysis of several ways of energy deposition including heating the gas-fill, extra scattered light and ablating the LEH membrane, revealed that too much laser energy were wasted in exploding the LEH membrane if we use a 1 ns square pulse. After we introduced a low power prepulse to intentionally exploding the membrane, the behavior of the x-ray flux from the gas-filled hohlraum is identical with the vacuum hohlraum. In subsequent studies, the motion of x-ray spot and corona plasma has also been studied using the gas-filled hohlraum. We obtained high quality data of the gas/wall interface and the boundary of the ablated wall near the LEH. The result agrees with that in simulation. However, there is a discrepancy between the experiment and the simulation in the spatial feature of the ablated wall near the LEH extracted from M-band x-ray image.