Session BO7: Hohlraum and X-Ray Cavity Physics I

The advanced hohlraum research project

We present results of a three-year study on alternate hohlraum designs. Several alternatives to cylindrical gas-filled hohlraums have been investigated. Proposed new hohlraum concepts utilize different hohlraum shapes, multiple laser entrance holes, and alternate materials such as metal foam walls. For each design we assess the radiation drive efficiency, the time-dependent drive symmetry, and laser-plasma interaction issues such as backscatter and crossed beam energy transfer. Results from supporting experiments on laser-heated foams are also summarized.   

Progress in Octahedral Spherical Hohlraum Study

In this talk, we report on our progress in octahedral spherical hohlraum study, including theoretical study, experimental study, and 3D simulation development. For theoretical study, we will address the concept, configuration, design and robust (sensitivity) of the novel octahedral spherical hohlraums with cylindrical Laser Entrance Holes (LEH), the proposal of using 4$\omega $- 2$\omega $ laser as ignition driven, and a point design of ignition target with a spherical hohlraum. For experimental study, we will address all experiments implemented on the ShenGuang(SG) laser facilities since 2014, including improvement of laser transport by using the cylindrical LEHs in the spherical hohlraums, first spherical hohlraum energetics, first experimental demonstration of low laser-plasma instabilities in gas-filled spherical hohlraums at laser injection angle designed for ignition target, how to realize the laser injections into 6LEH spherical hohlraum on SGIII laser facility which was designed for the cylindrical hohlraums, and first energetics of 6LEH spherical hohlraum on SGIII laser facility, etc. For 3D simulation, we will address the physics included in our 3D code and the 3D simulation results on the 6LEH octahedral spherical hohlraums with cylindrical LEHs.   

View Factor and Radiation-Hydrodynamic Simulations of Gas-Filled Outer-Quad-Only Hohlraums at the National Ignition Facility

A cylindrical National Ignition Facility hohlraum irradiated exclusively by NOVA-like [1] outer quads ($44.5^{\circ}$ and $50^{\circ}$ beams) is proposed to minimize laser plasma interaction (LPI) losses and avoid problems with propagating the inner ($23.5^{\circ}$ and $30^{\circ}$) beams. Symmetry and drive are controlled by shortening the hohlraum, using a smaller laser entrance hole (LEH), beam phasing the $44.5^{\circ}$ and $50^{\circ}$ beams [2-3], and correcting the remaining $P_4$ asymmetry with a capsule shim [4]. Ensembles of time-resolved view factor simulations [5] help narrow the design space of the new configuration, with fine tuning provided by the radiation-hydrodynamic code HYDRA. [1] L.~J.~Suter, et al., Phys.~Rev.~Lett.~73, 2328 (1994). [2] R.~E.~Turner, et al., Phys.~Plasmas~7, 333 (2000). [3] S.~G.~Glendinning, et al., Rev.~Sci. Instrum.~70, 536 (1999). [4] D.~S.~Clark, et al., Phys.~Plasmas~23, 072707 (2016). [5] J.~J.~MacFarlane, J. Quant.~Spectr.~Rad.~Transfer 81, 287 (2003).   

Simulations of super-ellipse hohlraum targets as a path to high neutron yields

Recently neutron yields in excess of $10^{16}$ have been achieved at the National Ignition Facility (NIF) using a low-density gas fill hohlraum and a subscale high-density-carbon capsule[1]. The laser power used was near the current maximum level allowed on the inner cones of the NIF laser. While more energy can be extracted from the laser to provide additional improvement on the neutron yield, a more efficient design is desired. A new effort has begun to investigate alternatives to the current cylinder-shaped hohlraum for driving larger capsules (1.1 mm outer radius). If these new hohlraums can preserve the implosion symmetry, the additional absorbed energy is expected to provide a path to high neutron yield and potential ignition. Super-ellipse hohlraums, a generalization of an earlier rugby hohlraum design[2], have the advantage of a larger waist diameter and reduced parasitic energy losses from the corners of cylindrical hohlraums while still being able to produce the required capsule drive at the current energy and power limits available at the NIF. We will present plausible designs of these hohlraums based on the Lamé mathematical construction, and discuss their prospects to reach high neutron gains. [1] NIF shot N170601 [2] P. Amendt et al Phys of Plasmas 22, 040703 (2015)   

Simulation of alternate hohlraum shapes for improved inner beam propagation in indirectly-driven ICF implosions

Recent indirectly-driven ICF experiments performed on the National Ignition Facility have shown that the propagation of the inner beam cones is impeded late in the laser pulse by the growth of a gold bubble, which is initiated at the location where the outer beams hit the hohlraum wall and which expands radially inward into the hohlraum as the implosion progresses. Late in time, this gold bubble intercepts a significant portion of the inner beams reducing the available energy reaching the waist of the hohlraum and affecting the implosion symmetry. Integrated hohlraum simulations of alternate hohlraum shapes using HYDRA are performed to explore options for reducing the impact of the gold bubble on inner beam propagation. The simulations are based on recent NIF implosions using High-Density Carbon (HDC) ablators, which have shown good performance, but which could benefit from improved inner beam propagation.   

Enhancing Hohlraum Design with Artificial Neural Networks

A primary goal of hohlraum design is to efficiently convert available laser power and energy to capsule drive, compression and ultimately fusion neutron yield. However, a major challenge of this multi-dimensional optimization problem is the relative computational expense of hohlraum simulations. In this work, we explore overcoming this obstacle with the use of artificial neural networks built off ensembles of hohlraum simulations. These machine learning systems emulate the behavior of full simulations in a fraction of the time, thereby enabling the rapid exploration of design parameters. We will demonstrate this technology with a search for modifications to existing high-yield designs that can maximize neutron production within NIF’s current laser power and energy constraints.   

Progress understanding how hohlraum foam-liners can be used to improve laser beam propagation through hohlraum plasmas

The expansion of a laser-heated hohlraum wall can quickly fill the cavity and reduce or prevent propagation of other laser beams into the hohlraum. To delay such plasma filling, ignition hohlraums have typically used a high-density gas-fill or have been irradiated with a short (< 10 ns) laser pulse; the former can cause laser plasma instabilities (LPI), while a short laser pulse limits the design space required to reach symmetric implosions. Foam-liners are predicted to mitigate wall motion in a low gas-fill hohlraum, and so would enable the hohlraum to usefully drive a capsule over a longer duration. On the National Ignition Facility we have been engaged in two types of experiments to study foam-lined hohlraums. The first aims to radiograph the expansion of a foam-lined Au wall in a cylindrical geometry and, using simulation, infer the location of the 1/4 $n_{crit}$ surface. We observe that a 20 mg/cc Ta$_2$O$_5$ foam, 200 µm thick delays the expansion of Au hohlraum wall by 0.5 - 0.7 ns. The second type introduces a Ta$_2$O$_5$ foam-liner into a hohlraum and are designed to measure the effect of the foam-liner on capsule drive.   

Constraining heat-transport models by comparison to experimental data in a NIF hohlraum

The accurate simulation of hohlraum plasma conditions is important for predicting the partition of energy and the symmetry of the x-ray field within a hohlraum. Electron heat transport within the hohlraum plasma is difficult to model due to the complex interaction of kinetic plasma effects, magnetic fields, laser-plasma interactions, and microturbulence. Here, we report simulation results using the radiation-hydrodynamic code, HYDRA, utilizing various physics packages (e.g., nonlocal Schurtz model [1], MHD, flux limiters) and compare to data from hohlraum plasma experiments which contain a Mn-Co tracer dot [2]. In these experiments, the dot is placed in various positions in the hohlraum in order to assess the spatial variation of plasma conditions. Simulated data is compared to a variety of experimental diagnostics. Conclusions are given concerning how the experimental data does and does not constrain the physics models examined. [1] G. P. Schurtz et al., Phys. Plasmas 7, 4238 (2000). [2] M. A. Barrios Garcia, et al., Phys. Plasmas 23, 056307 (2016).   

Study Plasma Stagnation in Laser-Driven Hohlraums

Understanding plasma stagnation in laser-driven hohlraums is important for inertial confinement fusion. It has been realized that the use of conventional single-species-averaged hydrodynamic codes for modelling stagnation is largely responsible for some disagreements between the experimental results and numerical simulations. A number of mechanisms which play important roles in this process have been missed in hydrodynamic simulations, including ion interpenetration and diffusion. Self-generated fields and plasma instabilities observed in connection with plasma blow-off seen at hohlraum laser entrance holes provides additional compelling experimental evidence of non-hydrodynamic processes. To explore such phenomena, a series of experiments was performed at the Omega laser facility. Data obtained from several diagnostics, including monoenergetic-proton radiography and x-ray imaging, are compared with modified three-dimensional hydrodynamic simulations, providing new insight into hohlraum stagnation and a more complete physical picture of hohlraum dynamics. This work was supported in part by US DOE, LLNL and LLE.   

NLTE atomic kinetics modeling in ICF target simulations

Radiation hydrodynamics (HYDRA) simulations using recently developed 1D spherical and 2D cylindrical hohlraum models have enabled a reassessment of the accuracy of energetics modeling across a range of NIF target configurations. Higher-resolution hohlraum calculations generally find that the X-ray drive discrepancies are greater than previously reported. We identify important physics sensitivities in the modeling of the NLTE wall plasma and highlight sensitivity variations between different hohlraum configurations (e.g. hohlraum gas fill). Additionally, 1D capsule only simulations show the importance of applying a similar level of rigor to NLTE capsule ablator modeling. Taken together, these results show how improved target performance predictions can be achieved by performing inline atomic kinetics using more complete models for the underlying atomic structure and transitions.   

Design of Thin-Au-Coating Sphere experiments on the OMEGA Laser to examine Hohlraum model

Experiments performed by LLNL on OMEGA studying X-ray conversion efficiencies for Au (Dewald et al. PoP 2008), resulted in a "liberal" flux limiter value of 0.15 (the High Flux Model (Rosen et al. HEDP 2011)) needed to match simulations with these measurements. However, simulations using this HFM do not fit NIF Hohlraum experiments (Jones et al. PoP 2017) and a much more restrictive f$=$0.03, related by the authors to Ion Acoustic Turbulence (IAT), was found to better fit the experimental data. This f$=$0.03 does not fit the Au-Sphere data (Rosen et al. APS/DPP conference 2015). We re-examine the Au-Sphere simulations accounting for Ion Acoustic Turbulence effect on the thermal electron flux inhibition and enhanced laser absorption near critical density (K. Ma et al. APS/DPP conference 2016). New experiments, using thin Au-coatings (0.1-0.5mic) instead of thick (7mic) Au layers, are proposed to explore time dependent Thermal (0-2KeV) and M-band (2-4KeV) emissions as a function of Au-Coating thickness, using various values and models for flux limiter, and laser absorption. These new experiments are expected to add more restrictions to the development of new models.   

Heat-Flux Measurements in Laser-Produced Plasmas Using Thomson Scattering from Electron Plasma Waves

An experiment was designed to measure heat flux in coronal plasmas using collective Thomson scattering. Adjustments to the electron distribution function resulting from heat flux affect the shape of the collective Thomson scattering features through wave-particle resonance. The amplitude of the Spitzer--H\"{a}rm electron distribution function correction term ($f_{\mathrm{1}})$ was varied to match the data and determines the value of the heat flux. Independent measurements of temperature and density obtained from Thomson scattering were used to infer the classical heat flux (\textbf{q }$=$ --$\kappa \nabla T_{\mathrm{e}})$. Time-resolved Thomson-scattering data were obtained at five locations in the corona along the target normal in a blowoff plasma formed from a planar Al target with 1.5 kJ of 351-nm laser light in a 2-ns square pulse. The flux measured through the Thomson-scattering spectra is a factor of $\sim $5 less than the $\kappa \nabla T_{\mathrm{e}}$ measurements. The lack of collisions of heat-carrying electrons suggests a nonlocal model is needed to accurately describe the heat flux. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.