Session JO7: ICF: Hohlraums

Overview of Inertial Fusion Energy research in Europe

I am leading a European consortium, funded under the EUROFusion Enabling Research grant entitled “Routes to High Gain Inertial Fusion Energy (RHoGIFE)”, comprising sixty eminent physicists from fifteen laboratories in nine nations. We are undertaking a series of collaborative experiments on existing European facilities to qualify new diagnostics, instruments and techniques, in preparation for deployment on PETAL/LMJ facility in the next decade. We have three main objectives: a) studying fundamental materials properties and laser-plasma interactions to acquire new insights into basic physics for ignition on MJ-scale facilities b) critically evaluating advanced alternative schemes for the high gain target designs that are required for inertial fusion energy, including the exciting new auxiliary heating approach developed by our consortium; and c) developing key IFE innovative materials, lasers and target fabrication technologies. I will describe key advances made by our consortium, concentrating on fundamental atomic physics, CBET and RT instability studies, auxiliary heating and alternative ignition schemes (electron- and ion- fast ignition).   

\textbf{Understand Hohlraum Physics from NIF Gated Laser-entrance-hole Images }

The ns-gated NIF laser-entrance-hole (LEH) imager provides a routine, non-perturbative measurement of hohlraum x-ray emission from inner and outer laser beam deposition regions. From the data we infer the plasma bubble trajectory and the ratio of brightness between inner and outer beams as a function of time. These experimental results are used to understand models of heat transport and hydrodynamic instability development used in the radiation-hydrodynamic codes. We will summarize the experimental data and its comparisons with models as a function of reduced model parameters for a variety of NIF ICF experiments. The agreements and discrepancies will be highlighted as well as new understanding and interpretations of the experimental data.   

Experimental study of hohlraum dynamics with x-ray spectroscopy of mid-Z tracer dopants on hohlraum wall

Achieving good control of symmetry is essential for the indict drive implosion experiments at NIF. Because of their longer path in the hohlraum, transport of the inner cone beams changes later in time. This late time reduction of inner cone power delivery is explained by absorption of the beam's power in the under-dense plasma ablated off the hohlraum wall and the capsule surface. However, experiments at lower hohlraum gas-fill show enhanced wall blow-in but less inner cone beam attenuation. We developed a new hypothesis that the plasma conditions in the reduced gas-fill allow greater beam transmission. To test this hypothesis we added time-resolved x-ray spectroscopy to the experiments to measure the electron temperature of the hohlraum plasma. We will present the experimental results and compare to the temperature predicted by radiation-hydrodynamic simulations.   

Tripling the energy coupling efficiency from hohlraum to capsule on NIF*

In the current cylindrical-hohlraum indirect drive schemes for ICF, a strong limitation is the inefficient (\textasciitilde 10{\%}) energy coupling from hohlraum to capsule, typically less than 200 kJ with laser drive energies up to 1.8 MJ. We report a NIF experiment demonstrating \textasciitilde 30{\%} energy coupling to an aluminum capsule in a rugby-shaped gold hohlraum (Ping, Smalyuk, Amendt, et al. Nature Physics 2019). Based on x-ray radiography measurements, the shell kinetic energy reaches 34 kJ with 1MJ drive at 0.7x subscale, consistent with \textasciitilde 300 kJ capsule energy coupling. More experiments were performed recently at larger, 0.9x scale with 1.5 MJ laser drive. The nuclear bang time and the shell velocity from simulations agree well with experimental data, indicating \textasciitilde 450 kJ coupling with 1.5MJ drive while keeping good shell symmetry. This high coupling efficiency can substantially increase the tolerance to residual imperfections and improve the prospects for ignition, both in mainline single-shell hot-spot designs and potential double-shell targets. * This work was performed under the auspices of the US DOE by LLNL under contract number DEAC52- 07NA27344, with partial support from the DOE OFES ECRP program.   

Integrated simulations of capsule implosions in low gas-fill hohlraums at the National Ignition Facility

Current capsule implosions at the National Ignition Facility (NIF) using high-density-carbon ablators and laser energies close to 2MJ have shown neutron yields in excess of 50 KJ. Improving on this performance requires a deeper understanding of the different degradation mechanism affecting the quality of NIF implosions. While it is likely that no single cause is responsible, the latest implosions have shown considerable fuel areal density variations consistent with a low-order mode 1 asymmetry. A current working hypothesis attributes these asymmetries to a combination of beam to beam variations in the laser delivery and possibly coupling to target features [1], such as target positioning and/or diagnostic windows needed for x-ray imaging. To understand the causes of these asymmetries a 3D integrated simulation model (using the code HYDRA) has been developed and used to investigate several relevant NIF implosions. Our simulations investigate and quantify the impact of the different portions of the laser pulse on implosion asymmetry and experimental observables. We will compare our results with measurements and quantify the impact of the laser and target features. [1] B. McGowan et al. Paper presented at IFSA 2019   

3D extended MHD simulations of laser driven hohlraums

Magnetisation of electrons in a hohlraum plasma modifies the transport of heat from the laser absorption regions to the ablation surface and the region of soft X-ray emission and can affect the efficiency with which laser energy is converted to X-rays. Within each laser heated region, the temperature and density gradients provide a source of spontaneously generated magnetic field. For multiple overlapping beams with different intensities such as in the NIF hohlraum this process produces an intrinsically 3D and irregular magnetic field structure producing anisotropic heat flow. Similarly in experiments where external magnetic fields are applied in order to suppress thermal conduction losses within the imploding fusion capsule, this also modifies heat transport within the hohlraum. While these applied magnetic fields are initially more regular than the self-generated fields, they become contorted by the wall expansion and capsule ablation and again result in an irregular structure driving anisotropic heat flow. We present results from 2D and 3D hohlraum simulations using the extended MHD code Gorgon. A simple copper hohlraum of NIF scale is modelled using a laser ray trace model and multi-group radiation transport. Results highlight the interplay between the field generation, the anisotropic heat flow and the Nernst term. Results with externally applied fields also investigate resistive diffusion in the dense plasma regions. The contributions of other extended MHD terms such as Righi-LeDuc and Ettingshausen heat flow are also evaluated.   

Analysis of Predictivity of Hohlraum Simulations of Implosion Experiments on the NIF

Simulations of NIF hohlraum implosion experiments are unable to reliably predict several key features of the resulting capsule implosion, particularly bang-time and shape. To better understand what parameters or physics models are necessary to improve predictivity, we simulate 20$+$ NIF ignition experiments in HYDRA, varying several key physics packages and parameters. The experiments are selected from four experimental campaigns, and include keyholes, convergent ablator symcaps, and layered implosions. The four campaigns include three using high-density carbon capsule ablator material: the ignition campaign using depleted Uranium hohlraums, BigFoot with Gold hohlraums, and the Hybrid-B campaign. The fourth campaign is 2-Shock campaign that uses plastic capsule ablator material. Parameters and packages to be tested include mesh resolution, laser scatter and absorption parameters, cross-beam energy transfer and backscatter, high fidelity opacity models of hohlraum wall material, and non-local heat conduction models. These simulations are performed with the HyPyD Hydra Python Deck, allowing a robust, standardized framework on which to field these surveys. Simulation synthetic diagnostic results are grouped into benchmark categories; nuclear (yield, ion temperature, down-scatter ratio), shape (x-ray bang-time P2, P4), bang-time, etc., and compared with experiments so the overall impact of any given physics package can be assessed across all campaigns.   

View Factor Study of 3D Low-Mode Asymmetries in Inertial Confinement Fusion Implosions

Inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF) seek to drive a spherical deuterium-tritium target to high temperatures and pressures. Fusion performance is significantly affected by the symmetry of the radiation environment produced inside the cylindrical hohlraum enclosure driven by 192 laser beams. Presently, full 3D ICF radiation-hydrodynamic calculations with sufficient resolution are very computationally expensive, limiting their utility. To rapidly assess the impact of drive asymmetry induced by laser power imbalance and hohlraum engineering features, we employ a view factor model [1]. The calculated low mode drive asymmetries are coupled with a Green's Function model of the capsule response [2] and results are compared against experimental data across a range of NIF experiments. [1] J. J. MacFarlane, J. Quant. Spectr. Rad. Transfer, 81, 287 (2003) [2] L. Masse, APS DPP Abstract TI3.004 (2018)   

Application of cross-beam energy transfer to control drive symmetry in CH, ICF implosions with a medium gas fill Hohlraum at the NIF

We present a demonstration of stagnated core symmetry control using a 1 {\AA} wavelength difference between inner and outer drive beams at the National Ignition Facility. Application of a wavelength difference utilizes the process of Cross-Beam Energy Transfer to increase the x-ray drive incident on the waist of the ICF capsule, producing symmetry control in implosions with a plastic ablator. We show these findings over seven experiments by comparing the case of no wavelength difference, $\Delta \lambda =$0 {\AA}, to that of $\Delta \lambda \quad =$ 1{\AA}. We experimentally show implosion symmetry control of \textasciitilde 24 um in Legendre mode 2 (P2) symmetry, based on experimental playbooks this corresponds to a 25{\%} increase of inner beam drive at the waist of the hohlraum.   

Understanding 3D Asymmetries In X-ray Drive At The National Ignition Facility Using a Simple View Factor Metric

Low mode 3D drive asymmetries are an important degradation mechanism for NIF indirect drive implosions. We will apply a simple static view factor model to assess sources of drive variability at the capsule. The sources include laser performance variation in the foot and peak, errors in laser/target positioning, losses from target diagnostic windows, variation in Cross Beam Energy Transfer (CBET), and drive losses due to Stimulated Brillouin Scattering (SBS). Each source can produce mode-1 drive imbalances of M$_{\mathrm{1}}$/M$_{\mathrm{0}}$ \textasciitilde 0.5{\%}. Combined the drive imbalances can impart a velocity of 100km/s to the imploded hotspot, degrading the yield. Mode-2 drive imbalances of P$_{\mathrm{2}}$/P$_{\mathrm{0}}$ \textasciitilde 0.5{\%} that can lead to hotspot shape changes of P$_{\mathrm{2}}$ \textasciitilde 5-$\mu $m from shot to shot, are also seen. We compare the view factor representation of the known drive asymmetries with measurements from the hotspot velocity and shape diagnostics, together with SBS observations, for multiple experiments with different ablators and target scales. These studies illuminate correlations and systematic trends in the facility performance and laser coupling including variability of CBET.   

Enhanced Laser-Energy Coupling with Small-Spot Distributed Phase Plates (SG5 650) in OMEGA Cryogenic Implosions

The ratio of the laser far-field spot diameter to the target diameter has been reduced in an attempt to mitigate cross-beam energy transfer and improve energy coupling. The 60 OMEGA beams were outfitted with new small spot (``SG5-650'') distributed phase plates (DPP's), with a diameter \textasciitilde 80{\%} of that of the standard SG5-850 DPP's, and used for cryogenic DT ice target implosions. The ablation-front trajectory, the backscattered laser energy, and the neutron bang time were found to be consistent with a 10{\%} increase in energy coupling. The hydrodynamic efficiency, defined as the ratio of the kinetic energy in the imploding shell to the laser energy, increased from 4.5{\%} to 5.0{\%}. However, an increase in hot electron production was observed, and evidence was seen in framing-camera images for increased hydrodynamic instabilities associated with the smaller DPP spots, limiting the implosion performance. Further experiments are required to study how to maintain the improved energy coupling while mitigating preheat and hydrodynamic instabilities. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Using directly driven beryllium spheres to study heat transport

Heat transport in integrated experiments can be difficult to study due to the complex interplay between radiation transport, laser-plasma interactions, and heat conduction. Self-generated magnetic fields, nonlocality, and micro-instabilities all alter the underlying heat flux. Here, we report experimental results of directly driven beryllium spheres at the Omega laser facility. Low Z beryllium reduces power emitted as x-rays to a small percentage of incident laser power. Incident laser intensity of $10^{14}$ W/cm$^2$ results in $\sim 96\%$ of laser energy coupled to the target, in agreement with radiation-hydrodynamics simulations which neglect cross-beam energy transfer (CBET). At $2.5\times10^{14}$ W/cm$^2$, coupled laser energy drops to $~87\%$ and it is believed that CBET results in a loss of $\sim10\%$ of the incident energy. Comparisons are made between measured densities and temperatures using Thomson scattering and 2D simulations which include Thomson self-heating from the probe beam. At drive intensity of $10^{14}$ W/cm$^2$, Thomson self-heating has roughly a $10\%$ effect on measured temperature. The impact of self-generated fields on heat transport in the 2D simulation is assessed.   

Thermal transport modeling of laser-irradiated spheres

In laser-fusion plasmas, classical Spitzer-Harm (SH) local thermal conduction is often used. A flux limiter is employed as an ad hoc fix to reduce the heat flux to more physical levels when the mean-free-paths of the heat flux carrying electrons is comparable to the temperature gradient scale length. This work studies the effect of non-local electron transport in the plasma corona surrounding direct-drive spheres at laser intensities ranging from $10^{14}-10^{15}$ W/cm$^{2}$. In order to highlight the thermal transport modeling, we consider low- to mid-Z materials (Be, Al, Cu), for which non-LTE kinetics are easier and less impactful on observables. One-dimensional spherical radiation-hydrodynamics simulations of the proposed experiments are performed using HYDRA. The thermal transport is modeled using the recently updated Schurtz-Nicolai-Busquet (SNB) reduced-order nonlocal model[1][2]. The HYDRA-SNB model exhibits good agreement with Vlasov-Fokker-Planck modeling, while bot h differ from SH transport, where differences in the thermal heat fluxes lead to hotter coronal electron temperatures.\\ $[1]$Brodrick et al., Phys. Plas. 24, 092309 (2017)\\ $[2]$Schurtz et al., Phys. Plas. 7, 4238 (2000)   

Influence of laser field on Coulomb logarithm with MD simulations

Coulomb logarithm, ($\ln(\Lambda)$), is involved in several important quantities of high energy density plasmas such as inverse bremsstrahlung absorption, electron-ion temperature relaxation frequency, electron conduction and more. Molecular-dynamic (MD) simulations of Plasmas of Ions and electrons in COulomb interaction have been carried out with our code PIeCO to measure numerically $\ln(\Lambda)[n_e, T_e]$ out of the velocity damping of electrons through ions. PIeCO reproduced results of MD simulations by Dimonte and Daligault [1] and theoretical results by BPS [2]. The work presented in this talk will focus on PIeCO simulations of the same type of plasmas subjected, in addition, to oscillating electric fields (to mimic a laser field with $\lambda$=351 nm and intensities ranging from I=$10^{11}$ to $10^{15}$ W/cm$^2$). The results of these MD simulations allowed to grasp the non-trivial variation of $\ln(\Lambda)$ with respect to $I$ and $\lambda$ (in addition to $n_e$ and $T_e$). A best fit of $\ln(\Lambda)[n_e, T_e, I, \lambda]$ involving all 4 parameters will be presented. \\ \\1. G. Dimonte and J. Daligault, PRL {\bf 101}, 135001 (2008)\\ 2. L.S. Brown et al., Phys. Reports {\bf 410}, 237 (2005)