Session TO07: ICF: Capsule Instabilities, Low & High-Mode

Live

A 3-D view of the hot-spot is crucial for understanding the evolution of the hot spot and the multidimensional effects that occur during inertial confinement fusion implosions. OMEGA currently has two time-gated x-ray imagers: a time-resolved Kirkpatrick--Baez x-ray microscope [F. J. Marshall et al., Rev. Sci. Instrum. 88, 093702 (2017)] and the single-line-of-sight, time-resolved x-ray imager (SLOS-TRXI) [W. Theobald et al., Rev. Sci. Instrum. 89, 10G117 (2018)]. A time-gated hot-spot x-ray imager is being developed for use on OMEGA as a third line of sight that will follow the SLOS-TRXI concept but will have improved spatial and temporal resolutions of $\le $5 $\mu $m and 10 ps, respectively. The diagnostic requirements and an optimized design will be presented. The self-emission from the hot spot from multiple lines of sight will be analyzed and combined to infer the electron temperature, internal pressure, and to create a low-mode reconstruction of the hot-spot region. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

Low mode 3D drive asymmetries ($l \quad =$ 1 and 2) are important degradation mechanisms for indirect drive implosions. A static view factor model is used to assess sources of drive asymmetry including laser variation in the foot and peak, laser/target positioning errors, target diagnostic window losses, Cross Beam Energy Transfer, and Stimulated Brillouin Scattering (SBS). Each source can produce $l \quad =$ 1 drive asymmetries of M$_{\mathrm{1}}$/M$_{\mathrm{0}}$ \textasciitilde 0.5{\%}. and in combination imploded hotspot velocities of 100km/s. Asymmetries in the thickness and composition of the capsule ablator also contribute to hotspot velocity. We compare the drive and target asymmetries from the laser diagnostics and target metrology, with measurements from the hotspot velocity, shape and SBS diagnostics. These studies illuminate systematic trends in the facility and target performance that can be used to understand impacts on experiments and potential mitigations.   

Live

Low mode asymmetry in ICF implosions has been recognized as a potential performance limiting factor. Recently a non-linear, but solvable, theory based upon the simple picture of a pair of asymmetric pistons has generated new insights and provides some practical formulae for estimating the degradation of an implosion due to mode-1 asymmetry and there are hints the model naturally extends to mode-2. Asymmetry of the implosion “shell,” as opposed to asymmetry of the hot-spot, is key to the model. By including time-dependent swing in the model, it is shown that a key variable in the model is the shell asymmetry fraction at stagnation, f, which interconnects DSR asymmetry measured by nuclear diagnostics, the neutron-time-of-flight measured “hot-spot” velocity, and the concept of RKE. The theory yields explicit expressions for the impact of asymmetry upon hot-spot diameter, stagnation pressure, hot-spot energy, inertial confinement-time, Lawson parameter, hot-spot temperature, fusion yield, and yield amplification. Agreement is found between the scalings coming from the theory and ICF implosion data from the NIF and to large ensembles of detailed simulations, making the theory handy for interpreting data.   

Live

To approach ignition in inertial confinement fusion implosions, large shell areal densities are required to confine the high-temperature fusion plasma during the disassembly phase. The presence of low modes in implosions leads to significant variations in the mass distribution of the cold shell. In this work, an analytic 3-D model for low modes is presented to describe the impact of low modes on the ignition threshold. A new form of the average areal densities is derived in terms of the harmonic mean to capture the hydrodynamics of a fast{\-}disassembly, thin-shell wall. The characteristic time scale defined by the second time derivative of the hot-spot volume at the time of minimum volume is shown to be a good candidate to represent the disassembly time scale. The presence of low-mode areal density asymmetry is shown to quench ignition by driving a higher rate of \textit{PdV} loss during the disassembly phase than the rate of alpha heating. An analytic relation between neutron yields and stagnation parameters including the areal density is derived.   

Live

We present a model detailing the sensitivity of inferred bulk hot-spot motion in inertial confinement fusion implosions in the presence of x-ray flux asymmetries. Bulk hot-spot motion is inferred through neutron time of flight diagnostics on the National Ignition Facility and are indicative of collective motion within the hot spot. This bulk hot-spot motion is most notably pronounced when in implosions with large spherical harmonic mode one (l$=$1) asymmetries but can also result from aneurisms in the dense shell. Implosions on the National Ignition Facility however typically comprise of multiple simultaneous asymmetries and the interaction of asymmetries can impact the observables. In this work we perform 3D HYDRA [1] simulations to investigate low mode x-ray flux asymmetry mode-coupling and observe the sensitivity of the direction and magnitude of the bulk hot-spot motion. We find oblate (pancake) stagnated shell geometries inhibit the motion to a larger degree than prolate (sausage) ones. We verify this relationship in the presence of multi-mode flux asymmetries and apply the knowledge to experimental data taken on the National Ignition Facility.   

Live

Distributed polarization rotators smooth laser speckle by splitting each OMEGA beam into two nearly co-propagating beams with orthogonal polarization. On target, the focal spots of these two beams are offset by \textasciitilde 90 ?m. The polarizations are balanced near the center of the intersection of the overlapping spots but not at the edges. Cross-beam energy transfer effects in these linearly polarized regions can significantly degrade the symmetry of absorbed energy and overall performance in an implosion. Predicted distributions of scattered light from an implosion are compared to measurements on multiple detectors where asymmetries of the order of tens of percent are commonly observed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

Achieving ignition at the National Ignition Facility (NIF) requires the kinetic energy of an imploding capsule to be maximally transferred to the central fuel to initiate thermonuclear burn. Deviation from sphericity at stagnation may significantly reduce the energy available leading to performance degradation. Current implosions at the NIF, using high-density-carbon (HDC) have shown residual hot-spot velocities in the direction of low areal density, suggesting the presence of mode-1 asymmetries [1]. From analysis of several DT shots, the asymmetries can be correlated to beam-to-beam laser delivery variations, target features (such as diagnostic windows) and recently ablator thickness variations [2,3]. However, some variability remains indicating other sources may be at play. Recent measurements have indicated that the of the individual ablator layers that make the HDC shell may have larger thickness variations than expected. We have used the code HYDRA to study these non-uniformities and found that implosions show sensitivity to thickness variations of the doped layer even for constant ablator thickness. [1] H. Rinderknecht et al., Phys. Rev. Lett. 124, 145002 (2020) [2] B. McGowan et al. Paper presented at IFSA 2019 [3] D. Casey et al. Paper submitted Phys. Rev. Lett.   

Live

Performance degradation in direct-drive inertial confinement fusion (ICF) implosions can be caused by several effects, one of which is Rayleigh-Taylor (RT) instability growth. Defects in ICF targets like inner surface voids and surface roughness create seeds for RT growth during the initial phase of implosions. Perturbations created by these defects are propagated along acoustic waves that reverberate within the shell. The presence of an ablator--ice interface creates reflected rarefaction and compression waves that can amplify these initial seeds and perturbations. A comprehensive understanding of the evolution of these particular waves and perturbations is required to characterize the influence of these internal defects. The evolution and amplification of 2-D perturbations and their acoustic wave propagation within a planar target will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

In inertial confinement fusion (ICF) implosions, the interface between the cryogenic DT fuel and the ablator is unstable to shock acceleration (the Richtmyer-Meshkov instability, RM) and constant acceleration (Rayleigh-Taylor instability, RT). If, however, the constant acceleration is in the direction of the lighter material (negative Atwood number), the RT instability produces oscillatory motion that can stabilize against RM growth. Theory and simulations suggest this scenario occurred at early times in the higher adiabat ``big-foot'' implosions, possibly explaining their favorable performance compared to 1D simulations. This characteristic can be included in newer, lower adiabat designs. These designs can potentially improve compression while minimizing ablator mixing into the DT fuel.   

Live

Hydrodynamic instabilities are major factor in degradation of spherical implosions in inertial confinement fusion (ICF). Instabilities at ablation front are some key contributors to overall stability of x-ray driven implosions. We present results of hydrodynamic instability experiments with high-density-carbon (HDC) ablators on National Ignition Facility (NIF). The unstable growth of pre-imposed modulations at various mode numbers was measured with x-ray radiography using Hydrodynamic Growth Radiography (HGR) platform. The experiments were conducted with Au and U hohlraums with \textasciitilde 25{\%} difference in M-band emission of x-ray drives. The dependence of the instability growth on M-band fraction will be presented.   

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The mixing of fuel and shell material in ICF implosions can affect implosion dynamics and even prevent ignition. Computational models for mixing have many free parameters that require calibration. Manufacturing defects and measurement errors complicate the calibration process. We present a framework for mix model calibration that accounts for uncertainty in the underlying system. We calibrate and assess the validity of a diffusion and a turbulence model for mix in a series of separated reactant experiments performed at the National Ignition Facility. We perform an ensemble of 1D simulations and account for manufacturing defects by varying the stoichiometry of the capsule shell. Uncertainty in the radiation drive is accounted for by varying the energy and spectrum of the simulated source. Mix model parameters are varied over a wide range, representing a lack of prior knowledge of their values. A surrogate model is constructed using data from simulations. Experimental measurements are used to infer a distribution for mix model parameters and the calibrated computational models are used to find predictive distributions of additional ``hidden'' experimental data.   

Live

The asymptotic evolution of the Rayleigh-Taylor (RT) instability has been widely studied, but a consensus is still lacking on the influence on initial conditions (ICs) on the final self-similar regime. A planar direct-drive platform has been deployed at the NIF to investigate the highly nonlinear stage of the ablative RT [1]. This platform allows to benchmark the nonlinear theory of the ablative RTI in a unchartered experimental regime. The initial seeds of the RT were formed using a well-characterized imprinting laser beam, and the growth of the optical depth modulations and the acceleration of the foil are observed using time-resolved x-ray radiography. We show that the mixing parameter of the nonlinear ablative RT has a strong dependence on ICs, as previously observed in classical hydrodynamics experiments. The variance can be attributed to the dominance of either bubble competition or bubble merger. We will adapt this platform to examine the effects of different materials on RT development. By varying the ratio of ablation velocity to acceleration, we will gain a new understanding of the role of ablation in the RT. This has deep implications across a range of HED physics research, including supernovae investigations, mix studies, and ICF capsule design. [1] A. Casner et al, Plasma Phys. Control. Fusion \textbf{60}, 014012 (2018).   

Live

Traditional indirect drive design is a two-step process: first a desired capsule, laser and hohlraum configuration are constructed via simulation, followed by a subsequent experimental test. However, due to variations in laser performance and material manufacturability (e.g. capsule geometry), the as-fired configuration will naturally deviate from the originally conceived ideal case. Furthermore, due to physics knowledge gaps, the inferred radiation environment in the experiment differs from that which was predicted by simulations (and around which the design was originally conceived). The combination of the design and experiment having both different boundary conditions and different physics can complicate the interpretation of results, obfuscating whether observations are inherent to the design itself (and therefore potentially fixable) or whether attributable to random shot-to-shot variation. In this work we aim to answer the question, “Can we engineer resilience to as-fired conditions into a capsule design?” We will explore this question in the context of re-optimizing high-performing as-fired NIF shots to include uncertainty due to manufacturability and laser drive. Given what we can infer about likely experimental conditions, can we design the best capsule to suit?   

Live

Recent inertial confinement fusion (ICF) measurements have highlighted the importance of 3D asymmetry effects on implosion performance. One prominent example is the bulk drift velocity of the deuterium-tritium plasma undergoing fusion (``hot spot''). Upgrades to the National Ignition Facility neutron time-of-flight diagnostics now provide v$_{\mathrm{bulk}}$ to better than 1 part in 10$^{\mathrm{4}}$, and enable cross-correlations with other measurements. This talk presents the impact of v$_{\mathrm{bulk}}$ on neutron yield, downscatter ratio, apparent ion temperature, electron temperature, and 2D x-ray and neutron emission. The benefits of using cross-diagnostic analysis to obtain 3D views of the plasma is highlighted. A comparison with modeling is made, and future concepts for measuring hot spot flows is presented. This research reflects the growing interest in 3D measurements of the national ICF community.