Session KI02: Invited: Inertial Confinement Fusion

Live

Nonuniformities present in the laser illumination and target in laser-direct-drive (LDD) inertial confinement fusion experiments lead to an asymmetric compression of the target, resulting in an inefficient conversion of shell kinetic energy to thermal energy of the hot-spot plasma. These multidimensional effects on hot-spot formation in LDD DT cryogenic implosions were examined using 3-D nuclear and x-ray diagnostics (i.e., having three or more diagnostic lines of sight) on the OMEGA laser. The neutron-averaged hot-spot velocity ($v_{\mathrm{hs}})$ and apparent ion temperature ($T_{\mathrm{i}})$ asymmetry are determined from neutron time-of-flight (nTOF) measurements of the primary D$-$T fusion neutron energy spectrum, while the compressed shell areal density surrounding the hot spot is inferred from measurements of the scattered neutron energy spectrum. The low-mode perturbations of the hot-spot shape were characterized from x-ray self-emission images recorded along three quasi-orthogonal lines of sight. This talk will present the first systematic study of the $v_{\mathrm{hs}}$ in LDD DT cryogenic implosions, including an interpretation of the experimental results using 3-D radiation-hydrodynamics simulations. Implosions with significant mode-1 asymmetries show large hot-spot velocities (\textgreater 100 km/s) in a direction consistent with the hot-spot elongation observed in x-ray images and the measured $T_{\mathrm{i}}$ asymmetry. Mode 1 laser-drive corrections have been applied through shifting the initial target location in order to mitigate the measured asymmetry. With the asymmetry corrected, a more-symmetric hot spot is observed with reduced $v_{\mathrm{hs}}$ and $T_{\mathrm{i}}$ asymmetry and an increase in fusion yield. Plans to improve implosion performance using these measurements will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

A novel target design for mitigating laser imprint and subsequent Rayleigh-Taylor growth is presented for use in high-energy-density and direct-drive inertial-confinement-fusion experiments. In this scheme, a thin gold membrane is offset from the main target by several hundred microns. A strong picket on the drive beams is incident upon this membrane to produce x rays which generate the initial shock through the target. The main drive follows shortly thereafter, passing through the ablated shell and directly driving the main target. The efficacy of this scheme is demonstrated in planar geometry through experiments performed at the OMEGA EP facility, showing an exponential-in-frequency reduction of the Rayleigh-Taylor instability growth, suppressing development by at least a factor of 5 for all wavelengths below $100\,\mathrm{{\mu}m}$. The next phase of research is focused on fielding a target in spherical geometry using a cone-in-shell configuration. The status of its development and the additional benefits resulting from such a geometry are discussed.   

Live

The performance of Inertial Confinement Fusion (ICF) implosions is governed by four key parameters implosion velocity, adiabat, inflight ablation pressure, and capsule size [1]. Experiments on the NIF have roughly bracketed the limits of these terms for current systems [2-4]. While optimizing these terms has previously enabled experiments at the NIF to achieve record hot spot energies and fusion yield, experiments to explore cliffs in performance indicate that these terms may have reached their near optimum for these platforms [5]. The goal of HYBRID campaigns is to increase energy delivered to the hot spot by increasing capsule scale and to determine the largest capsule size that can be fielded symmetrically within the current experimental limits at NIF. Here, we report on HYBRID-E experiments that have fielded the largest diamond capsule implosions driven symmetrically on the National Ignition Facility (NIF) (inner radius of $\sim $1100$\mu $m) to high velocities of up to 400 km/s. This was enabled by using a modest amount of cross beam transfer (CBET) and choosing hohlraum parameters in a semi-empirical way [6] to control symmetry at small case-to-capsule ratio (CCR). These experiments build on the work of previous campaigns, including HYBRID-B [7-9] which symmetrically fielded a $\sim $1050$\mu $m inner radius capsule implosion, without CBET, at a larger CCR and slower implosion velocity. We report record fuel kinetic energies and hot spot energies of \textasciitilde 15kJ by driving a 65$\mu $m thick DT layer to \textasciitilde 360 km/s in a 1100$\mu $m inner radius HDC capsule. This configuration currently holds the record for the highest neutron yield on NIF and gave several times higher yield than a direct comparison experiment using a 10$\mu $m thinner DT ice layer which showed more meteors, or glowing bright spots, that cool the implosion via ablator mix into the hot spot. This strong sensitivity to ice thickness could be due to instabilities seeded by defects in the capsules that contain thousands of voids. Experiments to increase hohlraum temperature using a smaller laser entrance hole (LEH) resulted in higher implosion velocities up to 400km/s. However, preliminary experiments at higher velocities again showed meteors. While the origin and mitigation of these meteors is still being investigated, ongoing work is being done to test better quality capsules, reduce ablation front growth factors, and further increase ice thickness at high implosion velocity. In the coming months we also plan to scan capsule scale (1050$\mu $m-1100$\mu $m inner radius) to determine the optimal capsule scale for the current laser capability of NIF with hot spot pressure being a primary metric. \begin{enumerate} \item O A Hurricane, \textit{et al,} Plasma Phys. Control. Fusion 61, 014033 (2019). \item T. Ma, et al, Phys. Rev. Lett. \textbraceleft $\backslash $bf 114\textbraceright , 145004 (2014). \item S. Le Pape, \textit{et al,} Phys. Rev. Lett., 120, 245003 (2018). \item D. T. Casey, \textit{et al,} Physics of Plasmas \textbraceleft $\backslash $bf 25\textbraceright , 056308 (2018). \item O. A. Hurricane, \textit{et al,} 26, 052704 (2019). \item D. A. Callahan, \textit{et al,} Physics of Plasmas 25, 056305 (2018). \item A. L. Kritcher, \textit{et al,} Phys. of Plasmas, submitted (2020). \item M. Hohenberger, \textit{et al,} Phys. of Plasmas, submitted (2020). \item A. Zylstra, \textit{et al,} Phys. of Plasmas, submitted (2020). \end{enumerate} * This work was performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC52-07NA27344 and Contract no. 89233218CNA000001. This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.   

Live

A key requirement for improving the ignition prospects on the NIF is increased capsule absorbed energy $E_{\mathrm{cap}}$. Three approaches toward this goal include the use of advanced hohlraums, implementing laser direct drive, and increasing the laser energy. This talk focuses on the first option to achieve 2-3x higher $E_{\mathrm{cap}}$ \textgreater 0.5 MJ with use of innovative hohlraums on the existing NIF laser. In contrast to the extensively used cylinder hohlraum, two non-standard geometries toward high $E_{\mathrm{cap}}$ are considered: the ``Frustraum'' (or double-cone hohlraum) [1] and the rugby-shaped hohlraum [2]. Both shapes can accommodate \textasciitilde 50{\%} larger capsules without sacrificing drive symmetry and peak drive, according to recent favorable NIF data. By significantly increasing$ E_{\mathrm{cap}}$, ignition can be achieved at lower fuel convergence and higher implosion adiabat. Experiments with cylinder hohlraums and nominal scale (\textasciitilde 1 mm radius) capsules to date have shown less fuel compression than expected [3], which is consistent with sources of degrading preheat and mix that hinder ignition. Operating with a high-volume and -adiabat capsule (``HVAC'') potentially provides a novel path towards ignition at the acceptable price of reduced energy gain. The HVAC mode of ignition conveniently spans a spectrum of states from near hot-spot to volume ignition, defined as when (1) the entire fuel is the hot spot and (2) the ablator provides the majority of inertial confinement of the igniting fuel. This talk covers in detail the close coupling of validated advanced hohlraum performance with prospects for realizing the HVAC ignition concept on the NIF. [1] P. Amendt \textit{et al}., PoP \textbf{26}, 082707 (2019); https://doi.org/10.1063/1.5099934 [2] Y. Ping, V. Smalyuk, P. Amendt \textit{et al}., Nature Phys.; https://doi.org/10.1038/s41567-018-0331-5 [3] A.L. Kritcher \textit{et al}., PoP \textbf{23}, 052709 (2016).