Session GO09: ICF: Alternative Technologies

Live

Spherical hohlraums, including tetrahedral hohlraums shot on OMEGA\footnote{ G.R. Bennett \textit{et al.}, Phys. Plasmas \textbf{7}, 2594 (2000).} and octahedral hohlraums (with six laser entrance holes on the faces of a cube) proposed by Lan \textit{et al.}\footnote{ K. Lan \textit{et al.}, Phys. Plasmas \textbf{21}, 010704 (2014).} promise significant uniformity advantages compared with conventional cylindrical hohlraums. This work advocates a minor rearrangement of the port locations of the 48~quads proposed for irradiating octahedral hohlraums on the SG4 laser. This will enable symmetric direct-drive implosions to be carried out in the same target chamber with minimal adjustments of the beam pointings (no more than about 12 degrees, in contrast to 35 degrees in typical National Ignition Facility direct-drive designs). View-factor calculations for octahedral hohlraums find essentially the same excellent performance as in Ref. 2, with the capsule nonuniformity ranging from 0.6{\%} (rms) at early times to \textless 0.1{\%} at later times.   

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A new configuration, first suggested by Farmer \textit{et al}.,\footnote{ W. A. Farmer \textit{et al.}, Phys. Plasmas \textbf{26}, 032701 (2019).} is proposed for spherical hohlraums on OMEGA in which seven laser entrance holes (LEH's) are used, five around the equator and one at each pole. This is known as the PEPR (pentagonal prism) hohlraum. A new view-factor code \textit{LORE}, based on the work of Schnittman et al.,\footnote{ J. D. Schnittman and R. S. Craxton, Phys. Plasmas \textbf{3}, 3786 (1996).} is used to model the PEPR hohlraum and compare its performance with the tetrahedral hohlraums shot on OMEGA by Bennett \textit{et al.}\footnote{ G. R. Bennett \textit{et al.}, Phys. Plasmas \textbf{7}, 2594 (2000).} For most albedos the PEPR hohlraum produces a factor-of-2 lower on-capsule nonuniformity. For high albedos this can be improved by another factor of 2 (to about 0.4{\%}) by a small increase in the polar LEH radii. The PEPR hohlraum is well matched to the OMEGA symmetry and could be a useful platform for studying the physics of spherical hohlraums.   

Live

Following the successful NIF experiment demonstrating \textasciitilde 30{\%} energy coupling to an aluminum capsule in a rugby-shaped gold hohlraum with a reverse ramp laser drive (Ping, Smalyuk, Amendt, et al. Nature Physics 2019), a series of NIF shots are being carried out using 3mm-diameter HDC capsules and 2-shock pulse shape to determine whether the high coupling can be maintained for a single-shell ignition design. The first energy walkup shot at 1.1MJ showed low backscatter and hot spot symmetry in good agreement with simulations. Results from subsequent experiments will be presented and the prospect on high-adiabat ignition enabled by high energy coupling (\textasciitilde 500kJ) will be discussed. * This work was performed under the auspices of the US DOE by LLNL under contract number DEAC52- 07NA27344.   

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The HotThick campaign at the National Ignition Facility (NIF) is a new effort to increase energy coupled to the implosion using a higher radiation temperature ($\sim$315 eV), generated by fielding the full NIF laser power and energy (480 TW, up to 1.9 MJ) in a small hohlraum (5.4 mm diameter) at small case to capsule ratio. The higher drive supports a thicker CVD diamond (HDC) ablator and DT ice layer than previous campaigns at the same capsule scale. Fielding a longer laser pulse at full power and energy in a smaller hohlraum presents significant hohlraum physics challenges, including mitigating backscattered laser power and maintaining implosion symmetry control. This work presents an overview of the design and first five shock timing and symmetry implosion experiments. The target radiation temperature of 315 eV was surpassed with 1.4 MJ of laser energy, and applying wavelength detuning of each outer cone (44$^{\circ}$ and 50$^{\circ}$) relative to the inner cones (23$^{\circ}$ and 30$^{\circ}$, so-called ``4 color'' cross-beam energy transfer) has successfully maintained backscatter at reasonable levels, equally balanced between all four cones. Future experiments will increase wavelength detuning to further mitigate flux asymmetries.   

Live

In recent experiments at the NIF, near-record neutron yields have been measured in modest convergence Polar Direct Drive (PDD) implosions of DT gas-filled capsules using only 55{\%} to 75{\%} of the available NIF laser energy [1]. These experiments represent the highest efficiency conversion of NIF laser energy to thermonuclear fusion output demonstrated to date. We propose to increase the yield still further by including a liquid DT layer on the inner surface of a large PDD capsule. The use of PDD enables \textasciitilde 10X more energy absorbed by the capsule (compared to indirect drive), and the use of a DT liquid layer [2] (rather than a DT ice layer) enables layered implosions with the reduced convergence [3] required for adequate symmetry with PDD. In this talk, we will compare the PDD DT liquid layer approach to the indirect drive DT ice layer concept and present a preliminary ignition design that has a modest convergence with conservative implosion parameters. 1. Elijah Kemp \textit{et al}, ``Development of high-efficiency, non-cryogenic, direct-drive neutron sources on the NIF,'' YO6.00014, 61st APS-DPP meeting (2019). 2. R. E. Olson \textit{et al}., ``First Liquid Layer ICF Implosions at the NIF,'' \textit{Phys. Rev. Lett}. \underline {117}, 245001 (2016). 3. B. M. Haines \textit{et al}., ``The effects of convergence ratio on the implosion behavior of DT layered ICF capsules,'' \textit{Phys. Plasmas} \underline {24}, 072709 (2017).   

Live

In exploding-pusher (XP) target implosions a significant fraction of the total neutron yield is generated by the outgoing reflected shock potentially augmented by yield due to compression of the fuel by the shell. Unlike ablatively driven implosions, these are low-convergence and characterized by low areal densities and high ion temperatures, and are insensitive to the perturbations that degrade high-convergence implosions, making them of interest for study of laser-energy coupling and as neutron sources for various applications. XP's are typically driven by simple (Gaussian, flattop, ramp) pulses. We present the results of a suite of optimizations using \textit{Telios,} designed to maximize the free-fall yield based on changes in pulse shape. For OMEGA plastic-shell 10-atm D2-filled targets, these pulses are predicted to provide significant (over 40{\%}) increase in total and free-fall yield relative to implosions driven by a flattop pulse of the same energy. Experimental data from implosions employing optimized pulses will be compared to those using a baseline flattop pulse. The physical causes of the predicted improvement in performance, including both multiple shock dynamics and hydrodynamic coupling, will be discussed.   

Live

A new class of ignition designs is proposed for inertial confinement fusion (ICF) experiments. These designs are based on the hot-spot--ignition approach, but instead of conventional targets that comprise of spherical shells with thin frozen deuterium--tritium (DT) layers, liquid DT spheres are used where the lower-density central region and higher-density shell are created dynamically by appropriately shaping the laser pulse. These offer several advantages, including simplicity in target production and lower sensitivity to physics uncertainty in shock interaction with the ice/vapor interface. The design evolution starts by launching an \textasciitilde 1-Mbar shock into a homogeneous DT sphere. After bouncing from the center, the reflected shock reaches the outer surface of the sphere and the shocked material starts to expand outward until its pressure drops below the ablation pressure. At this point, an adjustment shock is launched inward by supporting ablation pressure. This shock compresses the ablator and fuel, forming a shell. The shell then is accelerated and compressed by appropriately shaping the drive laser pulse, similar to the conventional thin-shell, hot-spot designs. This talk will discuss the feasibility of the new concept using hydrodynamic simulations.   

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Current experiments at NIF are investigating the use of room-temperature, large-diameter, thin-shelled plastic ablator targets to produce high neutron fluxes with fusion output approaching 100 kJ. Recent results have produced \textasciitilde 25 kJ of fusion output using \textasciitilde 1.1 MJ incident 351 nm light. Work done on the OMEGA laser demonstrated the efficacy of shell contouring to compensate for oblique laser pointings implicit with polar drive. We present the results of experiments performed on the NIF investigating the benefit of contoured shells to compensate for polar-drive pointings. Contours were derived from 2-D \textit{DRACO} simulations accounting for the effects of cross-beam energy transfer between the beams. Target shells implementing these contours were constructed at General Atomics and delivered to the NIF for implosion. We present experimental results and post-shot analysis of the shots with and without shell contours, comparing observables such as yield, burn history, and overall shell morphology with 2-D \textit{DRACO} simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

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Recent progress validating the concept of multi-shell direct-drive ignition has been made on NIF using both single shell and double shell polar direct drive (PDD) implosions. The intent of these experiments is to validate the outer ablator shell hydro-efficiency and the kinetic energy transfer efficiency between two concentric shells. The Revolver triple-shell ignition target requires high (\textgreater 90{\%}) coupling of direct-drive laser energy to its large 5-6 mm outer shell to demonstrate high hydro-efficiency (\textasciitilde 10{\%}) of PDD laser energy into inward kinetic energy for small laser beam to capsule radius ratios (\textasciitilde 1/3). Moreover, high implosion kinetic energy transfer efficiency between the outer two colliding shells (\textgreater 50{\%}) is needed. These experiments employed 5 mm outer shells driven with surface intensities and laser pulse lengths consistent with the Revolver ignition design. Outer shell trajectories were measured using both x-ray self-emission imaging and x-ray backlit radiography. The post-collision inner shell trajectory was measured with backlighting showing good agreement with simulation. The evolution of the outer shell joint also was observed for the first time. In contrast to indirect-drive double shells, no discernable impact on the inner shell from the outer shell joint was observed. Diagnostic measurements indicate low levels of scattered light from these targets. Details and comparisons with simulations will be shown. Research supported under LANL's LDRD Program project 20180051DR.   

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This work describes a new pulse-shaping methodology which, according to radiation hydrodynamic simulations, enables the benefits of Shock Ignition but without the requirement for high peak laser intensity and/or power. If this can be realized, this work indicates that Laser Direct Drive implosions could be fielded on existing facilities which would achieve gains of \textasciitilde 85 with a peak implosion velocity of \textless 330km/s. Shock Ignition appears an attractive route to achieving ignition and high gain via laser fusion. The strong shock enables ignition at implosion velocities below the self-ignition threshold, limiting growth of the ablative Rayleigh-Taylor instability. However, it does have potential disadvantages, caused by the need for high peak intensity to drive the strong shock. This work details a novel pulse shape which enables the creation of a very strong shock (\textgreater 1 Gbar), thereby enabling the benefits of Shock Ignition, but with a peak intensity of \textasciitilde 1.3x10$^{\mathrm{15}}$W/cm$^{\mathrm{2}}$. The reduced intensity may reduce deleterious parametric instabilities, while the reduced peak power requirements, would enable a large capsule (radius 1720\textmu m) to be fielded on NIF while remaining within the current 1.8MJ, 500TW limitations.   

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Odin is a 2D Arbitrary Lagrangian-Eulerian (ALE) code with tabulated equation of state, implicit thermal conduction and radiation, 3D supra-thermal (hot) electron model (\textit{Solodov, \& Betti Phys. Plasmas 15.4 (2008): 042707}), and the option of a simplified laser ray model; all developed at the University of Warwick with aid of the wider UK academic community. Using Odin we simulated a shock ignition target with the aims of investigating how susceptible a non-uniform late stage capsule is to hot electrons and whether the common asymmetry modes seen at NIF and OMEGA exacerbate the already significant issue that hot electrons pose to 1D ignition.\\ Our project builds on investigations from Atzeni et al. (\textit{EPJ D 73.11 (2019): 243}) and Colaïtis et al. (\textit{Phys. Plasmas 23.7 (2016): 072703}) looking at 2D non-uniform targets and hot electrons in shock ignition respectively. We model target sensitivity to hot-electron distributions, total energy and angular spread, starting within the safe margins of ignition according to the Ignition Threshold Factor (ITF) (\textit{Hohenberger et al. Phys. Plasmas 22.5 (2015): 056308}). In addition to the threat posed to 1D ignition we find non-uniform targets pose additional opportunity for detrimental effects from hot electrons.   

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Fast ignition driven by ions [1] constitutes a valid alternative to the more conventional fast ignition scheme, which leverages relativistic electrons [2]. In this work, we demonstrate that ion beams with charge and energy suitable to trigger the ignition spark could be generated via collisionless shocks in the expanded corona surrounding the compressed pellet. Performing two-dimensional simulations using the Particle-In-Cell code OSIRIS [3], we modelled the interaction of an intense laser pulse with the long scale-length corona plasma. Numerical results indicate that an electrostatic shock is launched as a consequence of the hole bored by the laser. The shock propagates upstream and accelerates protons to energies between 8 and 30 MeV. Considering a compressed Deuterium-Tritium pellet with density of 400 g/cm$^3$ and temperature of 5 keV, such protons can deposit the bulk of their energy in the core within a range of 0.3 - 1.2 g/cm$^2$. Finally, we show that for large enough laser spot-sizes, the proton beam contains a number of ions sufficient to create the hot spark that will drive the thermonuclear burn wave. [1] Roth et al., Phys. Rev. Lett. 86, 436 (2001). [2] Tabak et al., Phys. Plasmas 1, 1626 (1994). [3] Fonseca et al., Lect. Notes Comp. Sci. 2331, 342 (2002).   

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Staged Z-pinch is a potential high-energy gain fusion concept, where a high-atomic number liner implodes onto a DD or DT target using pulsed multi-MA current source. We used the two-dimensional code MACH2 for a comparative study of three different types of solid liners (Be, Cu, and Ag) imploding onto DD and DT target. Comparison is also made with experiments and simulations of Be-liner imploding on DD target, with and without preheating of MagLIF like target at the Sandia National Laboratory Z-facility. Our study shows that high-atomic number liners generate strong shock waves which facilitate target plasma preheating to few hundred eV before the final adiabatic compression of the target. This eliminates the auxiliary heating requirement for the Magneto-Inertial fusion concept. In addition, significantly higher yield is produced. Our modeling results suggest that fusion energy breakeven, and beyond, is possible in a high current machine like the Sandia National Laboratory Z-facility.   

Live

A directly-driven Revolver triple-shell capsule is designed to have two dynamical fuel implosion stages prior to ignition: a shock phase in which the fuel is pre-heated by a spherically converging shock, followed by an adiabatic compression phase in which the fuel is further heated to ignition temperatures. Employing the state-of-the-art, hybrid (kinetic-ion/fluid electron), multi-ion Vlasov-Fokker-Planck code, iFP\footnote{J. Comp. Phys., {\bf 297} 357 (2015); {\em ibid.} {\bf 318} 391 (2016); {\em ibid.} {\bf 339} 453 (2017); {\em ibid.} {\bf 365} 173 (2018).} -- as well as semi-analytic predictions from ideal hydrodynamics in spherical geometry -- we confirm this two phase picture. Critically, we find that shock kinetic effects and non-ideality are present in the course of the fuel implosion, but these effects do not change the overall dynamics (which is well described by ideal hydrodynamics theory).\footnote{see Phys.\ Plasmas {\bf 27}, 042704 (2020) for details}   

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Hydrodynamic stability is perhaps the most challenging physics issue confronting double shell inertial confinement fusion (ICF) targets from the achievement of robust thermonuclear burn. Double shell implosions at the National Ignition Facility (NIF) utilize the x-rays created by laser heating of Au hohlraums to drive the inward acceleration of the outer ablator shell toward an interior high-density (e.g., W) shell containing the DT fuel. At implosion speeds of 200 km/s, the high-Atwood-number inner shell quickly becomes Rayleigh-Taylor unstable to all perturbation wavelengths except those short enough to experience viscous dissipation. Previous simulations [J.L. Milovich et al., Phys. Plasmas 11 (2004)] have predicted that engineered density gradients can stabilize high-modes (\textgreater 200), however, their use for suppressing mid-mode (30-100) growth and feedthrough is unknown, which our simulations suggest are most damaging to shell integrity during stagnation. In this talk we will present computational results of enhanced in-flight aspect ratio (IFAR) double shell designs in terms of areal density growth factor using extended density gradients and designs for experiments at the National Ignition Facility (NIF) to validate the computational results.