Session NO7: Compression and Burn I

Kinetic Energy Transfer Process in a Double Shell Leading to Robust Burn

A goal of double shell capsule implosions is to impart sufficient internal energy to the D-T fuel at stagnation in order to obtain robust $\alpha$-heating and burn with low hot spot convergence, C.R. $<$ 10. A simple description of the kinetic energy transfer from the outer shell to the inner shell is found using shock physics and adiabatic compression, and compares well with 1D modeling. An isobaric model for the stagnation phase of the inner shell is used to determine the ideal partition of internal energy in the D-T fuel. Robust burn of the fuel requires, at minimum, that $\alpha$-heating exceeds the rate of cooling by expansion of the hot spot so that the yield occurs before the hot spot disassembles, which is then used to define a minimum requirement for robust burn. One potential advantage of a double shell capsule compared to single shell capsules is the use of a heavy metal pusher, which may lead to a longer hot spot disassembly time. We present these analytic results and compare them to 1D and 2D radiation-hydrodynamic simulations.   

Investigating the importance of low-mode symmetry on double shell performance

Implosions of hohlraum-driven double shell targets as an alternative inertial confinement fusion (ICF) concept are underway at the National Ignition Facility. The double shell system relies on a series of efficient energy transfer processes starting from thermal x-ray absorption by the outer shell, followed by collisional transfer of kinetic energy to an inner shell, and final conversion to fuel internal energy. Beyond these zero-dimensional processes double shells must also be designed for robust performance against hydrodynamic instability growth, engineering features, and implosion asymmetry. In this talk we will present simulation results on the shape transfer process between the outer shell and inner shell during collision. We will discuss the mechanisms involved in the shape transfer process and give numerical predictions on their importance to double shell designs.   

NIF Double Shell outer/inner shell collision experiments

Double shell capsules are a potential low convergence path to substantial alpha-heating and ignition on NIF, since they are predicted to ignite and burn at relatively low temperatures via volume ignition. Current LANL NIF double shell designs consist of a low-Z ablator, low-density foam cushion, and high-Z inner shell with liquid DT fill. Central to the Double Shell concept is kinetic energy transfer from the outer to inner shell via collision. The collision determines maximum energy available for compression and implosion shape of the fuel. We present results of a NIF shape-transfer study: two experiments comparing shape and trajectory of the outer and inner shells at post-collision times. An outer-shell-only target shot measured the no-impact shell conditions, while an ‘imaging’ double shell shot measured shell conditions with impact. The ‘imaging’ target uses a low-Z inner shell and is designed to perform in similar collision physics space to a high-Z double shell but can be radiographed at 16keV, near the viable 2DConA BL energy limit.   

Double shell planar experiments on OMEGA

The double shell project is aimed at fielding neutron-producing capsules at the National Ignition Facility (NIF), in which an outer low-Z ablator collides with an inner high-Z shell to compress the fuel. However, understanding these targets experimentally can be challenging when compared with conventional single shell targets. Halfraum-driven planar targets at OMEGA are being used to study physics issues important to double shell implosions outside of a convergent geometry. Both VISAR and radiography through a tube have advantages over imaging through the hohlraum and double-shell capsule at NIF. A number physics issues are being studied with this platform that include 1-d and higher dimensional effects such as defect-driven hydrodynamic instabilities from engineering features. Additionally, the use of novel materials with controlled density gradients require study in easily diagnosed 1-d systems. This work ultimately feeds back into the NIF capsule platform through manufacturing tolerances set using data from OMEGA.   

Evaluation of the Revolver Ignition Design at the National Ignition Facility Using Polar-Direct-Drive Illumination

The direct-drive ignition design \textit{Revolver}\footnote{ K. Molvig \textit{et al.}, Phys. Rev. Lett. \textbf{116}, 255003 (2016).} \quad employs a triple-shell target using a beryllium ablator, a copper driver, and an eventual gold pusher. Symmetric numerical calculations indicate that each of the three shells exhibit low convergence ($\sim 3\,\mbox{to}\,5)$ resulting in a modest gain ($G\sim 4)$ for $\sim 1.7$ MJ of incident laser energy. Studies are now underway to evaluate the robustness of this design employing polar direct drive (PDD) at the National Ignition Facility. Integral to these calculations is the leveraging of illumination conditioning afforded by research done to demonstrate ignition for a traditional PDD hot-spot target design.\footnote{ T. J. B. Collins \textit{et al.}, Bull. Am. Phys. Soc. \textbf{59}, 150 (2014).} Two-dimensional simulation results, employing nonlocal electron-thermal transport and cross-beam energy transport, will be presented that indicate ignition using PDD. A study of the allowed levels of long-wavelength perturbations (target offset and power imbalance) not precluding ignition will also be examined. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Enhancing Ignition Probability and Fusion Yield in NIF Indirect Drive Targets with Applied Magnetic Fields

Imposed magnetic fields of tens of Tesla that increase to greater than 10 kT (100 MGauss) under capsule compression may relax conditions for ignition and propagating burn in indirect-drive ICF targets. This may allow attainment of ignition, or at least significant fusion energy yields, in presently-performing ICF targets on the National Ignition Facility that today are sub-marginal for thermonuclear burn through adverse hydrodynamic conditions at stagnation. Results of detailed 2D radiation-hydrodynamic-burn simulations applied to NIF capsule implosions with low-mode shape perturbations and residual kinetic energy loss indicate that such compressed fields may increase the probability for ignition through range reduction of fusion alpha particles, suppression of electron heat conduction and stabilization of higher-mode RT instabilities. Optimum initial applied fields are around 50 T. Off-line testing has been performed of a hohlraum coil and pulsed power supply that could be integrated on NIF; axial fields of 58T were obtained. Given the full plasma structure at capsule stagnation may be governed by 3-D resistive MHD, the formation of closed magnetic field lines might further augment ignition prospects. Experiments are now required to assess the potential of applied magnetic fields to NIF ICF ignition and burn.   

Scaling of Liquid DT Layer Capsules to an ICF Burning Plasma

Recent experiments at the NIF demonstrated cryogenic liquid DT layer ICF implosions$^{\mathrm{1}}$. Unlike DT ice layer implosions, DT liquid layer designs can operate with low-to-moderate convergence ratio (12 \textless CR \textless 25), with a hot spot formed mostly or entirely from DT mass originating within the central, spherical volume of DT vapor$^{\mathrm{2}}$. With reduced CR, hot spot formation is expected to have improved robustness to instabilities and asymmetries$^{\mathrm{3}}$. In addition, the hot spot pressure (Pr) required for self-heating is reduced if the hot spot radius (R$_{\mathrm{hs}})$ is increased (Pr $\alpha $ R$_{\mathrm{hs}}^{\mathrm{-1}})$. With a reduction in the hot spot Pr requirement, the implosion velocity and fuel adiabat requirements are relaxed. On the other hand, with larger hot spot size, the hot spot energy requirement for self-heating (E$_{\mathrm{hs}})$ is increased (E$_{\mathrm{hs}} \quad \alpha $ R$_{\mathrm{hs}}^{\mathrm{2}})$, and the required capsule-absorbed energy is increased. In this presentation, we will discuss the hot spot energy, hot spot pressure, cold fuel adiabat, and capsule-absorbed energy requirements for achieving self-heating and propagating burn with hot spot CR\textless 20. $^{\mathrm{1}}$R. E. Olson \textit{et al}., Phys. Rev. Lett. \textbf{117}, 245001 (2016). $^{\mathrm{2}}$R. E. Olson and R. J. Leeper, Phys. Plasmas \textbf{20}, 092705 (2013). $^{\mathrm{3}}$B. M. Haines \textit{et al}., Phys. Plasmas \textbf{24} (2017).   

Capsule physics comparison of different ablators for NIF implosion designs

Indirect drive implosion experiments on the Naitonal Ignition Facility (NIF) have now tested three different ablator materials: glow discharge polymer (GDP) plastic, high density carbon (HDC), and beryllium. How do these different ablator choices compare in current and future implosion experiments on NIF? What are the relative advantages and disadvantages of each? This talk compares these different ablator options in capsule-only simulations of current NIF experiments and proposed future designs. The simulations compare the impact of the capsule fill tube, support tent, and interface surface roughness for each case, as well as all perturbations in combination. According to the simulations, each ablator is impacted by the various perturbation sources differently, and each material poses unique challenges in the pursuit of ignition.   

Subscale HDC implosions driven at high radiation temperature using advanced hohlraums

h $-abstract-$\backslash $pard Implosions using HDC ablators have received increased attention because of shorter pulse length and can access higher implosion velocity than CH ablators.$^{\mathrm{1}}$ Recent HDC midscale (979 m radius) implosion experiments have achieved DT neutron yields of 1.5e16.$^{\mathrm{2}}$ Our 2D simulations show that subscale (890 m radius) HDC capsules can achieve robust high-yield performance if driven at high enough radiation temperature 330 eV, because the penalty for less fuel mass can be offset by higher implosion velocity. To achieve 330 eV will likely require the use of innovative hohlraum concepts, e.g., subscale rugby-shaped hohlraum$^{\mathrm{3}}$ using 1.3 MJ of laser energy without incurring a risk of high laser backscatter. Radiation symmetry is currently under study. Confidence in our modeling of HDC implosions is high in part because our 2D modeling of recent HDC implosions experiments show good agreement with data. $\backslash $pard$\backslash $pard1. D. D.-M. Ho \textit{et al}., Journal of Phys.: Conf. Series, \textbf{717}(1), 012023 (2016). $\backslash $pard2. L. Berzak Hopkins, S. Le Pape \textit{et al}., this conference and paper forthcoming. . P. Amendt, D. D-M. Ho and O. S. Jones, Phys. Plasmas \textbf{22}, 040703 (2015). $\backslash $pard-/abstract-$\backslash $\tex   

Beryllium implosion experiments at high case-to-capsule ratio on the National Ignition Facility

Using beryllium as an ablator material has several potential advantages for inertial fusion because of its low opacity and thus higher ablation rate. This could enable novel designs taking advantage of the reduced ablation-front growth rate, or operating at lower radiation temperature. To investigate the integrated performance of beryllium implosions, we conducted a tuning campaign leading into DT layered implosions using a 900um radius capsule in a 6.72mm diameter hohlraum (case-to-capsule ratio CCR=3.7); the large CCR enables direct study of the 1-D implosion performance. The tuning campaign shots demonstrate excellent control over the shock timing and implosion symmetry at this CCR. Performance data from the DT experiments will also be discussed.   

Quantifying design trade-offs of beryllium targets on NIF

An important determinant of target performance is implosion kinetic energy, which scales with the capsule size. The maximum achievable performance for a given laser is thus related to the largest capsule that can be imploded symmetrically, constrained by drive uniformity. A limiting factor for symmetric radiation drive is the ratio of hohlraum to capsule radii, or case-to-capsule ratio (CCR). For a fixed laser energy, a larger hohlraum allows for driving bigger capsules symmetrically at the cost of reduced peak radiation temperature ($T_r$). Beryllium ablators may thus allow for unique target design trade-offs due to their higher ablation efficiency at lower $T_r$. By utilizing larger hohlraum sizes than most modern NIF designs, beryllium capsules thus have the potential to operate in unique regions of the target design parameter space. We present design simulations of beryllium targets with a large $\mathrm{CCR}=4.3 \sim 3.7$. These are scaled surrogates of large hohlraum low Tr beryllium targets, with the goal of quantifying symmetry tunability as a function of CCR.   

Relationship between symmetry and laser pulse shape in low-fill hohlraums at the National Ignition Facility

Typically in indirect-drive inertial confinement fusion (ICF) hohlraums cryogenic helium gas fill is used to impede the motion of the hohlraum wall plasma as it is driven by the laser pulse. A fill of \textasciitilde 1 mg/cc He has been used to significantly suppress wall motion in ICF hohlraums at the National Ignition Facility (NIF); however, this level of fill also causes laser-plasma instabilities (LPI) which result in hot electrons, time-dependent symmetry swings and reduction in drive due to increased backscatter. There are currently no adequate models for these phenomena in codes used to simulate integrated ICF experiments. A better compromise is a fill in the range of 0.3\textasciitilde 0.6 mg/cc, which has been shown to provide some reduction in wall motion without incurring significant LPI effects[1]. The wall motion in these low-fill hohlraums and the resulting effect on symmetry due to absorption of the inner cone beams by the outer cone plasma can be simulated with some degree of accuracy with the hydrodynamics and inverse Bremsstrahlung models in ICF codes. We describe a series of beryllium capsule implosions in 0.3 mg/cc He fill hohlraums that illustrate the effect of pulse shape on implosion symmetry in the ``low-fill'' regime. In particular, we find the shape of the beginning or ``foot'' of the pulse has significant leverage over the final symmetry of the stagnated implosion. [1]G.N. Hall et al., Phys Plasmas \textbf{24}, 052706 (2017)   

Imaging and spectroscopy of copper dopant migration of indirectly driven Beryllium capsule implosion on the National Ignition Facility.

Using beryllium, as an ablator material for indirectly driven inertial fusion, requires the use of a Copper dopant to block preheat from the hohlraum M-band radiation. However, due to the microstructure and imperfections of the capsule, some of the copper may be injected into the core of the implosion, affecting the yield and performance. Alternatively, the copper dopant may blow into the ablated plasma affecting the hohlraum performance as well. We will present some of data on time integrated imaging of the copper dopant into the core of the capsule using either the 2-dimensional multiple monochromatic imaging of the implosion, as well as the 1D spectrally resolved imaging of the copper dopant emission. In either case we found that the copper did migrate to the hot core, while fewer copper ions ablated into the hohlraum.   

Spatially resolved x-ray fluorescence spectroscopy of beryllium capsule implosions at the NIF

Beryllium ablators used in indirectly driven inertial confinement fusion implosions are doped with copper to prevent preheat of the cryogenic hydrogen fuel. Here, we present analysis of spatially resolved copper K-$\alpha$ fluorescence spectra from the beryllium ablator layer. It has been shown that K-$\alpha$ fluorescence spectroscopy can be used to measure plasma conditions of partially ionized dopants in high energy density systems [1]. In these experiments, K-shell vacancies in the copper dopant are created by the hotspot emission at stagnation, resulting in K-shell fluorescence at bang time. Spatially resolved copper K-$\alpha$ emission spectra are compared to atomic kinetics and radiation code simulations to infer density and temperature profiles. [1] M. J. MacDonald et al, J. Appl. Phys. 120, 125901 (2016).   

Demonstration of high coupling efficiency to Al capsule in rugby hohlraum on NIF

A new design of the double-shell approach predicts a high coupling efficiency from the hohlraum to the capsule, with \textasciitilde 700 kJ in the capsule instead of \textasciitilde 200kJ in the conventional low-Z single-shell scheme, improving prospects of double-shell performance. A recent experiment on NIF has evaluated a first step toward this goal of energy coupling using 0.7x subscale Al capsule, Au rugby hohlraum and 1MJ drive. A shell velocity of 150 $\mu $m/ns was measured, DANTE peak temperature of 255 eV was measured, and shell kinetic energy of 36 kJ was inferred using a rocket model, all close to predictions and consistent with 330kJ of total energy coupled to the capsule. Data analysis and more results from subsequent experiments will be presented. In the next step, an additional 2x increase of total coupled energy up to \textasciitilde 700 kJ is projected for full-scale 2-MJ drive in U Rugby hohlraum.