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The outer surface of the high-Z inner shell in the double shell configuration of inertial confinement fusion experiments experiences Rayleigh–Taylor instability growth during the implosion process due to inverted density and pressure gradients between a highly compressed foam interstitial layer and the accelerating dense inner shell. Graded density layers have long been known to reduce instability growth rates. In this study, we employ high-fidelity radiation hydrodynamic simulations to demonstrate this improved stability when grading beryllium into tungsten. We first characterize the response to L-band preheat of these layers using a newly calibrated radiation drive. While graded layer capsules suffer reduced performance (here, measured as DD neutron yield from a CD foam fuel) in 1D simulations due to reduced kinetic energy coupling and reduced fuel compression, they suffer less of a performance drop when 2D instabilities are accounted for. With the improved stability of graded layers, we explore the performance of capsules with larger fuel radii and thinner shells as a preliminary study to find new designs in which graded layers produce the highest yields. [ABSTRACT FROM AUTHOR]

Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs. .sup.1-3), during the so-called epoch of reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs.sup.4-7, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies.sup.8-12 and companions of rare quasars.sup.13,14. These studies conclude that the contribution of dust-obscured galaxies to the cosmic star formation rate density at z > 6 is sub-dominant. Recent ALMA and Spitzer observations have identified a more abundant, less extreme population of obscured galaxies at z = 3-6 (refs. .sup.15,16). However, this population has not been confirmed in the reionization epoch so far. Here, we report the discovery of two dust-obscured star-forming galaxies at z = 6.6813 [plus or minus] 0.0005 and z = 7.3521 [plus or minus] 0.0005. These objects are not detected in existing rest-frame UV data and were discovered only through their far-infrared [C ii] lines and dust continuum emission as companions to typical UV-luminous galaxies at the same redshift. The two galaxies exhibit lower infrared luminosities and star-formation rates than extreme starbursts, in line with typical star-forming galaxies at z [almost equal to] 7. This population of heavily dust-obscured galaxies appears to contribute 10-25% to the z > 6 cosmic star formation rate density. Two serendipitously detected dust-obscured galaxies are reported at z = 6.7 and 7.4, with estimates that such galaxies provide an additional 10-25% contribution to the total star formation rate density at z > 6., Author(s): Y. Fudamoto [sup.1] [sup.2] [sup.3] , P. A. Oesch [sup.1] [sup.4] , S. Schouws [sup.5] , M. Stefanon [sup.5] , R. Smit [sup.6] , R. J. Bouwens [sup.5] , [...]

Double shell inertial confinement fusion is a concept for achieving robust thermonuclear burn that uses dense metal shells to compress deuterium-tritium (DT) fuel to fusion conditions. Double shell implosions are typically indirectly driven and involve a target that consists of a low-Z ablator, a foam layer, and a high-Z pusher surrounding the DT fuel. The goal of the campaign is to achieve a volumetric burn as radiation losses from the DT fuel are trapped by the opaque high-Z shell. The overall performance of double shell implosions relies on the efficient collisional transfer of kinetic energy between layers. The efficiency of this transfer (and therefore the overall performance of a given implosion) is degraded by the presence of low-mode asymmetries. P2 asymmetries are often observed in spatially resolved 2D radiographs of nominal double shell implosions. This work discusses three such experiments: one with an oblate P2 asymmetry, one with a prolate P2 asymmetry, and one with an approximate spherical symmetry. After performing a shape analysis of the oblate and prolate implosions to quantify asymmetries, these experimental results were compared with the results of hydrodynamic simulations for the two experiments. Differences between the experiment and simulation were then used to design an approximately spherical implosion by altering the incident laser cone fraction. Radiographs from the experiment that implemented the modified cone fraction show evidence of an implosion that is approximately spherical until bang time. This design is intended to serve as a point design for future studies that will seek to optimize various aspects of the double shell target. [ABSTRACT FROM AUTHOR]

An interface is Rayleigh–Taylor (RT) unstable when acceleration pushes a less dense material into a more dense one, and the growth of the instability is governed partly by the Atwood number gradient. Double-shell inertial confinement fusion capsules have a foam spacer layer pushing on an inner capsule composed of a beryllium tamper and high-Z inner shell, and so have RT unstable interfaces that require benchmarking. To this end, the results of a planar shock experiment with beryllium/tungsten targets are presented. One target had the normal bilayer construction of beryllium and tungsten in two distinct layers; the second target had the beryllium grading into tungsten with a quasi-exponential profile, motivated by the potential for reduced RT growth with the gradient profile. Simulations mimic the shock profiles for both targets and match the shock velocity to within 5%. These results validate the ability of our simulations to model double-shell capsules with bilayer or graded layer Be/W inner shells, which are needed to design future experiments at the National Ignition Facility. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/Submillimeter Array (ALMA) [C  ii ] and ∼158 |$\rm \mu m$| continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; L IR = |$1.5^{+0.8}_{-0.5} \times 10^{12}$| L⊙) at z  = 7.31, spectroscopically confirmed by the Reionization Era Bright Emission Line Survey (REBELS) ALMA Large Programme. REBELS-25 has a significant stellar mass of |$M_{*} = \mbox{$ 8^{+4}_{-2} \times 10^{9} $}{} ~\mbox{M$_\odot $}{}$|⁠. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR |$= \mbox{$199^{+101}_{-63}$}{} ~ \mbox{M$_\odot $}~ \mbox{${\rm yr}$}^{-1}$|⁠. This is about four times the SFR estimated from an extrapolated main sequence. We also infer a [C  ii ]-based molecular gas mass of |$M_{{\rm H}_{2}} = \mbox{$5.1^{+5.1}_{-2.6} \times 10^{10}$}{} ~\mbox{M$_\odot $}{}$|⁠ , implying a molecular gas depletion time of |$t_{\rm depl, {\rm H}_{2}} = \mbox{$0.3^{+0.3}_{-0.2} $}{}$| Gyr. We observe a [C  ii ] velocity gradient consistent with disc rotation, but given the current resolution we cannot rule out a more complex velocity structure such as a merger. The spectrum exhibits excess [C  ii ] emission at large positive velocities (∼500 km s−1), which we interpret as either a merging companion or an outflow. In the outflow scenario, we derive a lower limit of the mass outflow rate of 200 |$\mbox{M$_\odot $}~ \mbox{${\rm yr}$}^{-1}$|⁠ , which is consistent with expectations for a star-formation-driven outflow. Given its large stellar mass, SFR, and molecular gas reservoir ∼700 Myr after the big bang, we explore the future evolution of REBELS-25. Considering a simple, conservative model assuming an exponentially declining star formation history, constant star formation efficiency, and no additional gas inflow, we find that REBELS-25 has the potential to evolve into a galaxy consistent with the properties of high-mass quiescent galaxies recently observed at z ∼ 4. [ABSTRACT FROM AUTHOR]

Bayesian optimization has shown promise for the design optimization of inertial confinement fusion targets. Specifically, in Vazirani et al. [Phys. Plasmas 28, 122709 (2021)], optimal designs for double shell capsules with graded inner shells were identified using one-dimensional xRAGE simulation yield calculations. While the machine learning models were able to accurately learn and predict one-dimensional simulation target performance, using simulations with higher fidelity would improve design optimization and better match with the expected experimental performance. However, higher fidelity physics modeling, i.e., two-dimensional xRAGE simulations, requires significantly larger computational time/cost, usually at least an order of magnitude, in comparison with one-dimensional simulations. This study presents a multi-fidelity Bayesian optimization, in which the machine learning model leverages low-fidelity (one-dimensional xRAGE) and high-fidelity (two-dimensional xRAGE) simulations to more accurately predict "pre-shot" target performance with respect to the expected experimental performance. By building a multi-fidelity Bayesian optimization framework coupled with xRAGE, the low-fidelity and high-fidelity simulations are able to inform one another, such that we have: (1) improved physics modeling in comparison with using low-fidelity simulations alone, (2) reduced computational time/cost in comparison with using high-fidelity simulations alone, and (3) more confidence in the expected performance of optimized targets during real-world experiments. In the future, we plan to use this robust multi-fidelity Bayesian optimization methodology to expedite the design of graded inner shells further and eventually full capsules as a part of the current double shell campaign at the National Ignition Facility. [ABSTRACT FROM AUTHOR]

Laser plasma instabilities (LPI) reduce driver-target coupling, alter implosion symmetry, and therefore can fundamentally limit fusion performance in inertial confinement fusion (ICF). Developing a predictive modeling capability for LPI effects can critically advance the success of the field. We perform vector particle-in-cell simulations of multi-speckled laser beams undergoing stimulated Raman scattering (SRS) at various densities and intensities relevant to mainly indirectly driven and a subset of parameter space for directly driven ICF systems, focusing on the regimes with intensities above the onset of electron trapping. Based on the wavenumber of the SRS daughter electron plasma wave, we identify several regions with underpinning SRS saturation physics: the electron-trapping dominated region with intermediate k λ D values, the strong Landau damping region at larger k λ D values, and the region where the Langmuir decay instability arises at lower k λ D values. We develop a nonlinear SRS reflectivity model that features the base trapping-dominated scaling of (k λ D) − 4 and its modifications. Electron trapping deforms the initialized electron distribution functions, and we have developed a new δf-Gaussian-mixture algorithm for an accurate characterization of the trapped hot electron population. With this SRS hot electron description, we construct a nonlinear energy deposition model and a hot electron source model—based on a modified Manley–Rowe relation—suitable for including SRS effects as a sub-grid module in a high-fidelity ICF design code. [ABSTRACT FROM AUTHOR]

The nonlinear physics of cross-beam energy transfer (CBET) for multi-speckled laser beams is examined using large-scale particle-in-cell simulations for a range of laser and plasma conditions relevant to indirect-drive inertial confinement fusion (ICF) experiments. The time-dependent growth and saturation of CBET involve complex, nonlinear ion and electron dynamics, including ion trapping-induced enhancement and detuning, ion acoustic wave (IAW) nonlinearity, oblique forward stimulated Raman scattering (FSRS), and backward stimulated Brillouin scattering (BSBS) in a CBET-amplified seed beam. Ion-trapping-induced detuning of CBET is captured in the kinetic linear response by a new δf-Gaussian-mixture algorithm, enabling an accurate characterization of trapping-induced non-Maxwellian distributions. Ion trapping induces nonlinear processes, such as changes to the IAW dispersion and nonlinearities (e.g., bowing and self-focusing), which, together with pump depletion, FSRS, and BSBS, determine the time-dependent nature and level of CBET gain as the system approaches a steady state. Using VPIC simulations at intensities at and above the onset threshold for ion trapping and the insight from the time-dependent saturation analyses, we construct a nonlinear CBET model from local laser and plasma conditions that predicts the CBET gain and the energy deposition into the plasma. This model is intended to provide a more accurate, physics-based description of CBET saturation over a wide range of conditions encountered in ICF hohlraums compared with linear CBET gain models with ad hoc saturation clamps often used in laser ray-based methods in multi-physics codes. [ABSTRACT FROM AUTHOR]

Before the end of the Epoch of Reionization, the Hydrogen in the Universe was predominantly neutral. This leads to a strong attenuation of Ly α lines of z ≳ 6 galaxies in the intergalactic medium. Nevertheless, Ly α has been detected up to very high redshifts (z ∼ 9) for several especially UV luminous galaxies. Here, we test to what extent the galaxy's local environment might impact the Ly α transmission of such sources. We present an analysis of dedicated Hubble Space Telescope (HST) imaging in the CANDELS/EGS field to search for fainter neighbours around three of the most UV luminous and most distant spectroscopically confirmed Ly α emitters: EGS-zs8-1, EGS-zs8-2, and EGSY-z8p7 at z spec = 7.73, 7.48, and 8.68, respectively. We combine the multiwavelength HST imaging with Spitzer data to reliably select z ∼ 7–9 galaxies around the central, UV-luminous sources. In all cases, we find a clear enhancement of neighbouring galaxies compared to the expected number in a blank field (by a factor ∼3–9×). Our analysis thus reveals ubiquitous overdensities around luminous Ly α emitting sources in the heart of the cosmic reionization epoch. We show that our results are in excellent agreement with expectations from the Dragons simulation, confirming the theoretical prediction that the first ionized bubbles preferentially formed in overdense regions. While three UV luminous galaxies already have spectroscopic redshifts, the majority of the remaining fainter, surrounding sources are yet to be confirmed via spectroscopy. JWST follow-up observations of the neighbouring galaxies identified here will thus be needed to confirm their physical association and to map out the ionized regions produced by these sources. [ABSTRACT FROM AUTHOR]

Hohlraums are hollow cylindrical cavities with high-Z material walls used to convert laser energy into uniform x-ray radiation drives for inertial confinement fusion capsule implosions and high energy density physics experiments. Credible computational modeling of hohlraums requires detailed modeling and coupling of laser physics, hydrodynamics, radiation transport, heat transport, and atomic physics. We report on improvements to Los Alamos National Laboratory's xRAGE radiation-hydrodynamics code in order to enable hohlraum modeling. xRAGE's Eulerian hydrodynamics and adaptive mesh refinement make it uniquely well suited to study the impacts of multiscale features in hohlraums. In order to provide confidence in this new modeling capability, we demonstrate xRAGE's ability to produce reasonable agreement with data from several benchmark hohlraum experiments. We also use xRAGE to perform integrated simulations of a recent layered high density carbon capsule implosion on the National Ignition Facility in order to evaluate the potential impacts of the capsule support tent, mixed cell conductivity methodologies, plasma transport, and cross-beam energy transfer (XBT). We find that XBT, seeded by plasma flows in the laser entrance hole (LEH), causes a slight decrease in energy coupling to the capsule and that all of these impact the symmetry of the x-ray drive such that they have an appreciable impact on the capsule implosion shape. [ABSTRACT FROM AUTHOR]

ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature T d remains mostly unconstrained due to the few available FIR continuum data at redshift |$z$| > 5. This introduces large uncertainties in several properties of high- |$z$| galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous [C  |$\scriptstyle \rm II$| ] 158-μm line and underlying dust continuum measurements, we derive T d in the continuum and [C  |$\scriptstyle \rm II$| ] detected |$z$| ≈ 7 galaxies in the ALMA Large Project REBELS sample. We find 39 < T d < 58 K, and dust masses in the narrow range M d = (0.9−3.6) × 107 M⊙. These results allow us to extend for the first time the reported T d(⁠|$z$|⁠) relation into the Epoch of Reionization. We produce a new physical model that explains the increasing T d(⁠|$z$|⁠) trend with the decrease of gas depletion time, t dep = M g/SFR, induced by the higher cosmological accretion rate at early times; this hypothesis yields T d ∝ (1 + |$z$|⁠)0.4. The model also explains the observed T d scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, T d only depends on the gas column density (metallicity), |$T_{\rm d} \propto N_{\rm H}^{1/6}$| (T d ∝  Z −1/6). REBELS galaxies are on average relatively transparent, with effective gas column densities around N H ≃ (0.03−1) × 1021 cm−2. We predict that other high- |$z$| galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated T d ≫ 60 K, are significantly obscured, low-metallicity systems. In fact, T d is higher in metal-poor systems due to their smaller dust content, which for fixed L IR results in warmer temperatures. [ABSTRACT FROM AUTHOR]

We analyse FIR dust continuum measurements for 14 galaxies (redshift z ≈ 7) in the ALMA Reionization Era Bright Emission Line Survey (REBELS) Large Program to derive their physical properties. Our model uses three input data, i.e. (a) the UV spectral slope, β, (b) the observed UV continuum flux at 1500 Å, F 1500, (c) the observed continuum flux at |$\approx 158\, \mu$| m, F 158, and considers Milky Way (MW) and SMC extinction curves, along with different dust geometries. We find that REBELS galaxies have 28−90.5 per cent of their star formation obscured; the total (UV+IR) star formation rates are in the range |$31.5 \lt {\rm SFR}/({\rm M}_\odot \, {\rm yr}^{-1}) \lt 129.5$|⁠. The sample-averaged dust mass and temperature are |$(1.3\pm 1.1)\times 10^7 \, \mathrm{M}_\odot$| and 52 ± 11 K, respectively. However, in some galaxies dust is particularly abundant (REBELS-14, |$M^{\prime }_{\rm d} \approx 3.4 \times 10^7 \, \mathrm{M}_\odot$|⁠), or hot (REBELS-18, |$T^{\prime }_{\rm d} \approx 67$| K). The dust distribution is compact (<0.3 kpc for 70 per cent of the galaxies). The inferred dust yield per supernova is |$0.1 \le y_{\rm d}/\, \mathrm{M}_\odot \le 3.3$|⁠ , with 70 per cent of the galaxies requiring |$y_{\rm d} \lt 0.25 \, \mathrm{M}_\odot$|⁠. Three galaxies (REBELS-12, 14, 39) require |$y_{\rm d} \gt 1 \, \mathrm{M}_\odot$|⁠ , which is likely inconsistent with pure SN production, and might require dust growth via accretion of heavy elements from the interstellar medium. With the SFR predicted by the model and a MW extinction curve, REBELS galaxies detected in [C  ii ] nicely follow the local L CII−SFR relation, and are approximately located on the Kennicutt–Schmidt relation. The sample-averaged gas depletion time is |$0.11\, y_{\rm P}^{-2}$| Gyr, where y P is the ratio of the gas-to-stellar distribution radius. For some systems, a solution simultaneously matching the observed (β, F 1500, F 158) values cannot be found. This occurs when the index Im  = (F 158/ F 1500)/(β − βint), where βint is the intrinsic UV slope, exceeds |$I_m^{*}\approx 1120$| for an MW curve. For these objects, we argue that the FIR and UV emitting regions are not co-spatial, questioning the use of the IRX–β relation. [ABSTRACT FROM AUTHOR]

Advances in machine learning provide the ability to leverage data from expensive simulations of high-energy-density experiments to significantly cut down on computational time and costs associated with the search for optimal target designs. This study presents an application of cutting-edge Bayesian optimization methods to the one-dimensional (1D) design optimization of double shell graded layer targets for inertial confinement fusion experiments. This investigation attempts to reduce hydrodynamic instabilities while retaining high yields for future NIF experiments. Machine learning methods can use predictive physics simulations to identify graded layer designs from within the vast design space that demonstrate high predicted performance, including novel designs with high uncertainty in performance that may hold unexpected promise. By applying machine learning tools to the simulation design, we map the trade-off between 1D yield and instability, specifically isolating parameter ranges, which maintain high performance while showing significantly improved Rayleigh–Taylor stability over the point design. The groundwork laid in this study will be a useful design tool for future NIF experiments with graded layer targets. [ABSTRACT FROM AUTHOR]