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DOE OSTI2018

Application of compressed magnetic fields to the ignition and thermonuclear burn of inertial confinement fusion targets

Application of axial seed magnetic fields in the range 20-100 T that compress to greater than 10,000 T (100 MG) under typical NIF implosion conditions may significantly relax the conditions required for ignition and propagating burn in NIF ignition targets that are degraded by hydrodynamic instabilities. Such magnetic fields can: (a) permit the recovery of ignition, or at least significant alpha particle heating, in submarginal NIF targets that would otherwise fail because of adverse hydrodynamic instability growth, (b) permit the attainment of ignition in conventional cryogenic layered solid-DT targets redesigned to operate under reduced drive conditions, (c) permit the attainment of volumetric ignition in simpler, room-temperature single-shell DT gas capsules, and (d) ameliorate adverse hohlraum plasma conditions during laser drive and capsule compression. In general, an applied magnetic field should always improve the ignition condition for any NIF ignition target design.

47 OTHER INSTRUMENTATION↗

DOE OSTI2022

Hohlraum used as a single turn solenoid to generate seed magnetic field for inertial confinement fusion

Perkins, Lindsay John↗

DOE OSTI2023

Hohlraum used as a single turn solenoid to generate seed magnetic field for inertial confinement fusion

Perkins, Lindsay John↗

DOE OSTI2018

Implementation of a 1-2 keV point-projection x-ray spectrometer on the National Ignition Facility

A point-projection soft X-ray Opacity Spectrometer (OpSpec) has been implemented to measure X-ray spectra from ~1 to 2 keV on the National Ignition Facility (NIF). Measurement of such soft X-rays with open-aperture point-projection detectors is challenging because only very thin filters may be used to shield the detector from the hostile environment. OpSpec diffracts X-rays from 540 to 2100 eV off a potassium (or rubidium) acid phthalate (KAP or RbAP) crystal onto either image plates or, most recently, X-ray films. A “sacrificial front filter” strategy is used to prevent crystal damage, while 2 or 3 rear filters protect the data. Since May 2017, OpSpec has been recording X-ray transmission data for iron-magnesium plasmas on the NIF, at “Anchor 1” plasma conditions (temperature ~150 eV, density ~7 × 10 21 e - /cm 3 ). Upgrades improved OpSpec’s performance on 6 NIF shots in August and December 2017, with reduced backgrounds and 100% data return using filter stacks as thin as 2.9 μm (total). We conclude that photometric noise is beginning to meet requirements, and further work will reduce systematic errors.

47 OTHER INSTRUMENTATION↗

DOE OSTI2016

A direct-drive exploding-pusher implosion as the first step in development of a monoenergetic charged-particle backlighting platforn at the National Ignition Facility

A thin-glass-shell, D 3 He-filled exploding-pusher inertial confinement fusion implosion at the National Ignition Facility (NIF) has been demonstrated as a proton source that serves as a promising first step toward development of a monoenergetic proton, alpha, and triton backlighting platform at the NIF. Among the key measurements, the D3He-proton emission on this experiment (shot N121128) has been well-characterized spectrally, temporally, and in terms of emission isotropy, revealing a highly monoenergetic (ΔE/E~4%) and isotropic source (~3% proton fluence variation and ~0.5% proton energy variation). On a similar shot (N130129, with D 2 fill), the DD-proton spectrum has been obtained as well, illustrating that monoenergetic protons of multiple energies may be utilized in a single experiment. In conclusion, these results, and experiments on OMEGA, point toward future steps in the development of a precision, monoenergetic proton, alpha, and triton source that can readily be implemented at the NIF for backlighting a broad range of high energy density physics (HEDP) experiments in which fields and flows are manifest, and also utilized for studies of stopping power in warm dense matter and in classical plasmas.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

DOE OSTI2025

Nitrogen fixation in Pampinifervens , a new species-rich genus of Aquificaceae that inhabits a wide pH range in terrestrial hot springs

Members of the Aquificota play important roles in hydrogen, sulfur, and carbon cycles in geothermal systems as ubiquitous and abundant chemolithoautotrophs; however, their roles in the nitrogen cycle are poorly defined. Here, we show that Aquificaceae isolate T-2 T from Tengchong, China, fixes nitrogen based on growth with dinitrogen as the sole nitrogen source and incorporation of 15 N atoms when grown with 15 N 2 . We further show evidence that suggests its nitrogen fixation (nif) genes are transcribed in situ in a nearby spring. We propose the name Pampinifervens diazotrophicum gen. et sp. nov., with strain T-2 T (= JCM 35475 T = DSM 116324 T ) as the nomenclatural type. Its close relative designated Pampinifervens florentissimum sp. nov. T-8 T (= CGMC 1.5214 T = JCM 33569 T ) does not fix nitrogen but respires nitrate. Analysis of 104 metagenome-assembled genomes from 84 metagenomes allowed us to circumscribe the genus Pampinifervens and identify at least 16 sympatric Pampinifervens species that dominate the Aquificaceae populations in diverse hot springs in southwest China, with distinct species globally. Analysis of these Pampinifervens genomes revealed variable presence of nif and denitrification genes and frequent gene loss and horizontal transfer. Genes for pH homeostasis were also present in Pampinifervens genomes, with a proliferation in species from Tengchong, consistent with the wide pH growth range of both Pampinifervens isolates and the prevalence of Pampinifervens in both alkaline and acidic springs. The dynamic evolution of nif and denitrification genes, along with pH homeostasis genes, may facilitate the diversification of Pampinifervens into diverse springs with different nitrogen availability.

Aquificae↗

DOE OSTI2017

Conceptual design of initial opacity experiments on the national ignition facility

Accurate models of X-ray absorption and re-emission in partly stripped ions are necessary to calculate the structure of stars, the performance of hohlraums for inertial confinement fusion and many other systems in high-energy-density plasma physics. Despite theoretical progress, a persistent discrepancy exists with recent experiments at the Sandia Z facility studying iron in conditions characteristic of the solar radiative–convective transition region. The increased iron opacity measured at Z could help resolve a longstanding issue with the standard solar model, but requires a radical departure for opacity theory. To replicate the Z measurements, an opacity experiment has been designed for the National Facility (NIF). The design uses established techniques scaled to NIF. A laser-heated hohlraum will produce X-ray-heated uniform iron plasmas in local thermodynamic equilibrium (LTE) at temperatures ${\geqslant}150$ eV and electron densities ${\geqslant}7\times 10^{21}~\text{cm}^{-3}$ . The iron will be probed using continuum X-rays emitted in a ${\sim}200$ ps, ${\sim}200~\unicode[STIX]{x03BC}\text{m}$ diameter source from a 2 mm diameter polystyrene (CH) capsule implosion. In this design, $2/3$ of the NIF beams deliver 500 kJ to the ${\sim}6$ mm diameter hohlraum, and the remaining $1/3$ directly drive the CH capsule with 200 kJ. Calculations indicate this capsule backlighter should outshine the iron sample, delivering a point-projection transmission opacity measurement to a time-integrated X-ray spectrometer viewing down the hohlraum axis. Preliminary experiments to develop the backlighter and hohlraum are underway, informing simulated measurements to guide the final design.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2014

Measurements of fuel and ablator ρR in Symmetry-Capsule implosions with the Magnetic Recoil neutron Spectrometer (MRS) on the National Ignition Facility

The Magnetic Recoil neutron Spectrometer (MRS) on the National Ignition Facility (NIF) measures the neutron spectrum in the energy range of 4–20 MeV. This paper describes MRS measurements of DT-fuel and CH-ablator ρR in DT gas-filled symmetry-capsule implosions at the NIF. DT-fuel ρR's of 80–140 mg/cm{sup 2} and CH-ablator ρR's of 400–680 mg/cm{sup 2} are inferred from MRS data. The measurements were facilitated by an improved correction of neutron-induced background in the low-energy part of the MRS spectrum. This work demonstrates the accurate utilization of the complete MRS-measured neutron spectrum for diagnosing NIF DT implosions.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2014

Hydrodynamic instabilities in beryllium targets for the National Ignition Facility

Beryllium ablators offer higher ablation velocity, rate, and pressure than their carbon-based counterparts, with the potential to increase the probability of achieving ignition at the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)]. We present here a detailed hydrodynamic stability analysis of low (NIF Revision 6.1) and high adiabat NIF beryllium target designs. Our targets are optimized to fully utilize the advantages of beryllium in order to suppress the growth of hydrodynamic instabilities. This results in an implosion that resists breakup of the capsule, and simultaneously minimizes the amount of ablator material mixed into the fuel. We quantify the improvement in stability of beryllium targets relative to plastic ones, and show that a low adiabat beryllium capsule can be at least as stable at the ablation front as a high adiabat plastic target.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2015

Simulation study of 3–5 keV x-ray conversion efficiency from Ar K-shell vs. Ag L-shell targets on the National Ignition Facility laser

Tailored, high-flux, multi-keV x-ray sources are desirable for studying x-ray interactions with matter for various civilian, space and military applications. For this study, we focus on designing an efficient laser-driven non-local thermodynamic equilibrium 3–5 keV x-ray source from photon-energy-matched Ar K-shell and Ag L-shell targets at sub-critical densities (∼n{sub c}/10) to ensure supersonic, volumetric laser heating with minimal losses to kinetic energy, thermal x rays and laser-plasma instabilities. Using HYDRA, a multi-dimensional, arbitrary Lagrangian-Eulerian, radiation-hydrodynamics code, we performed a parameter study by varying initial target density and laser parameters for each material using conditions readily achievable on the National Ignition Facility (NIF) laser. We employ a model, benchmarked against Kr data collected on the NIF, that uses flux-limited Lee-More thermal conductivity and multi-group implicit Monte-Carlo photonics with non-local thermodynamic equilibrium, detailed super-configuration accounting opacities from CRETIN, an atomic-kinetics code. While the highest power laser configurations produced the largest x-ray yields, we report that the peak simulated laser to 3–5 keV x-ray conversion efficiencies of 17.7% and 36.4% for Ar and Ag, respectively, occurred at lower powers between ∼100–150 TW. For identical initial target densities and laser illumination, the Ag L-shell is observed to have ≳10× higher emissivity per ion per deposited laser energy than the Ar K-shell. Although such low-density Ag targets have not yet been demonstrated, simulations of targets fabricated using atomic layer deposition of Ag on silica aerogels (∼20% by atomic fraction) suggest similar performance to atomically pure metal foams and that either fabrication technique may be worth pursuing for an efficient 3–5 keV x-ray source on NIF.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2016

The design of the optical Thomson scattering diagnostic for the National Ignition Facility [The preliminary design of the optical Thomson scattering diagnostic for the National Ignition Facility]

Here, the National Ignition Facility (NIF) is a 192 laser beam facility designed to support the Stockpile Stewardship, High Energy Density and Inertial Confinement Fusion (ICF) programs. We report on the design of an Optical Thomson Scattering (OTS) diagnostic that has the potential to transform the community’s understanding of NIF hohlraum physics by providing first principle, local, time-resolved measurements of under-dense plasma conditions. The system design allows operation with different probe laser wavelengths by manual selection of the appropriate beam splitter and gratings before the shot. A deep-UV probe beam (λ 0 -210 nm) will be used to optimize the scattered signal for plasma densities of 5 × 10 20 electrons/cm 3 while a 3ω probe will be used for experiments investigating lower density plasmas of 1 × 10 19 electrons/cm 3 . We report the phase I design of a two phase design strategy. Phase I includes the OTS telescope, spectrometer, and streak camera; these will be used to assess the background levels at NIF. Phase II will include the design and installation of a probe laser.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2016

Using VISAR to assess the M-band isotropy in hohlraums

In laser based radiation flow experiments, drive variability can often overwhelm the physics sensitivity that one seeks to quantify. Hohlraums can help by providing a more symmetrized, Planckian-like source. However, at higher temperatures, the hohlraum’s actual emission can deviate significantly from a truly blackbody, Lambertian source. At the National Ignition Facility (NIF), Dante provides the best quantification of hohlraum output. Unfortunately, limited diagnostic access coupled with NIF’s natural symmetry does not allow for Dante measurements at more than two angles. As part of the CEPHEUS campaign on NIF, proof-of-principle experiments to better quantify the gold M-band isotropy were conducted. These experiments positioned beryllium/aluminum mirrors at differing angles, offset from the hohlraum. Filtering removes the thermal emission of the hohlraum and the remaining M-band radiation is preferentially absorbed in the aluminum layer. The subsequent hydrodynamic motion is measured via VISAR. Although indirect, this M-band measurement can be made at any angle.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2017

The potential of imposed magnetic fields for enhancing ignition probability and fusion energy yield in indirect-drive inertial confinement fusion

We examine the potential that imposed magnetic fields of tens of Tesla that increase to greater than 10 kT (100 MGauss) under implosion compression may relax the conditions required for ignition and propagating burn in indirect-drive inertial confinement fusion (ICF) targets. This may allow the attainment of ignition, or at least significant fusion energy yields, in presently performing ICF targets on the National Ignition Facility (NIF) that today are sub-marginal for thermonuclear burn through adverse hydrodynamic conditions at stagnation [Doeppner et al., Phys. Rev. Lett. 115, 055001 (2015)]. Results of detailed two-dimensional 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 potential stabilization of higher-mode Rayleigh-Taylor instabilities. Optimum initial applied fields are found to be around 50 T. Given that the full plasma structure at capsule stagnation may be governed by three-dimensional resistive magneto-hydrodynamics, the formation of closed magnetic field lines might further augment ignition prospects. Experiments are now required to further assess the potential of applied magnetic fields to ICF ignition and burn on NIF.

Perkins, L. J.↗

DOE OSTI2018

Development and modeling of a polar-direct-drive exploding pusher platform at the National Ignition Facility

High-intensity laser facilities, such as the National Ignition Facility (NIF), allow the experimental investigation of plasmas under extreme, high-energy-density conditions. Motivated by validating models for collisional heat-transfer processes in high-energy-density plasmas, we have developed an exploding pusher platform for use at the NIF in the polar-direct-drive configuration. The baseline design employs a 3 mm-diameter capsule, an 18 μm-thick CH ablator, and Ar-doped D 2 gas to achieve several keV electron-ion temperature separations with relatively low convergence ratios. In an initial series of shots at the NIF—N160920–003, -005, and N160921–001—the ratio of the laser intensity at different polar angles was varied to optimize the symmetry of the implosion. In this work, we summarize experimental results from the shot series and present pre- and post-shot analysis. Although the polar-direct-drive configuration is inherently asymmetric, we successfully tuned a post-shot 1D model to a set of key implosion performance metrics. The post-shot model has proven effective for extrapolating capsule performance to higher incident laser drive. Overall, the simplicity of the platform and the efficacy of the post-shot 1D model make the polar-direct-drive exploding pusher platform attractive for a variety of applications beyond the originally targeted study of collisional processes in high-energy-density plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2018

The dilation aided single–line–of–sight x–ray camera for the National Ignition Facility: Characterization and fielding

Crystal x–ray imaging is commonly used in inertial confinement fusion and laser-plasma interaction applications as it has advantages compared to pinhole imaging, such as higher signal throughput, better achievable spatial resolution, and chromatic selection. Yet, currently used x–ray detectors are only able to obtain a single time resolved image per crystal. The dilation aided single–line–of–sight x–ray camera described here was designed for the National Ignition Facility (NIF) and combines two recent diagnostic developments, the pulse dilation principle used in the dilation x-ray imager and a ns–scale multi-frame camera that uses a hold and readout circuit for each pixel. This enables multiple images to be taken from a single–line–of–sight with high spatial and temporal resolution. Presently, the instrument can record two single–line–of–sight images with spatial and temporal resolution of 35 μm and down to 35 ps, respectively, with a planned upgrade doubling the number of images to four. Here we present the dilation aided single–line–of–sight camera for the NIF, including the x–ray characterization measurements obtained at the COMET laser, as well as the results from the initial timing shot on the NIF.

47 OTHER INSTRUMENTATION↗

DOE OSTI2018

On the system stability and calibration of the image plate/scanner system for plasma diagnosis at the National Ignition Facility

At the National Ignition Facility (NIF), storage phosphor image plates (IP) are used extensively for recording x-rays, charged particles, and neutrons. For x-ray imaging and spectroscopy, absolute and relative calibrations are important for extracting plasma information from the diagnostics. We use Fuji MS, SR, and TR image plates that have been cut to fit custom diagnostic envelopes. The image plates are scanned on a General Electric FLA 7000 IP flying spot scanner. Calibrations for sensitivity, spatial scale, and temperature dependent fade are applied. During a set of recent calibrations, we noticed large shifts in the absolute calibration of the image plate system. The possible source of these shifts is discussed. We discuss scanner stability and a method for calibration. We discuss the fade and temperature effects of the image plates and how this correction is applied within the NIF environment. Here, we also compare our NIF GE FLA 7000 IP scanner with a new General Electric Amersham Typhoon IP scanner.

47 OTHER INSTRUMENTATION↗

DOE OSTI2019

The Crystal Backlighter Imager: A spherically bent crystal imager for radiography on the National Ignition Facility

The Crystal Backlighter Imager (CBI) is a quasi-monochromatic, near-normal incidence, spherically bent crystal imager developed for the National Ignition Facility (NIF), which will allow inertial confinement fusion capsule implosions to be radiographed close to stagnation. This is not possible using the standard pinhole-based area-backlighter configuration, as the self-emission from the capsule hotspot overwhelms the backlighter signal in the final stages of the implosion. The CBI mitigates the broadband self-emission from the capsule hot spot by using the extremely narrow bandwidth inherent to near-normal-incidence Bragg diffraction. Implementing a backlighter system based on near-normal reflection in the NIF chamber presents unique challenges, requiring the CBI to adopt novel engineering and operational strategies. The CBI currently operates with an 11.6 keV backlighter, making it the highest energy radiography diagnostic based on spherically bent crystals to date. For a given velocity, Doppler shift is proportional to the emitted photon energy. At 11.6 keV, the ablation velocity of the backlighter plasma results in a Doppler shift that is significant compared to the bandwidth of the instrument and the width of the atomic line, requiring that the shift be measured to high accuracy and the optics aligned accordingly to compensate. Experiments will be presented that used the CBI itself to measure the backlighter Doppler shift to an accuracy of better than 1 eV. These experiments also measured the spatial resolution of CBI radiographs at 7.0 μm, close to theoretical predictions. Results will be presented from an experiment in which the CBI radiographed a capsule implosion driven by a 1 MJ NIF laser pulse, demonstrating a significant (>100) improvement in the backlighter to self-emission ratio compared to the pinhole-based area-backlighter configuration.

47 OTHER INSTRUMENTATION↗

DOE OSTI2019

Investigation and modeling of optics damage in high-power laser systems caused by light backscattered in plasma at the target

High-power laser facilities may be restricted in operations by the risk of damage to optics due to the backscattering of laser light from the intended target. A portion of this backscattered light can re-enter the beam line and at sufficient fluence may damage (burn) optical apparatus. Here, the observed cumulative burn pattern on mirrors at the National Ignition Facility (NIF) due to stimulated Brillouin scattering (SBS) at the target is explained using detailed simulations. Our methodology involves using a measurement of a phase plate profile installed at the NIF to provide the laser input to a three-dimensional simulation of laser-plasma interaction in a target typical of that fielded at the NIF. Light scattered by SBS at the target is then followed back to the final aperture of the beam line, back through the phase plate, and to a mirror where damage typically occurs. We find that mirror damage patterns are largely dictated by the phase imprint of the phase plate on the returning SBS light. Our simulations show that the instantaneous SBS signal at the mirror varies on a picosecond time scale and is also highly modulated in space. Temporal averaging of the simulated SBS light pattern on the mirror reveals a pattern in close agreement with the experimentally observed damage. Finally, understanding these burn patterns (distribution, modulation depth) may lead to future phase plate designs that limit damage throughout the optics assembly of high-powered laser facilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗