Session GO11: ICF: Neutron Diagnostics and Pushered Single Shell

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Analysis of primary DD and DT neutron spectra in ICF experiments usually relies on a hydrodynamic approximation in which it is assumed that the reacting ions have a Maxwellian distribution of velocities. This greatly simplifies the kinematics of reacting ions and means the primary spectra shapes are functions of ion temperature and fluid velocity only. The richness of spectra shapes can then be ascribed to temporal and spatial variations of these quantities. However, the validity of the hydrodynamic approximation remains uncertain, particularly for implosions with a low areal density. In this work, this is investigated using kinetic ion simulations of capsule implosions. An algorithm is developed to calculate the primary neutron spectrum for arbitrary ion distributions that are fully-resolved in phase space. The algorithm is deterministic and includes the effects of the differential cross-section, allowing the sensitivity of the primary spectra to the ion distribution function to be accurately studied. The algorithm is implemented in the multi-ion VFP code iFP and implosions are simulated at a range of areal densities in a spherically-symmetric geometry. The neutron spectra from the fully kinetic calculation are compared with spectra calculated using the hydrodynamic approximation.   

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The LANL neutron imaging team has been providing neutron images of inertial confinement fusion implosions at the National Ignition Facility (NIF) for almost 10 years. The neutron imaging system has recently evolved into a comprehensive nuclear imaging suite including multiple neutron imagers, in addition to x-ray and gamma imaging. A powerful diagnostic tool, the neutron image precisely determines the region of fusion fuel actively undergoing fusion, providing information on the fuel shape at ignition and associated performance limitations. Since the delivery of the first primary neutron images at NIF in 2011, the nuclear diagnostic has grown from one instrument to three dedicated lines of sight with comprehensive imaging power. The diagnostic suite now encompasses: three orthogonal primary images allowing for limited-view 3D hotspot tomography, two down-scattered images facilitating fuel density reconstruction, colinear x-ray imaging for direct neutron/ x-ray comparison studies, and colinear gamma imaging to provide the remaining ablator shape. We will show recent results that highlight the diagnostic power of the comprehensive nuclear imaging suite.   

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The effects of mode 1 drive asymmetries on the moments of the primary DT and DD fusion neutron spectra have been well studied both theoretically and experimentally. However, these measurements only probe the conditions of the fusing plasma within the hotspot. The drive asymmetries also create a corresponding asymmetry in the dense fuel conditions. Recent theoretical work [1] has shown that the velocity and temperature of the dense fuel can be extracted from the back scattered neutron spectra. Additionally, the areal density asymmetry will affect the shape of the down scattered neutron spectrum. In this talk, we will discuss the development of an analysis of the scattered neutron spectrum over an extended energy range. This novel analysis will be used to simultaneously extract both the direction and amplitude of the areal density asymmetry and the hydrodynamic conditions of the dense fuel. This will aid in quantifying the reduced confinement time and residual kinetic energy of the implosion at stagnation. References [1] A. J. Crilly, et al., Physics of Plasmas 27, 012701 (2020)   

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A novel neutron detector is being implemented for 1-ps time-resolved measurement of the neutron-production history and spectrum at the MTW and LFEX short-pulse lasers, and the OMEGA and NIF long-pulse lasers. This new diagnostic is based on the Pockels-effect concept and will provide at least 10\texttimes better time resolution than current diagnostics. In the context of short-pulse-laser experiments, this diagnostic will provide a fundamental understanding of the production period and spectrum of the emitted neutrons, which is essential in the areas of life sciences, condensed-matter physics, fusion-material studies, and stellar nucleosynthesis of heavy elements. In the context of ICF, this diagnostic will provide critical experimental information about the dynamics of the hot-spot formation, fuel assembly and alpha heating in an ICF implosion, as encoded in the time evolution of neutron yield and hot-spot ion temperature. This work was supported in part by the U.S. DOE, the MIT/NNSA CoE, and LLE.   

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Analysis of neutron time-of-flight (nTOF) data in cryogenic DT experiments is often conducted over a relatively small range of neutron energies corresponding to DD and DT primary neutrons and therefore often employs instrument response functions (IRFs) corresponding to monoenergetic 2.45- or 14.03-MeV neutrons. Some analyses, such as those focused on nTOF data corresponding to nuclear physics experiments as well as recent analyses of areal densities in cryogenic experiments, span a much wider range of neutron energies. These analyses require the use of an energy-dependent IRF for accurate treatment of the data. This work describes construction of the energy-dependent IRF and application of this IRF in a forward fit via matrix multiplication. The process is detailed for the xylene nTOFs used at the Omega Laser Facility. To demonstrate the effects of the energy-dependent IRF on inferred quantities of interest, the forward-fit analysis is applied to synthetic data using the energy-dependent IRF as well as monoenergetic IRFs. 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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Neutron time-of-flight diagnostic techniques have been employed for over 50 years of fusion experiments. In 2016 Munro[1] documented the non-thermal contributions to the neutron fusion peak spectral variance leading to the result that a single sample along a single line-of-sight is insufficient to determine T$_{\mathrm{ion}}$ in all but the most unlikely of conditions. This ambiguity is endemic to all systems of rapid fuel assemby due to incomplete conversion of directed kinetic energy into heat. Use of multiple lines-of-sight and measurements can reduce the ambiguity and uncertainty to a level sufficient to achieve a desired task. Presented is a strategy for a new nToF suite for the Z-facility at Sandia National Laboratory, in Albuqueque, NM. This new suite leverages technologies from the National Ignition Facility at Lawrence Livermore National Laboratory and deploys these in a geometric configuration that enables T$_{\mathrm{ion}}$ measurement using either D$_{\mathrm{2}}$ or DT fusing plasmas. The strategy and logic for the design, along with estimates of precision will be presented. [1] ``Impact of temperature-velocity distribution on fusion peak shape'', D. H. Munro et al. Phys. Plasmas, 24, 056301, (2017).   

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An integrated neutron diagnostics suite is developed for the 3 MA MJOLNIR dense plasma focus (DPF) that emphasizes collective functionality over individual measurements. Instead of merely maximizing yield, the unique mission space motivates new success metrics such as radiation pulse width and spot size, necessitating more intricate diagnostics. Radiation diagnostics diagnose neutron flux and timing: A PMT-coupled scintillator 6 m away and SIPM-coupled scintillators 2 m away provide information about the neutron flux envelope. Beryllium, Bromine, and Yttrium activation detectors monitor the yield, the scattering contribution, and the dose. An in-vessel diamond radiation detector monitors X-ray signals for multiple pinches which can spread the neutron flux envelope. A 16-frame, 3 ns exposure framing camera monitors pinch stagnation and breakup time and location. These data are synchronized with the neutron information to calculate neutron fluence. Light gates along the anode corroborate the rundown velocity and will be used in the future to synchronize diagnostics with the neutron arrival time by monitoring positions along the run-in. Prepared by LLNL under Contract DE-AC52-07NA27344.   

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Neutrons from D-T fusion reactions in the hot spot of inertial confinement fusion (ICF) implosions elastically scatter deuterons and tritons as they transit the surrounding dense fuel layer. The energy of these ``knock-on'' charged particles depends on their angle of scattering and ranges up to 12.5 (10.6) MeV for deuterons (tritons), respectively. The most-energetic particles are forward scattered and indicate the shape of the hot spot, whereas lower-energy particles are made by sidescattering and ranging in the fuel and contain information about the distribution of mass around the hot spot. We report on the design and first results of a knock-on deuteron imager (KoDI) to record spectrally resolved images of these scattered particles from laser-direct-drive cryogenic ICF implosions on the OMEGA laser. The source is imaged using a multi-penumbral aperture with 35\texttimes magnification. A CR-39 detector provides energy resolution in the resulting images, from which the symmetry of the hot spot and of areal density in the hemisphere of the converged fuel layer facing the camera are inferred. Image plates record a co-aligned \textgreater 10 keV x-ray image of the implosion using the same aperture. Results from OMEGA cryogenic implosions are presented.   

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Knock-on deuteron imaging is a new diagnostic technique that images the source distribution of deuterons that are elastically scattered by primary neutrons generated in a D-T gas-filled ICF implosion. By energy-resolving these deuteron images, obtained in three near-orthogonal directions, information about the morphology of the hot spot and high-density D-T fuel can be obtained. Experimental demonstrations of the concept were successfully conducted at the OMEGA laser facility. In these experiments, several absolutely co-aligned penumbral knock-on deuteron images, with different energy bands, were obtained in the three different directions, from which the hot spot and surrounding high-density shell could be reconstructed. Reconstruction techniques have been developed that will be presented here, along with examples of resulting images. These results demonstrate that this technique works well.   

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Hydrodynamic instabilities and mix are of major interest in the field of High Energy Density physics. Graded High-Z Pushered Single Shells (PSS) capsules are predicted to achieve high neutron yields via core tamping and radiation trapping that increase confinement time and reduce bremmstrahlung losses compared to conventional inertial confinement fusion implosions. Adverse effects of core cooling by fuel gas-pusher mix are mitigated using a pure thin Be anti-mix layer. Experiments were performed recently on the National Ignition Facility (NIF) using novel capsules with graded Be/Cr metal shells that reach 50{\%} Cr concentration in the pusher and have a graded transition to pure Be ablator to mitigate hydrodynamic instabilities. Be/Cr capsules are hydrodynamic surrogates for Be/Mo capsules that are predicted to achieve fusion yields similar to ICF implosions using 6 mg/cc gas DT fuel. First Be/Cr experiments are focused on shock timing, early symmetry and implosion shape tuning. The opaque capsule implosions are characterized by neutron imaging and dedicated hard x-ray (\textless 30 keV) radiography using NIF's Advanced Radiographic Capability (ARC). The results of these first experiments and effects on mix models will be discussed.   

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The Pushered Single Shell concept uses a mid- or high-Z pusher at the fuel-ablator interface together with a graded density profile to enhance confinement time, thus making the ignition threshold more accessible. This concept is being studied at the National Ignition Facility~using a~hohlraum driven with 192 laser beams~[Dewald, et al. Phys. Plasmas (2019)].~Initial experiments with novel opaque Be/Cr graded capsules (50{\%} Cr in the pusher layer) utilize the Advanced Radiographic Capability (ARC) short pulse laser [J. M. Di Nicola, et al. Proc. of SPIE (2015)]~to measure the symmetry of the implosion near peak velocity. Two Au 25\textmu m diameter wire backlighters placed inside a two-dimensional plastic parabolic structure [R Tommasini et. al., to be submitted] are illuminated by the ARC beamlets to generate two inflight hard x-ray bremmstrahlung (30-70 keV) radiographs recorded on image plates and on the AXIS diagnostic [G. N. Hall et al Rev. Sci. Instrum. (2014)]. Backlighter performance and implosion symmetry results from ARC will be presented.   

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Criteria for determining the onset of thermonuclear ignition typically balance the rate of self-heating due to alpha particle deposition against losses from heat conduction, radiation and expansion against the confining mass. ~With the recognition that ICF stagnation is in fact a dynamic process, one can consider these criteria as a balance between the rate of net energy production and the disassembly rate of the hot spot. ~This is commonly formulated as a relation between the product of stagnation pressure and confinement time (P-tau) and the hot spot temperature, a form that highlights the role of the stagnated mass on the confinement time.~ A variation on standard laboratory ICF designs, the Pushered Single Shell (PSS)[1,2] takes advantage of enhanced confinement (tau) to relax the requirements on hot spot pressure and temperature in order to achieve robust self-heating. ~We describe current designs for PSS implosions on the NIF and illustrate their advantage in terms of ignition criteria with comparisons against non-pushered implosion designs. [1]D.C. Wilson et al., Fusion Technology \textbf{38}, pp. 16-21, July 2000 [2]D.D. Ho et al., APS-DPP 2018 PO6.00011   

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Beryllium ablators with inner layer doped with increasing Mo concentration towards the center can increase ρR with the benefit of radiation trapping. Configurations of this type of “high-ρR PSS” implosions with acceptable RT growth were reported.1 Based on this concept, we report new developments showing promises for achieving ignition that cannot be otherwise obtained using conventional approaches. (1) High-ρR and slow disassembly of the hotspot allow the use of DT liquid foam with high gas fill (1.5 – 2.0 mg/cc) and low-convergence ratio (< 20) implosions to deliver a few MJ in 2D. (2) Large-scale capsule (1500 m) gives high yield at lower implosion velocities since ρR increases with scale. However, YoC is lower which is apparently caused by the higher growth factor for large scales. Methods to improve the YoC will be discussed. (3) The high-ρR PSS allows the use of nominal-scale Be capsule with realistic drive to achieve ignition and high-yield in 2D while configurations using conventional Be ablators cannot. 1. D. Ho et al., APS-DPP PO6.00011(2018) and BO4.00010 (2019).   

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To achieve ignition, Inertial Confinement Fusion (ICF) implosions must converge symmetrically as any asymmetries can have deleterious effects on the implosion performance. As such, the measurement of asymmetries has long been an important tool for assessing and improving the performance of ICF implosions. Traditionally this has been accomplished through a combination of neutron and x-ray imaging and neutron down-scatter measurements. On the NIF, surrogate D$_{\mathrm{2}}$ and D$^{\mathrm{3}}$He filled implosions are often used, for reasons of practicality, in place of fully integrated DT layered experiments. These experiments lack the performance required for neutron imaging and must rely entirely on x-ray imaging to measure implosion asymmetries. However, directional neutron spectra from secondary DT reactions also encode information about implosion symmetry, opening the door for alternative and complementary asymmetry measurements. In this work, we demonstrate this capability using four neutron time of flight spectrometers on the NIF. This work was supported in part by the U.S. DOE, the MIT/NNSA CoE, and LLNL.