Session GO07: ICF: Nuclear Diagnostics

Inferring shell kinematics during fusion burn through precise gamma ray timing

Near peak compression, inertial confinement fusion implosions release both deuterium–tritium (DT) fusion gamma rays and neutron induced gamma rays from carbon from of the remaining ablator shell. The gamma reaction history diagnostic makes a time resolved measurement of both. Across many recent implosions, the carbon gamma ray peak arrives systematically 10 ps later compared to DT fusion burn. The timing shift is consistent with the carbon areal density increasing throughout the peak of the fusion burn, implying that the carbon portion of the capsule continues to converge. A model finds that the observed timing shift is consistent with a 4π averaged carbon ablator inward velocity of 80 μm/ns for the contemporary National Ignition Facility implosions. The timing shift is possibly related to the energy balance of the implosion, with the expectation that a high performing, igniting capsule would see the carbon gamma rays arrive before the DT fusion peak.   

Understanding background in gas Cherenkov detectors and future detector development

Gas Cherenkov Detectors (GCD) provide direct measurements of fusion reaction rate and other processes in inertial confinement fusion (ICF). The Cherenkov mechanism utilized in GCDs is inherently fast. However, detectors have been limited in temporal resolution by even state-of-the art photomultiplier tubes (PMT). The pulse dilation – photomultiplier tube (PD-PMT) fielded on the GCD-3 at the National Ignition Facility (NIF) provides unprecedented temporal resolution of 10 ps in gamma reaction history measurements in inertial confinement fusion experiments. First measurements of DT fusion gammas are showing slowly decaying tail feature corrupting the data quality. Detailed understanding of the cause and mitigation strategies for the tail are needed for both improving the data quality and designing future gas Cherenkov detectors for high background environments such as NIF or the Z-machine at Sandia National Laboratory.   

Not Participating

The joint LANL/LLNL nuclear imaging team has transformed the 2D neutron imaging technique for inertial confinement fusion implosions at the National Ignition Facility into a full 3D fuel characterization diagnostic suite over the past decade. With advances in detector technology, as well as sophisticated analysis codes, the nuclear imaging team now delivers physics information on the three-dimensional hot spot shape, cold fuel density, and remaining ablator position. Three neutron imaging lines-of-sight allow for three-dimensional hot spot tomography, and cold fuel density reconstruction – showing performance-limiting shape asymmetries. Applying our three-dimensional limited-view tomography technique to X-ray images visualizes contaminant mix into the hot spot. Gamma imaging interpretation aided by simulations shows the remaining ablator position, and potential jetting of remaining mass into the fuel. A decade of progress has grown the nuclear imaging effort from a single diagnostic into a powerful diagnostic suite that provides comprehensive information on the fuel assembly at stagnation.   

Gamma Imaging of Inertial Confinement Fusion Implosions

The nuclear imaging team has recently acquired the first gamma images of inertial confinement fusion implosions at the National Ignition Facility. The gamma image provides crucial information in our quest to fully characterize the inertial confinement fuel assembly at stagnation. Gamma imaging is a powerful technique that visualizes both gamma radiation emitted directly in Deuterium-Tritium (DT) fusion reactions as well as gammas produced when DT fusion neutrons scatter on Carbon atoms in the remaining ablator of the fuel capsule. The resulting image provides valuable information on the location of the remaining ablator location and mass, as well as potential contamination of the hot spot. We present recent gamma imaging data, analysis techniques, and interpretations illustrating the capabilities of this novel diagnostic. See N. Hoffman's talk at this conference for associated model calculations.   

Using GEANT4 to investigate performance of the NIF Gamma Imaging System

The National Ignition Facility at Lawrence Livermore National Laboratory is aiming to achieve the self-heated and sustained fusion of deuterium and tritium using over 2 MJ of laser energy. The required pressure and temperatures are generated at the centre of an imploded DT capsule surrounded by a thin ablator material, typically high-density carbon. During the burn history, neutrons stream out through the remnants of the ablated carbon shell and undergo 12C(n,n’γ)12C, resulting in the emission of 4.4 MeV gammas. Imaging of the carbon 4.4 gammas has been achieved through a modification to the NIF neutron imaging system, enabling determination of the spatial distribution and volumetric size of the remaining shell. Large size and significant asymmetries in the gamma images are expected to correlate with poor compression and reduced fusion yield, making the gamma imaging system a critical tool for diagnosing implosion performance. In this work we discuss how the GEANT4 Monte Carlo code has been used to investigate key parameters of the system, such as the spatially and energy-dependent point spread functions and scintillator sensitivity, and how synthetic images can be used to stress the likelihood maximization image analysis routines.   

Novel neutron imaging aperture for ICF

The passive imaging of high-energy x-rays and neutrons is a useful diagnostic in laser-driven fusion as well as laboratory astrophysics experiments which aim to study small samples of transient electron-positron plasmas.

Here we demonstrate a coded aperture with scatter and partial attenuation included, which we have dubbed a ‘CASPA’.  We compare CASPAs to the more common method of pinhole imaging, confirming the well-known throughput increase of coded apertures, and show that the decoding algorithm relaxes the need for a thick substrate.  This is demonstrated with a 511 keV x-ray source through ray-tracing and Geant4 simulations to show how partial attenuation of the source by the CASPA allows for a superior signal to noise ratio with respect to an equivalent standard pinhole system. 

We explore the potential applications in ICF, using NIF-like sources and geometries in Geant4 simulations to discuss the viability of this technique for measurements of implosion asymmetry.  Finally, after suggesting CASPA corrections to previous coded aperture maximum-likelihood expectation-maximization models, we discuss the reduction in substrate thickness of a CASPA based system in comparison to current NIF architecture, and its impact on imaging capabilities.   

Suprathermal ion distributions in burning plasmas on the NIF

For hydrodynamic plasmas described by Maxwellian ion distributions, a well-described relationship exists between the mean (<E>) and variance of the neutron energy spectrum¹. On the National Ignition Facility (NIF), the addition of the quartz Cherenkov neutron time-of-flight (nToF) spectrometers and a fifth nToF line-of-sight has extended the ability to measure <E> to a precision of 5 parts in 100,000 for high yield DT shots. Observations using this new capability on NIF shots with the highest neutron yield have revealed a significant deviation from the hydrodynamic relationship and disagreement with radiation-hydrodynamic simulations.

Experimental results are described and compared to predicted neutron spectra for a range of candidate ion distributions. This supports the claim that suprathermal ions are required to increase the mean of the neutron kinetic energy faster than the variance. The result suggests that the significant a-heating present in a burning plasma is sufficient to generate non-Maxwellian DT ion distributions. LLNL-ABS-824483

¹. D. H. Munro, Nucl. Fusion 56 036001 (2016) https://doi.org/10.1088/0029-5515/56/3/036001   

Diagnosing Ion Distributions using Primary Neutron Spectra

Recent measurements of primary neutron spectra on the OMEGA and NIF facilities have demonstrated unexpected features - the ratio of first to second spectral moments was higher than expected for hydrodynamic plasma in which the reacting ions have a Maxwellian distribution. These observations occurred in different plasma regimes. For NIF, the observation occurred in layered cryogenic DT experiments in which a burning plasma was achieved. For OMEGA, the observation occurred in low pressure, gas-filled, glass capsules.

This work investigates the feasibility of using primary neutron spectra measurements to identify the form of ion distribution functions in ICF plasmas. The iFP code is used to simulate OMEGA experiments. This shows ions accelerated by the shock create non-Maxwellian ion distributions that produce the observed spectra. These distributions are anisotropic in velocity space. However, the observed ratio of moments could also be obtained from isotropic, non-Maxwellian distribution functions. In contrast, the observed ratio of spectral moments from the NIF experiments can only be obtained from anisotropic, non-Maxwellian ion distribution functions. This result provides a stricter test for hypotheses seeking to explain the physical cause of the observed spectra.   

Inference of Isotropic and Anisotropic Flow in Laser Direct-Drive Cryogenic DT Implosions on OMEGA

Cryogenic DT cryogenic targets are imploded on the OMEGA Laser System using laser direct drive inertial confinement fusion (ICF).  Efficient conversion of the shell kinetic energy to the internal hot-spot energy is an essential requirement in ICF fusion implosions.  The spectral moments of the neutron distribution from a fusing deuterium-tritium (DT) plasma are used to interpret the spectral moments of the separate DT and DD fusion reactants^1,2.  Broadening of the second moment not attributed to the thermal temperature of the fusing plasma is a signature of isotropic and anisotropic flow within the hot-spot fuel assembly.  The inferred isotropic flow is used to calculate the residual kinetic energy fraction (f_RKE) and the influence of isotropic flows on the yield degradation will be investigated.  This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Fusion Neutron Energy Spectrum Measurements in Kinetic Plasmas

The first and second moments of the primary deuterium-tritium (DT) and deuterium–deuterium (DD) fusion neutron energy spectrum generated by thermonuclear plasmas with apparent ion temperatures between 2 and 18 keV have been measured on the OMEGA 60 Laser. For the low-temperature, more-hydrodynamic-like plasmas, both the DT and DD neutron energy spectrum measurements are consistent with predictions from hydrodynamic plasma models, which assume the plasma has a Maxwellian ion velocity distribution. For the high-temperature, more-kinetic-like plasmas, the DD neutron energy spectrum measurements are inconsistent with the hydrodynamic plasma model predictions. Post-shot Vlasov–Fokker–Plank (VFP) simulations predict the presence of a bimodal ion velocity distribution near peak neutron production in the high-temperature experiments, which causes the neutron energy spectrum emitted from these plasmas to deviate from the hydrodynamic, Maxwellian plasma predictions. The bimodal distribution is caused by a large counter-streaming ion population, which forms as the diffuse shock front converges in these implosions. The DD neutron energy spectrum measurements are consistent with the VFP simulation results and provide evidence of a non-Maxwellian ion velocity distribution in these thermonuclear plasmas. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

3D morphology of the hot spot and shell of warm ICF implosions at OMEGA

The 3D morphology of the hot spot and surrounding high-density shell of warm implosions at OMEGA have been measured using knock-on deuteron imaging (KODI). Data is presented from experiments in which several absolutely co-aligned penumbral images of knock-on deuterons were obtained in three nearly orthogonal directions. The KODI technique utilizes the fact that neutrons from DT-fusion reactions in the central hot spot of an ICF implosion elastically scatter deuterons as they transit the surrounding material. The energy of these knock-on deuterons depends on the scattering angle, which means that energy-resolving knock-on deuteron images provides information about different parts of the implosion. The most energetic knock-on deuterons are forward-scattered and probe the shape of the central hot spot, whereas lower-energy knock-on deuterons are made by side-scattering or ranging in the shell and carry information about the dense shell around the hot spot. These measurements provide new insights into the causes and effects of low-mode asymmetries in ICF implosions.   

Determining fuel areal density distributions from nuclear scattering signatures

It is important to measure the magnitude and uniformity of the compressed fuel as it significantly affects the performance of inertial confinement fusion (ICF) implosions [1]. We describe a Monte-Carlo based method to determine fuel areal density distributions from measurements of the yields of 14 MeV neutrons and the ratio of down scattered neutrons to 14 MeV neutrons (dsr) along a number of lines-of-site (LOS) around the NIF target chamber. The compressed fuel is modeled as a spherical shell whose directionally dependent areal density is specified by the 9 coefficients of a spherical harmonic series summed to L = 2. The Monte-Carlo fires 14 MeV neutrons through the shell to determine the yields and dsr’s at each detector position and is repeatedly run under the supervision of a Levenberg-Marquardt minimizer that adjusts the 9 coefficients to find the areal density that produces the best match to the measurements. We present areal density distributions for selected ICF implosions at NIF and a series of 2-D HYDRA simulations.

 

[1] O. A. Hurricane, et al., Phys. Plasmas, 24, 092706 (2017)   

MixIT: designing a 1D ion temperature imager for ICF

The MixIT project will establish the first steps toward a new and transformative ICF diagnostic. By coupling neutron time-of-flight and neutron imaging techniques, the project will demonstrate the first ever, direct measurement of the 1D temperature profile of an ICF plasma. From the temperature information that this diagnostic provides, a quantitative measurement of the mix of performance-limiting material into the hot spot can be extracted and used to inform and optimize the design of ICF targets and drive conditions.  This talk will focus on key aspects of MixIT instrument design and design considerations.   

MixIT: First spatially-resolved ion temperature measurement in ICF

Ion temperature is a key plasma variable and performance indicator in inertial confinement fusion. However, due to technological and physics limitations, all ion temperature measurements to date have been spatially integrated over the target assembly as viewed from a single line of sight. We present the methodology for, and results from, a proof-of-concept spatially-resolved ion temperature measurement conducted on ICF implosions, and path to a spatially-resolved, quantitative mix diagnostic.