Session PO04: ICF: Measurement and Diagnostic Techniques

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The neutron imaging lines of sight currently in place at the National Ignition Facility were all designed in the pre-ignition era. With shots now exceeding DT neutron yields of 10¹⁷ on a regular basis, the high flux and larger burn regions may necessitate new pinhole designs. This talk will focus on the framework of a new pinhole design model geared towards higher yield shots and the impacts of metrology errors on the interpretation of results.   

Chasing symmetry: 3D burn volumes of recent ignition shots at the National Ignition Facility

The transition of inertial confinement fusion into the ignition era is setting new challenges for diagnostics but allowing researchers a glimpse into never before accessed physics regimes. The nuclear imaging diagnostic suite at the National Ignition Facility opens a window into the shape characteristics of fusion implosions and fully characterizes the assembly at stagnation. This diagnostic has significantly advanced our understanding of performance limitations in ICF and is now providing images of the first successful ignition shots. This talk will look at hot spot shapes of recent high yield shots at NIF.   

Source Localization via Neutron Imaging and Machine Learning

Neutron imaging systems are important diagnostic tools for characterizing the physics of inertial confinement fusion reactions at the National Ignition Facility (NIF). In particular, neutron images give diagnostic information on the size, symmetry, and shape of the fusion hot spot and surrounding cold fuel. Images are formed via collection of neutron flux from the source using a system of aperture arrays and scintillator-based detectors. Currently reconstruction of fusion source geometry from collected neutron images is accomplished by solving a Maximum Likelihood Expectation (MLE) problem via Expectation Maximization (EM). To accurately reconstruct the source geometry, the source location must be known. However, the knowledge of a priori source locations is limited by machine precision and experimental controls.

 

To overcome these physical limitations, the source location must be determined computationally from neutron image data. In this talk we show how to use neural networks to predict source locations in inertial confinement fusion reactions.  We provide experimental demonstrations of our methods on both non-noisy and noisy data. We also discuss the effects of training data sizes and the importance of choosing training data that covers the entire range of possible source locations.   

Neutron imaging estimation of nuclear radchem diagnostics

Nuclear reactions induced by neutrons are utilized ubiquitously as diagnostics in fusion plasma experiments. By doping the contained plasma (or its boundary) with elements which are susceptible to various neutron-induced reactions, one can extract information about the burn by measuring the isotopes produced in threshold reactions such as (n,2n) neutron knock-out, and zero-threshold reactions such as (n,γ) neutron capture. At the National Ignition Facility (NIF), three-dimensional reconstructions of the (time-integrated) fusion burn are obtained using neutron imaging. By utilizing this data, we can estimate local densities and neutron fluences, thereby allowing us to further estimate the total number of induced reactions for each of the diagnostics of interest. We can then compare these estimates with the experimentally obtained isotope counts. In this presentation, we will illustrate this technique as well as provide explicit comparison with experimentally measured reaction products.   

Imaging DD fusion neutrons using a coded aperture of sub-mean-free-path thickness

Neutron imaging is a useful tool with applications ranging from NIF implosion geometry to material radiography for industrial and security applications.  Pinhole arrays typically used for such imaging require thick substrates to obtain good contrast along with a small pinhole diameter to obtain good resolution, resulting in pinholes that have large aspect ratios. This leads to expensive pinhole arrays that have small solid angles and are difficult to align.  We have previously proposed a coded aperture with scatter and partial attenuation (CASPA), that relaxes the requirement of thick substrates for good image contrast.

Here, we show results using a 3.5 mm resolution 6.8 mm thick tungsten CASPA to image the neutron source of a dense plasma focus (DPF).  The LLNL MJOLNIR DPF produces roughly 10¹¹ DD (2.5 MeV) fusion neutrons that have a mean-free-path of 23.8 mm through tungsten, which are typically imaged with pinholes through approximately 110 mm thick substrates to generate absolute imaging contrast.  Using conventional cross-correlation decoding techniques, our reconstruction was successful in imaging a source diameter of 5.2 ± 1.8 mm, limited by the aperture cell size.  We discuss how to optimise the CASPA imaging system to reduce noise in the reconstructed image, how to improve the imaging resolution, and the limitations on achievable resolution.   

Using Real-Time Nuclear Activation Detectors for Measuring DD-Neutron Yields on the NIF

The NIF has 48 Real-Time Nuclear Activation Detectors (RTNADs) distributed around the target chamber capable of measuring DT neutron yields with high precision. In this work we extend this functionality to DD neutrons using a nuclear reaction that occurs in the detector's scintillator material. The corresponding decay of the activated material has a very short half life of 5 seconds which necessitates rapid data collection immediately following an experiment. In this regime, deadtime can be very high (>50%) adding significant uncertainty in the measurement. To combat this, we have developed a deadtime model that can self-consistently describe the measured data. Initial results show reasonable agreement with DD neutron yields from neutron time-of-flight spectrometers (nTOFs).   

Optically multiplexed neutron time-of-flight technique for ion temperature imaging in inertial confinement fusion

Laser-driven inertial confinement fusion experiments are susceptible to contamination of the hotspot by high-Z material during compression that impedes thermonuclear burn. This mix can be quantified using the ion temperature distribution of the implosion given the predicted deviation of the contaminant, but current neutron time-of-flight (nTOF) diagnostics only measure the spatially integrated temperature. This project probes the feasibility of using time multiplexed optical fibers to record nTOF spectra from segmented thin scintillators, which could be used to collect spatially resolved ion temperature measurements with a single photomultiplier tube (PMT). A prototype detector was constructed using 20 multiplexed fiber channels coupled to a monolithic EJ-262 plastic scintillator and a Photek PMT210. nTOF pulses were successfully measured through all channels at the expected time separations on experiments at the OMEGA laser. Design methodology, material selection, experimental results, and data analysis are discussed. Next steps are now aimed at applying this technique to previous work demonstrating a 1D spatially resolved ion temperature imager on OMEGA in order to demonstrate quasi-2D ion temperature imaging at NIF with a cost-efficient detector.   

Neutron yields inferred from neutron time-of-flight (nToF) traces for the Magnetized Liner Inertial Fusion (MagLIF) experiments.

Accurately measuring neutron yields is crucial for understanding the performance and scaling of the MagLIF experiments performed at the Z Pulsed Power Facility. This work presents a technique for measuring neutron yields from neutron time-of-flight (nToF) detectors, which are fielded at radial and axial locations around the neutron source. Each detector consists of a fast plastic scintillator coupled to one or more photomultiplier tubes (PMTs). Outputs from PMTs are digitized on multiple channels with GHz-bandwidth oscilloscopes. The technique for measuring neutron yields involves calculating the charge deposited from neutrons incident on the detector and measuring detector-specific calibration factors to convert the charge into incident neutrons. The charge from incident neutrons is calculated by subtracting a background and integrating the neutron component on the digitized nToF trace. Detector-specific calibration factors are measured at Sandia National Laboratories Ion Beam Laboratory, using a setup designed to generate neutrons through beam-target fusion reactions. Yields measured from nToF traces can be applied to calculate the degree of spatial isotropy for neutron emission and will be compared with neutron activation yields. Prepared by LLNL under Contract DE-AC52-07NA27344. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.   

Simulating the neutron energy spectrum for future deuterium-tritium gas filled magnetized liner inertial fusion experiments on Z

Introducing tritium into the initial gas fill used in the magnetized liner inertial fusion (MagLIF) experiments performed at the Sandia National Laboratories Z machine will enable new neutron energy spectrum measurements to be made. When a trace (~1%) amount of tritium is introduced into the fuel the primary deuterium-deuterium (DD) and deuterium-tritium (DT) fusion signal intensities are expected to be comparable, enabling simultaneous measurements of the DD and DT yield and apparent ion temperature on a single neutron time of flight (nTOF) detector. As the tritium fraction is increased the primary DT signal intensity will quickly surpass that of the DD. Therefore, gated nTOF detectors will be required to measure the full dynamic range of the neutron energy spectrum in high tritium fraction (>10%) experiments. Furthermore, at higher tritium fractions a large number of the primary DT neutrons will scatter both elastically and inelastically off of the beryllium liner and generate a substantial down scattered signal which will act as a background for the primary DD signal. This scattered background signal will make inferring the primary DD yield and apparent ion temperature difficult for high tritium fraction experiments. MCNP simulations of MagLIF implosions with different tritium fractions will be presented and different experimental observables will be considered.   

Anomalous Magnification and Distortion of Knock-On Deuteron Images

Knock-on deuterons, produced when fusion neutrons elastically scatter deuterons in the fuel assembled around the hot spot of an inertial confinement fusion implosion, encode information about the morphology of the assembled fuel. These measurements can diagnose 3-D asymmetries in the imploded fuel at stagnation, which degrade implosion performance. The knock-on deuteron imager is a diagnostic under development at the Omega Laser Facility to image these particles. Initial data include several image distortions, however, that prevent analysis of the images. In this talk, we describe these image distortions, present how they change with experimental parameters, and discuss previous and ongoing attempts to mitigate them. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856 and Department of Energy under Award Number DE-SC0020431.   

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Uncovering 3D structure of the stagnated fusion fuel and liner mix in Magnetized Liner Inertial Fusion (MagLIF) experiments is critical to understanding target performance. However, accurate diagnosis of 3D structure in MagLIF experiments on the Z Machine has been limited by extremely sparse data, often two or less 2D images of self-emission at stagnation. Here we present an approach to accurately reconstruct the morphology of stagnation columns with just two orthogonal views using a set of basis functions learned from training volumes with quasi-helical structures relevant to stagnation plasmas. Reconstructions of emission volumes generated from radiation-magnetohydrodynamic simulations show more accurate morphology compared to when using other bases and forms of regularization. In addition, the learned basis provides accurate estimates of fuel volume, which are necessary for inferences of stagnation pressures. We also present the first 3D reconstructions of experimental MagLIF stagnation plasmas. The approach is applicable to sparse-view 3D reconstruction of other Z-pinch and HED plasmas.   

Pulse dilation system upgrade for Gamma Reaction History

The Gamma Reaction History (GRH) diagnostic has provided a deuterium tritium (DT) fusion reaction history at the National Ignition Facility (NIF). The gammas measured from the DT fusion reaction providing crucial performance parameters for Inertial Confinement Fusion (ICF) such as fusion bang time and fusion burn width. With increased temporal resolution, the shape of the reaction history i.e., the burn profile, can be resolved. The burn profile increases our understanding of onset and truncation of thermonuclear burn as well as puts tighter limits simulations. The Pulse Dilation – Photomultiplier Tube (PD-PMT) provides 10 ps temporal resolution, enough band width for resolving the burn profile. The pulse dilation technology has been tested on the GRH sister detector NIF - Gas Cherenkov Detector (GCD). Recently one GRH gas cell was upgraded with a PD-PMT system. Lessons learned from the GCD testbed were incorporated in the design of the coupling of GRH with a PD-PMT. In this presentation an overview of the GRH upgrade project as well as characterization of GRH/PD-PMT system and burn profile data acquired from ignition class shots will be presented.   

Using Gamma Rays to Measure Reaction-in-flight Interactions

In inertial confinement fusion, the high DT fusion neutron flux creates up-scattered deuterons and tritons with MeV energies, some of which undergo DT fusion for a second time. These reaction-in-flight (RIF) interactions are valuable because they probe the stopping power of the plasma, an interesting metric for the effect of mix and its resulting increase on the electron density. RIF reactions have historically been measured through up-scattered (>15 MeV) neutrons through activation techniques and neutron time of flight detectors. I'll present an alternative technique to potentially measure the RIF interaction - the DT fusion gamma ray pathway. The promise of the RIF gamma ray pathway may allow a time resolved measurement, a metric of how the electron density in the burning hot spot evolves over time. New nuclear physics calculations were developed to calculate the MeV DT fusion gamma ray pathway and a diagnostic requirement needed to make such a measurement defined.   

Simulated Gamma-Ray Images of NIF Capsule Implosions

Gamma-ray imaging of burning inertial confinement fusion capsules give information on where the ablator is during the final stages of the implosion. The ablator areal density (rhoR) can be inferred through gamma-ray detection if the capsule’s total neutron yield is measured [1]. Capsule implosions on the National Ignition Facility (NIF) have recently achieved ignition [2] with repeat experiments achieving much lower yields than expected. Degradation mechanisms are a leading theory as to why there is such variability in the repeats. Recent simulation work has shown that capsule defects lead to collections of jets in the shell that reduce overall compression of the DT fuel [3-4]. We present high resolution 2D simulations using radiation-hydrodynamics code xRAGE [5] of NIF shots N211024 and N211107 using clean/degraded capsules. We compare simulated rhoR and gamma-ray images with experiments.

References

[1] N.M. Hoffman et al, Rev. Sci. Instrum., 81, 10D332 (2010)

[2] H. Abu-Shawareb et al, PRL, 129, 075001 (2022)

[3] B. M. Haines et al, Phys. Plasmas, 29, 042704 (2022)

[4] B. M. Haines et al, Phys. Plasmas, 26, 012707 (2019)

[5] M. L. Gittings et al, Comput. Sci. Discov., 1, 015005 (2008)   

3D gamma-ray reconstructions of inertial confinement fusion implosions

The Nuclear Imaging System (NIS), located at the National Ignition Facility (NIF), captures neutron, X-ray and gamma-ray images of Inertial Confinement Fusion (ICF) driven implosions. Today, the nuclear imaging suite consists of three nearly orthogonal lines of sight (LoS) allowing 3D reconstruction of neutron source distributions. Furthermore, two of those LoS are equipped to capture gamma-ray images which can help characterize the remaining ablator of the fuel capsule. From the X-ray and gamma-ray images, implosion information - such as the presence of higher Z material - can be extracted to assess fusion efficiency. With its neutron, X-ray and gamma-ray image reconstruction capabilities, NIS can provide critical insight on the mechanisms that may limit implosion performance.