Neutron time of flight (nToF) detectors for inertial fusion experiments.

Neutrons generated in Inertial Confinement Fusion (ICF) experiments provide valuable information to interpret the conditions reached in the plasma. The neutron time-of-flight (nToF) technique is well suited for measuring the neutron energy spectrum due to the short time (100 ps) over which neutrons are typically emitted in ICF experiments. By locating detectors 10s of meters from the source, the neutron energy spectrum can be measured to high precision. We present a contextual review of the current state of the art in nToF detectors at ICF facilities in the United States, outlining the physics that can be measured, the detector technologies currently deployed and analysis techniques used.

Observation of Hydrodynamic Flows in Imploding Fusion Plasmas on the National Ignition Facility.

Inertial confinement fusion implosions designed to have minimal fluid motion at peak compression often show significant linear flows in the laboratory, attributable per simulations to percent-level imbalances in the laser drive illumination symmetry. We present experimental results which intentionally varied the mode 1 drive imbalance by up to 4% to test hydrodynamic predictions of flows and the resultant imploded core asymmetries and performance, as measured by a combination of DT neutron spectroscopy and high-resolution x-ray core imaging. Neutron yields decrease by up to 50%, and anisotropic neutron Doppler broadening increases by 20%, in agreement with simulations. Furthermore, a tracer jet from the capsule fill-tube perturbation that is entrained by the hot-spot flow confirms the average flow speeds deduced from neutron spectroscopy.

Converting existing optical detectors into fast x-ray detectors.

The very short burn time and small size of burning plasmas created at advanced laser-fusion facilities will require high-spatial-resolution imaging diagnostics with fast time resolution. These instruments will need to function in an environment of extremely large neutron fluxes that will cause conventional diagnostics to fail because of radiation damage and induced background levels. One solution to this challenge is to perform an ultrafast conversion of the x-ray signals into the optical regime before the neutrons are able to reach the detector and then to relay image the signal out of the chamber and into a shielded bunker, protected from the effects of these neutrons. With this goal in mind, the OMEGA laser was used to demonstrate high-temporal-resolution x-ray imaging by using an x-ray snout to image an imploding backlighter capsule onto a semiconductor. The semiconductor was simultaneously probed with the existing velocity interferometry system for any surface reflector (VISAR) diagnostic, which uses an optical streak camera and provided a one-dimensional image of the phase in the semiconductor as a function of time. The phase induced in the semiconductor was linearly proportional to the x-ray emission from the backlighter capsule. This approach would then allow a sacrificial semiconductor to be attached at the end of an optical train with the VISAR and optical streak camera placed in a shielded bunker to operate in a high neutron environment and obtain time-dependent one-dimensional x-ray images or time-dependent x-ray spectra from a burning plasma.

Interpolating individual line-of-sight neutron spectrometer measurements onto the "sky" at the National Ignition Facility (NIF).

Nuclear diagnostics provide measurements of inertial confinement fusion implosions used as metrics of performance for the shot. The interpretation of these measurements for shots with low mode asymmetries requires a way of combining the data to produce a "sky map" where the individual line-of-sight values are used to interpolate to other positions in the sky. These interpolations can provide information regarding the orientation of the low mode asymmetries. We describe the interpolation method, associated uncertainties, and correlations between different metrics, e.g., Tion, down scatter ratio, and hot-spot velocity direction. This work is also related to recently reported studies [H. G. Rinderknecht et al., Phys. Rev. Lett. 124, 145002 (2020) and K. M. Woo et al., Phys. Plasmas 27, 062702 (2020)] of low mode asymmetries. We report an analysis that makes use of a newly commissioned line of sight, a scheme for incorporating multiple neutron spectrum measurement types, and recent work on the sources of implosion asymmetry to provide a more complete picture of implosion performance.

Understanding the effects of neutron scattering for neutron-yield-isotropy measurements at the NIF.

Neutron-yield diagnostics at the NIF have been upgraded to include 48 detectors placed around the NIF target chamber to assess the DT-neutron-yield isotropy for inertial confinement fusion experiments. Real-time neutron-activation detectors are used to understand yield asymmetries due to Doppler shifts in the neutron energy attributed to hotspot motion, variations in the fuel and ablator areal densities, and other physics effects. In order to isolate target physics effects, we must understand the contribution due to neutron scattering associated with the different hardware configurations used for each experiment. We present results from several calibration experiments that demonstrate the ability to achieve our goal of 1% or better precision in determining the yield isotropy.

Three-dimensional diagnostics and measurements of inertial confinement fusion plasmas.

Recent inertial confinement fusion measurements have highlighted the importance of 3D asymmetry effects on implosion performance. One prominent example is the bulk drift velocity of the deuterium-tritium plasma undergoing fusion ("hotspot"), vHS. Upgrades to the National Ignition Facility neutron time-of-flight diagnostics now provide vHS to better than 1 part in 104 and enable cross correlations with other measurements. This work presents the impact of vHS on the neutron yield, downscatter ratio, apparent ion temperature, electron temperature, and 2D x-ray emission. The necessary improvements to diagnostic suites to take these measurements are also detailed. The benefits of using cross-diagnostic analysis to test hotspot models and theory are discussed, and cross-shot trends are shown.

Real-time nuclear activation detectors for measuring neutron angular distributions at the National Ignition Facility (invited).

The Real Time Nuclear Activation Detector (RTNAD) array at NIF measures the distribution of 14 MeV neutrons emitted by deuterium-tritium (DT) fueled inertial confinement fusion implosions. The uniformity of the neutron distribution is an important indication of implosion symmetry and DT shell integrity. The array consists of 48 LaBr3(Ce) crystal gamma-ray spectrometers mounted outside the NIF target chamber, which continuously monitor the slow decay of the 909 keV gamma-ray line from activated 89Zr located in Zr cups surrounding each crystal. The measured decay rate dramatically increases during a DT implosion in proportion to the number of 14 MeV neutrons striking each Zr cup. The neutrons produce activated 89Zr through an (n, 2n) reaction on 90Zr, which is insensitive to low energy neutrons. The neutron flux along the detector line-of-sight at shot time is determined by extrapolating the fitted 909 keV decay curve back to shot time. Automatic analysis algorithms were developed to handle the non-stop data stream. The large number of detectors and the high statistical accuracy of the array enable the spherical harmonic modes of the neutron angular distribution to be measured up to L ≤ 4 to provide a better understanding of implosion dynamics. In addition, these data combined with measurements of the down-scattered neutrons can be used to derive fuel areal density distributions. This paper will describe the RTNAD hardware and analysis procedures.

The five line-of-sight neutron time-of-flight (nToF) suite on the National Ignition Facility (NIF).

Measurement of the neutron spectrum from inertial confinement fusion implosions is one of the primary diagnostics of implosion performance. Analysis of the spectrum gives access to quantities such as neutron yield, hot-spot velocity, apparent ion temperature, and compressed fuel ρr through measurement of the down-scatter ratio. On the National Ignition Facility, the neutron time-of-flight suite has been upgraded to include five independent, collimated lines of sight, each comprising a high dynamic range bibenzyl/diphenylacetylene-stilbene scintillator [R. Hatarik et al., Plasma Fusion Res. 9, 4404104 (2014)] and high-speed fused silica Cherenkov detectors [A. S. Moore et al., Rev. Sci. Instrum. 89, 10I120 (2018)].

Optimal choice of multiple line-of-sight measurements determining plasma hotspot velocity at the National Ignition Facility.

The measurement of plasma hotspot velocity provides an important diagnostic of implosion performance for inertial confinement fusion experiments at the National Ignition Facility. The shift of the fusion product neutron mean kinetic energy as measured along multiple line-of-sight time-of-flight spectrometers provides velocity vector components from which the hotspot velocity is inferred. Multiple measurements improve the hotspot velocity inference; however, practical considerations of available space, operational overhead, and instrumentation costs limit the number of possible line-of-sight measurements. We propose a solution to this classical "experiment design" problem that optimizes the precision of the velocity inference for a limited number of measurements.

Neutron Time-of-Flight Measurements of Charged-Particle Energy Loss in Inertial Confinement Fusion Plasmas.

Neutron spectra from secondary ^{3}H(d,n)α reactions produced by an implosion of a deuterium-gas capsule at the National Ignition Facility have been measured with order-of-magnitude improvements in statistics and resolution over past experiments. These new data and their sensitivity to the energy loss of fast tritons emitted from thermal ^{2}H(d,p)^{3}H reactions enable the first statistically significant investigation of charged-particle stopping via the emitted neutron spectrum. Radiation-hydrodynamic simulations, constrained to match a number of observables from the implosion, were used to predict the neutron spectra while employing two different energy loss models. This analysis represents the first test of stopping models under inertial confinement fusion conditions, covering plasma temperatures of k_{B}T≈1-4 keV and particle densities of n≈(12-2)×10^{24} cm^{-3}. Under these conditions, we find significant deviations of our data from a theory employing classical collisions whereas the theory including quantum diffraction agrees with our data.

Generating keV ion distributions for nuclear reactions at near solid-density using intense short-pulse lasers.

Our understanding of a large range of astrophysical phenomena depends on a precise knowledge of charged particle nuclear reactions that occur at very low rates, which are difficult to measure under relevant plasma conditions. Here, we describe a method for generating dense plasmas at effective ion temperatures >20 keV, sufficient to induce measurable charged particle nuclear reactions. Our approach uses ultra-intense lasers to drive micron-sized, encapsulated nanofoam targets. Energetic electrons generated in the intense laser interaction pass through the foam, inducing a rapid expansion of the foam ions; this results in a hot, near-solid density plasma. We present the laser and target conditions necessary to achieve these conditions and illustrate the system performance using three-dimensional particle-in-cell simulations, outline potential applications and calculate expected nuclear reaction rates in the D(d,n) and 12C(p,γ) systems assuming CD, or CH aerogel foams.

Using multiple neutron time of flight detectors to determine the hot spot velocity.

An important diagnostic value of a shot at the National Ignition Facility is the resultant center-of-mass motion of the imploding capsule. This residual velocity reduces the efficiency of converting laser energy into plasma temperature. A new analysis method extracts the effective hot spot motion by using information from multiple neutron time-of-flight (nToF) lines-of-sight (LoSs). This technique fits a near Gaussian spectrum to the nToF scope traces and overcomes reliance on models to relate the plasma temperature to the mean energy of the emitted neutrons. This method requires having at least four nToF LoSs. The results of this analysis will be compared to an approach where each LoS is analyzed separately and a model is used to infer the mean energy of the emitted neutrons.

Ab initio response functions for Cherenkov-based neutron detectors.

Neutron time-of-flight diagnostics at the NIF were recently outfitted with Cherenkov detectors. A fused silica radiator delivers sub-nanosecond response time and is optically coupled to a microchannel plate photomultiplier tube with gain from ∼1 to 104. Capitalizing on fast time response and gamma-ray sensitivity, these systems can provide better than 30 ps precision for measuring first moments of neutron distributions. Generation of ab initio instrument response functions (IRFs) is critical to meet the <1% uncertainty needed. A combination of Monte Carlo modeling, benchtop characterization, and in situ comparison is employed. Close agreement is shown between the modeled IRFs and in situ measurements using the NIF's short-pulse advanced radiographic capability beams. First and second moments of neutron spectra calculated using ab initio IRFs agree well with established scintillator measurements. Next-step designs offer increased sensitivity and time-response.

A fused silica Cherenkov radiator for high precision time-of-flight measurement of DT γ and neutron spectra (invited).

A fused silica Cherenkov radiator has been implemented at the National Ignition Facility to provide a new high precision measurement of the time-of-flight spectrum of 14.1 MeV DT fusion neutrons. This detector enables a high precision (<30 ps) co-registered measurement of both a thresholded γ-ray and a neutron spectrum on a single record. Other methods typically require γ and neutron signals to be co-registered via other diagnostics and/or dedicated timing experiments. Analysis of the co-registered γ and neutron signals allows precise extraction of the mean neutron energy and bulk hot-spot velocity, both of which were not possible with prior scintillator technologies. Initial measurements demonstrate the feasibility of this measurement and indicate that combined detection of neutrons and γ-rays on multiple lines-of-sight should enable the bulk vector velocity of the implosion hot-spot to be determined to ≈5 km/s and reduced uncertainty in the spectral width ≈0.1 keV.

Velocity correction for neutron activation diagnostics at the NIF.

The velocity distribution of the hotspot in an inertial confinement fusion implosion changes the energy spectra of fusion neutrons emitted from the experiment as a function of viewing angle. These velocity-induced spectral changes affect the response of neutron activation diagnostics (NADs) positioned around the experiment and must be accounted for to correctly extract information about areal density (ρR) asymmetry from the data. Three mechanisms through which average hotspot velocity affects NAD activation are addressed: change in activation cross section due to the Doppler shift of the mean neutron energy, kinematic focusing of neutron fluence, and change in the scattering cross section due to the Doppler shift. Using the hotspot velocity inferred from neutron time-of-flight measurements of D-T and D-D fusion neutrons, the hotspot velocity is shown to account for the observed NAD activation asymmetry in a calibration shot with negligible fuel ρR. A robust method to evaluate uncertainties in spherical-harmonic fits to the NAD data due to the velocity correction and detector uncertainty is discussed.

Testing a Cherenkov neutron time-of-flight detector on OMEGA.

A Cherenkov neutron time-of-flight (nTOF) detector developed and constructed at Lawrence Livermore National Laboratory was tested at 13 m from the target in a collimated line of sight (LOS) and at 5.3 m from the target in the open space inside the OMEGA Target Bay. Neutrons interacting with the quartz rod generate gammas, which through Compton scattering produce relativistic electrons that give rise to Cherenkov light. A photomultiplier tube (PMT) transferred the Cherenkov light into an amplified electrical signal. The Cherenkov nTOF detector consists of an 8-mm-diam, 25-cm quartz hexagonal prism coupled with a Hamamatsu gated PMT R5916U-52. The tests were performed with DT direct-drive implosions with cryogenic and room-temperature targets, producing a wide range of neutron yields and ion temperatures. The results of the tests and comparison with other nTOF detectors on OMEGA are presented. In the collimated LOS at 13 m from the target, the Cherenkov nTOF detector demonstrated good precision measurement in both the yield and ion temperature for DT yields above 3 × 1013.

Characterization of photodetector temporal response for neutron time-of-flight (nToF) diagnostics at the National Ignition Facility.

The temporal response of a microchannel plate photomultiplier tube used in the suite of neutron time of flight (nToF) diagnostics at the National Ignition Facility has been characterized to reduce uncertainty in, and understanding of, shot parameters obtained from nTOF data. A short pulse laser, neutral density glass filters, and electrical attenuators were used to gather statistically significant samples of photodetector impulse response functions (IRF) in rapid succession. Individual components have been absolutely calibrated to minimize systematic uncertainties. The zeroth (collected charge), first (transit time), and second central moments (transit time spread) of the IRF were calculated as either the bias voltage or the amount of light incident on the detector was varied. Timing reference was provided by a monitor photodiode viewing a pickoff of the incident laser pulse. The primary sources of uncertainty are jitter in the monitor photodiode and the statistical variation across our measurement period. The spreads in the first moment, with respect to the timing photodiode, and the square root of the second central moment were found to be less than 50 ps and 150 ps, respectively.

Uncertainty analysis of response functions and γ -backgrounds on Tion and t0 measurements from Cherenkov neutron detectors at the National Ignition Facility (NIF).

Cherenkov radiators deployed to measure the neutron time-of-flight spectrum have response times associated with the neutron transit across the detector and are free from long time response tails characteristic of scintillation detectors. The Cherenkov radiation results from simple physical processes which makes them amenable to high fidelity Monte Carlo simulation. The instrument response function of neutron time-of-flight systems is a major contributor to both the systematic and statistical uncertainties of the parameters used to describe these spectra; in particular, the first and second moments of these distributions are associated with arrival time, t0, and ion temperature, Tion. We present the results of uncertainty analysis showing the significant reduction of the uncertainty in determining these quantities in the Cherenkov detector system recently deployed at NIF. The increased sensitivity to gamma radiation requires additional consideration of the effect of this background to the uncertainties in both t0 and Tion.

High-Performance Indirect-Drive Cryogenic Implosions at High Adiabat on the National Ignition Facility.

To reach the pressures and densities required for ignition, it may be necessary to develop an approach to design that makes it easier for simulations to guide experiments. Here, we report on a new short-pulse inertial confinement fusion platform that is specifically designed to be more predictable. The platform has demonstrated 99%+0.5% laser coupling into the hohlraum, high implosion velocity (411 km/s), high hotspot pressure (220+60 Gbar), and high cold fuel areal density compression ratio (>400), while maintaining controlled implosion symmetry, providing a promising new physics platform to study ignition physics.

Fluence-compensated down-scattered neutron imaging using the neutron imaging system at the National Ignition Facility.

The Neutron Imaging System at the National Ignition Facility is used to observe the primary ∼14 MeV neutrons from the hotspot and down-scattered neutrons (6-12 MeV) from the assembled shell. Due to the strong spatial dependence of the primary neutron fluence through the dense shell, the down-scattered image is convolved with the primary-neutron fluence much like a backlighter profile. Using a characteristic scattering angle assumption, we estimate the primary neutron fluence and compensate the down-scattered image, which reveals information about asymmetry that is otherwise difficult to extract without invoking complicated models.