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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

A new neutron time-of-flight detector for yield and ion-temperature measurements at the OMEGA Laser Facility

A new neutron time-of-flight (nTOF) detector for deuterium–deuterium (DD)-fusion yield and ion-temperature measurements was designed, installed, and calibrated for the OMEGA Laser Facility. This detector provides an additional line of sight for DD neutron yield and ion-temperature measurements for yields exceeding 1 × 10 10 with higher precision than existing detectors. The nTOF detector consists of a 90-mm-diam, 20-mm-thick BC-422 scintillator and a gated Photek photomultiplier tube (PMT240). The PMT collects scintillating light through the 20-mm side of the scintillator without the use of a light guide. There is no lead shielding from hard x rays in order to allow the x-ray instrument response function of the detector to be measured easily. Instead, hard x-ray signals generated in implosion experiments are gated out by the PMT. The design provides a place for glass neutral-density filters between the scintillator and the PMT to avoid PMT saturation at high yields. The nTOF detector is installed in the OMEGA Target Bay along the P8A sub-port line of sight at a distance of 5.3 m from the target chamber center. In addition to DD measurements, the same detector can be used to measure the neutron yield and ion temperature from deuterium–tritium (DT) implosion targets in the 5 × 10 10 to 2 × 10 12 yield range. Here, the design details and the calibration results of this nTOF detector for both D 2 and DT implosions on OMEGA will be presented.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Design of a Multi-Monochromatic X-ray Imager (MMI) for Kr K-shell line emission

We report the Multi-Monochromatic X-ray Imager (MMI) is a time-gated spectrometer used in implosion experiments at the OMEGA laser facility. From the data, electron temperature and density spatial distributions can be obtained at different implosion times. Previous MMI designs used Ar K-shell emission (3–6 keV) as a spectroscopic tracer and provided a spectral resolution of around 20 eV. However, Ar K-shell line emission becomes less useful at electron temperatures above 2 keV due to over-ionization. Kr K-shell (12–16 keV) has been shown to be an attractive alternative to diagnose hot implosion cores in recent publications. The purpose of this paper is to show a new point design that allows the MMI to detect this higher photon energy range with suitable spectral resolution. The algorithm used to find the optimal design couples a ray-tracing code and an exhaustive parameter space search. This algorithm may be useful as a tool to find optimal MMI designs for other purposes, i.e., other spectral regions for other spectroscopic tracers. So, the main change between the two designs is the replacement of the multi-layer mirror with a flat Bragg Ge (220) crystal. The final Kr K-shell MMI design has a photon energy range from 12 to 16.1 keV.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

A comparison of past and present computational methods for shape analysis of double-shell x-ray radiographs

Implosion symmetry is a key requirement in achieving a robust burning plasma in inertial confinement fusion experiments. In double-shell capsule implosions, we are interested in the shape of the inner shell as it pushes on the fuel. Shape analysis is a popular technique for studying said symmetry during implosion. Combinations of filtering and contour-finding algorithms are studied for their promise in reliably recovering Legendre shape coefficients from synthetic radiographs of double-shell capsules with applied levels of noise. A radial lineout max(slope) method when used on an image pre-filtered with non-local means and a variant of the marching squares algorithm are able to recover p 0 , p 2 , and p 4 maxslope Legendre shape coefficients with mean pixel discrepancy errors of 2.81 and 3.06, respectively, for the noisy synthetic radiographs we consider. Here, this improves upon prior radial lineout methods paired with Gaussian filtering, which we show to be unreliable and whose performance is dependent on input parameters that are difficult to estimate.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Role of initial conditions in plasma-current coupling of gas-puff Z-pinches

Azimuthal magnetic field measurements obtained during the implosion phase of an oxygen gas-puff Z-pinch on a 500 kA peak current and 180 ns rise time linear transformer driver are presented. While a fraction of the driver current was measured within the imploding plasma, key initial conditions were found to significantly impact the delivery of current to the plasma load. The electrode geometry was modified to assist the initial dielectric breakdown and resulted in improved shot reproducibility. Optimization of the gas injection plenum pressure and timing resulted in an increase in the current coupling parameter, defined as the ratio of the measured value of $B$ $θ$ to the expected value, from 50% to 75%. Furthermore, the degree of radial expansion of the gas puff in the load region, which is suspected to lead to the observed current loss during the implosion, was reduced by shortening the valve opening duration. Additionally, a pre-embedded axial magnetic field of up to 0.2 T was found to have no significant impact on the plasma-current coupling of the oxygen implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Measuring and simulating ice–ablator mix in inertial confinement fusion

Fuel–ablator mix has been established as a major performance degrading effect in the burning plasma regime of recent inertial confinement fusion (ICF) experiments. As such, the study of fuel–ablator mix with experiments and simulations can provide valuable insight for our understanding of these experiments and establish a path for even higher yields and increased robustness. Here we present a novel high-yield experimental ICF design that is motivated by recent experiments measuring ice–ablator mix with a CH ablator instead of a high-density carbon (HDC) ablator. Here we review these experiments in more detail and describe the modeling assumptions and parameters used to obtain agreement with the data from implosion and burn simulations with mix. Using this mix model calibrated a posteriori to the experimental data, we design an implosion that uses a CH ablator that is predicted to achieve better performance than a recent experiment that achieved net target gain of 1.5 in HDC. Because hydrodynamic instabilities are greatly reduced with this new design, we also expect a high reproducibility at the same implosion adiabat as current record yield experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Outer shell symmetry for double shell capsules with aluminum ablators

Double shell targets are a promising potential avenue to obtain robust neutron yield at current laser facilities. Similar to single shell designs, double shells require the symmetric implosion of an ablator in order to uniformly compress and heat a fuel volume, with the goal of achieving thermonuclear burn. Significant differences between double and single shells include the usage of an aluminum ablator as well as a reverse ramp laser pulse. In addition, double shells require a different convergence than single shells for fuel ignition. Numerical implosion studies at various energies with comparisons to experimental outcomes are required to gain confidence that simulations can capture the ablator shape from subscale to full scale. The current work builds on previous implosion experiments conducted at 1-MJ laser energy to confirm achieved ablator symmetry at 1.25 and 1.5 MJ. Average ablator P2 and P4 shapes measured in these experiments are within 5% of the simulated shape, which merits the platforms for further experimental studies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Design and analysis of dudded fuel experiments at the National Ignition Facility

Recent experiments conducted at the National Ignition Facility (NIF) within the past 2 years have achieved the burning plasma state and exceeded the Lawson criterion for the first time in the laboratory. Here, we report on a set of experiments where the deuterium and tritium (DT) ice layers were replaced with dudded tritium, hydrogen, and deuterium (THD) fuel mixtures to remove the influence of alpha-heating on hot spot dynamics. The hot spot compression and yield in the absence of alpha particle self-heating were measured to assess the proximity of NIF implosions toward the ignition cliff. We find that the “burn-off” Lawson parameters χnoα inferred from the THD experiments are in good agreement with the inferences from postshot simulations of the DT-layered implosions. The THD for burning plasma shot N210307 yielded χnoα≈0.88±0.03 while the THD for ignition shot N210808 yielded χnoα≈1.04±0.04. These results also provide important context for the observed variability in the repeat attempts of ignition shot N210808 since implosions on the ignition cliff are expected to exhibit very large variations in the fusion yield from small changes in the initial conditions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Code-to-code comparison between FLASH and HYDRA in gas-puff Z-pinch modeling

The numerical modeling of gas-puff Z pinches involves the nonlinear coupling of a broad range of complex, multi-physics phenomena that makes such simulations challenging. The challenge is further compounded by nonlinear instabilities that can impact the dynamics of imploding gas-puff Z pinches, such as the magneto Rayleigh–Taylor instability (MRTI). If the growth rate and amplitude of the latter is comparable to the relevant timescales and properties of the imploding plasma, the MRTI can dramatically alter implosion dynamics, dictate pinch stability, and govern the plasma properties achievable in pulsed-power-driven laboratory experiments. National Laboratories and academic teams have developed numerical tools that can accurately model Z-pinch configurations and provide reliable design capabilities that can guide experimental choices and assist in interpreting experimental results. Most such tools, however, are not broadly available. Here, we present newly developed Z-pinch simulation capabilities of the publicly available FLASH code, applied in the study of MRTI growth and dynamical effects in gas-puff implosions. To verify the new implementations, we perform a comparison of FLASH gas-puff implosion simulations with previously published calculations with the HYDRA code from Lawrence Livermore National Laboratory, which have been validated with experimental data from the CESZAR pulsed-power driver at the University of California, San Diego. The experiments involved double- and triple-nozzle configurations, in an experimental attempt to stabilize the pinch to the MRTI. The code-to-code comparison shows similar results between the FLASH and HYDRA simulations, supporting the use of FLASH in the modeling of future gas-puff Z-pinch experiments at CESZAR.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Studies of localized mix and asymmetry caused by the stalk in inertial fusion experiments at OMEGA

In an inertially confined fusion experiment, a capsule of fusion fuel is compressed to high temperatures and densities using lasers, creating a hot plasma in which thermonuclear fusion reactions take place. To achieve efficient conversion of laser energy to fuel heating, the implosion must be as spherically symmetric as possible. One common source of asymmetry that degrades performance is the stalk and glue spot that hold the capsule in place before the experiment. The exact nature and magnitude of this perturbation had not been previously quantified. In this work, inertially confined fusion experiments were executed with an additional stalk on the capsule to measure and quantify the effect of the stalk and glue spot on the symmetry and performance. It is found that the stalk drives low-mode asymmetry by shadowing the laser drive on the part of the shell near the glue spot, enhancing the hot-spot velocity toward the stalk by 40–80 km/s and elongating the hot-spot along the stalk axis. It is also found that localized mix due to the additional stalk has a negligible effect on implosion performance. Two-dimensional radiation-hydrodynamic simulations using the xRAGE code reproduce the direction of the experimentally observed change to hot-spot velocity and implosion performance metrics, but not the observed elongation, suggesting that this feature results from an interaction between stalk perturbations and other preexisting asymmetry seeds, such as laser drive asymmetry.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Burn mode signatures enabled by high dynamic range fusion reaction history

The evolution of the fusion burn of a compressed inertial confinement fusion (ICF) implosion gives information about the evolution of the temperature, mass, and volume of the hot spot. Currently, the fusion reaction history has been measured with about a decade of dynamic range, giving information about just the peak of the fusion burn. There are proposals for extending the dynamic range to 1000×, measuring the rising edge of the burn earlier in time. Using fusion hot spot theory and a set of ICF simulations, we identify and categorize different stages of fusion burn and what signatures could be measured. For ice layered implosions, the details, conditions, and dynamics of the fusion burn propagation could be observed. For double shell and OMEGA scale implosions, the evolution of the final compression and rebounding shock before ignition could be observed. Higher dynamic range expands investigation into the logarithmic derivative, α, as a signature of various mechanisms.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Plasma Jet Driven Magneto-Inertial Fusion (PJMIF)

We report that PJMIF is the only embodiment of magneto-inertial fusion that has the unique combination of standoff implosion and high implosion velocity (50 km/s - 150 km/s). It uses inexpensive plasma guns for all plasma formation and implosion. It has potential for relatively high repetition rate from 1 to 2 Hz. Its configuration is compatible with the use of a thick liquid wall that doubles as a tritium breeding blanket as well as a coolant for extracting the heat out of the fusion reactor. Its operational parameter space allows for the possibility of using a suffciently dense target plasma for the target plasma to have a high beta. If such a high beta plasma could be realized, it would help to suppress micro and MHD instabilities, giving its target plasma classical transport and energy confinement characteristics. Its open geometry and moderate time and spatial scales provide convenient diagnostics access. Diagnostics accessibility, high shot rate and low cost per shot should enable quick resolution of technical issues during development, thus the potential for enabling rapid R&D of PJMIF. There are a number of challenges, however, for PJMIF including being at a very early stage of development, developing the required plasma guns, dealing with potential liner non-uniformities, clearing the chamber of residual high-Z gas between shots, and developing the repetitive pulsed power component technologies. Over the last three years, the development of the Plasma Liner Formation Experiment (PLX-alpha) have been undertaken to explore the physics and demonstrate the formation of a spherical liner by the merging of a spherical array of plasma jets. Two- and three- jet merging experiments have been conducted to study the interactions of the jets. Lastly, six- and seven-jet experiments have been performed to form a piece of the plasma liner. A brief status report on this development is provided in this paper.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Correcting Hohlraum Drive Asymmetry with Glow Discharge Polymerization Coated Capsule Shims

In inertial confinement fusion target design, the shape discrepancy between the cylindrical hohlraum and the spherical capsule creates a low mode asymmetry in the implosion. One way to correct such asymmetry is to shim the target capsule surface with extra mass in specific locations following a 3D P4 Legendre mode. Previously, the desired surface pattern was precision machined out of the capsule. The resulting 2DConA experiments that investigated the implosion’s shape demonstrated the shimming method’s success. However, machining leaves large defects on the capsule surface which will degrade neutron yield in a DT implosion. An alternative shimming approach is to grow the pattern on the capsule surface using a glow discharge polymerization coating process in a stencil lithography application. In this study, we discuss the fabrication, characterization and challenges of making shimmed target capsules with this new method.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Staged Z-pinch experiment at the double-EAGLE pulsed power facility

The staged Z-pinch is a potential high-energy gain fusion concept where a high atomic number liner implodes on a deuterium target using a pulsed multi-MA current source. Over the past several years this concept has been studied on 0.5–1.0 MA facilities using Ar and Kr gas puffs injected at R = 1.2 cm. Neutron yield up to (2.5 ± 0.34) x 10 10 was measured, Ruskov et al (2023 IEEE Trans. Plasma Sci. 51 3310–6). Here, in this study, we present experimental results from the 4 MA, 110 ns current rise time Double-EAGLE facility at L3Harris (currently, Fisica Inc.) where a larger radius nozzle created gas density profiles peaked at R = 2.5 cm. Modeling with the MACH2 and FLASH codes indicates that the larger radius allows stronger acceleration of the liner plasma and generation of shock waves that preheat a target plasma layer next to the liner to temperatures $T_i >$1 keV. The resulting counter thermal pressure on the liner plasma limits the growth of the Magneto-Rayleigh–Taylor (MRT) instability at the liner-vacuum boundary and the implosion proceeds in a relatively stable manner. Near bang time the enormous target plasma thermal pressure smoothens the MRT perturbations developed during the earlier implosion stages. Time integrated x-ray pinhole images with cutoff energy of 100 eV confirm that a long (∼3 cm), stable and uniform high energy density plasma column is formed in the final implosion stage. Consistent neutron yield in the 10 10 –10 11 range was measured for both Ar and Kr liners imploding on a deuterium target.

Physics - Plasma physics↗
DOE OSTI2017

Ion kinetic dynamics in strongly-shocked plasmas relevant to ICF

Implosions of thin-shell capsules produce strongly-shocked (M > 10), low-density (ρ ~1 mg/cc -1 ), high-temperature (T i ~keV) plasmas, comparable to those produced in the strongly-shocked DT-vapor in inertial confinement fusion (ICF) experiments. A series of thin-glass targets filled with mixtures of deuterium and Helium-3 gas ranging from 7% to 100% deuterium was imploded to investigate the impact of multi-species ion kinetic mechanisms in ICF-relevant plasmas over a wide range of Knudsen numbers (N K ≡ λ ii /R). Anomalous trends in nuclear yields and burn-averaged ion temperatures in implosions with N K > 0.5, which have been interpreted as signatures of ion species separation and ion thermal decoupling, are found not to be consistent with single-species ion kinetic effects alone. Experimentally inferred Knudsen numbers predict an opposite yield trend to those observed, confirming the dominance of multi-species physics in these experiments. In contrast, implosions with N K ~ 0.01 follow the expected yield trend, suggesting single-species kinetic effects are dominant. In conclusion, the impact of the observed kinetic physics mechanisms on the formation of the hotspot in ICF experiments is discussed.

42 ENGINEERING↗
DOE OSTI2020

Magnetized directly-driven ICF capsules: increased instability growth from non-uniform laser drive

Simulations anticipate increased perturbation growth from non-uniform laser heating for magnetized direct-drive implosions. At the capsule pole, where the magnetic field is normal to the ablator surface, the field remains in the conduction zone and suppresses non-radial thermal conduction; in unmagnetized implosions this non-radial heat-flow is crucial in mitigating laser heating imbalances. Single-mode simulations show the magnetic field particularly amplifying short wavelength perturbations, whose behavior is dominated by thermal conduction. The most unstable wavelength can also become shorter. 3D multi-mode simulations of the capsule pole reinforce these findings, with increased perturbation growth anticipated across a wide range of scales. Finally, the results indicate that high-gain spherical direct-drive implosions require greater constraints on the laser heating uniformity when magnetized.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Enhancements in Laser-Direct-Drive Nuclear Performance with Target Radius

Inertial confinement fusion is likely to require significant improvements in technology and design for large targets to implode at high driver energies. Here, to assess the potential for benefits, we report on nuclear performance as a function of target radius 𝑅 t in direct-drive cryogenic implosions as performed on the OMEGA laser. The neutron yield and areal density are found to increase as 𝑅 5.0±0.2 t and 𝑅 1.8±0.2 t , respectively, and exhibit a much stronger dependence on target radius than previous studies have assumed. As this paper demonstrates, these types of sensitivities should be expected when implosions are unstable, and have been degraded by a range of multidimensional effects in 3D. If the imperfections in a target and laser system are fixed in magnitude, it follows that larger implosions have higher relative quality, and approach criteria to ignite much more quickly than commonly appreciated.

inertial confinement fusion↗
DOE OSTI2025

Effect on compression of lowering the design adiabat in the SQ-n campaign

A set of experiments in the SQ-n campaign attempts to increase the compression in layered implosions by reducing the design adiabat from 3.0 down to ∼ 2.6. The experiments utilize nanocrystalline high density carbon ablators driven by a ramped laser pulse drive. Contrary to expectations, the lower design adiabat layered implosion reaches the same level of compression, within the measurement uncertainty, as the previous set of SQ-n layered implosions at the higher design adiabat. Here, we examine potential explanations for this result, including incomplete melting or refreezing of the ablator after the first shock passes into the fuel layer, reproducibility, and dopant areal density. We also discuss additional potential paths to reaching lower adiabat including increased dopant areal density, ramped dopant profiles, and increased initial capsule size which are also based on experimental campaign results from the National Ignition Facility.

Physics - Plasma physics↗
DOE OSTI2019

Measurement of the B 10 ( α , n 0 ) 13 N cross section for 2.2 < E α < 4.9 MeV and its application as a diagnostic at the National Ignition Facility

We present that the National Ignition Facility (NIF) provides the opportunity to study nuclear reactions under controlled conditions at high temperatures and pressures at a level never before achieved. However, the timescale of the deuterium-tritium (DT) implosion is only a few nanoseconds, making data collection and diagnostics very challenging. One method that has been proposed for obtaining additional information about the conditions of the implosion is to activate a dopant material using the α particles produced from the DT fuel as a diagnostic. The yield of the activated material can give a measure of the mixing that occurs in the capsule. One of the reactions that has been proposed is 10 B(α,n) 13 N as it produces a radioactive reactant product with a convenient half-life of ≈10 min. Although this reaction has several advantages for the application at hand, it has not seen much study in the present literature, resulting in large uncertainties in the cross section. Furthermore, for the current application, the cross section must be well characterized. With this motivation, the 10 B(α,n) 13 N cross section has been remeasured for 2.2 < E α < 4.9MeV with the angle-integrated ground-state cross section reported for the first time. Lastly, the present results, combined with previous measurements, allow for a determination of the cross section to a significantly higher degree of accuracy and precision than obtained previously and are shown to be consistent with thick-target measurements. Preliminary calculations are performed to test the feasibility of this reaction as a diagnostic for a NIF implosion.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗