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At least 163 records · Page 9

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↗

Structure of supported DPPC/cholesterol bilayers studied via X-ray reflectivity

The electron density profile of bilayers of DPPC/cholesterol mixtures supported on semiconductor grade silicon substrates were studied with the objective of determining how the proximity of a solid interface modifies the phase diagram of mixed bilayers. The bilayers were studiedin situimmersed in waterviasynchrotron X-ray reflectivity (XRR). Measurements were performed as a function of temperature through the main phase transition and cholesterol mole fractions up to 40%. Analysis of XRR yields the bilayer thickness, roughness and leaflet asymmetry. We find that the structure of the pure DPPC bilayers in the gel phase is in agreement with previous X-ray measurements of supported bilayers depositedviavesicle fusion and multilamellar vesicles but show more clearly defined features than measurements made on films formed using Langmuir-Blodget Langmuir-Shaffer (LB) deposition. Examination of bilayer thicknessvs.temperature shows that the melting temperature for supported bilayers is shifted upwards by approximately 4 degrees C relative to multilamellar vesicles and that the melting temperature decreases with increasing cholesterol content up to 20%. For pure DPPC bilayers the leaflets melt in two stages with the distal leaflet melting first. For cholesterol concentrations of 10% and 20% there is no clear indication of separate melting. Finally. for 33% and 40% cholesterol content no clear transition is seen in the bilayer thickness, but an abrupt change in roughness indicates possible microdomain formation in the 40% cholesterol sample.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Portable and Adaptable Neutron Diagnostics for Advancing Fusion Energy Science Addendum

Activation detectors developed at LLNL for measuring real-time neutron fluence from fusion sources are used in the broader fusion community. The recommended fluence operating range of this diagnostic is 5x10 2 – 1x10 6 n/cm2. The upper limit on this fluence range is set by the dead time caused by data transfer between the detector and data acquisition computer. Delaying the start of counting is a possible strategy to operate these detectors in higher fluences.

42 ENGINEERING↗

LaBr 3 : Ce self-activation analysis for measuring fast neutron fields

Measurement of the fast neutron production rate in deuterium–tritium (D–T) fusion reactions is important for applications such as active interrogation, fusion diagnostics, and borehole logging. Such measurements are typically performed by neutron activation analysis of metal foils, especially copper. Copper foil activation analysis requires efficiency and energy calibrations of the detector used to measure the foil, and it relies on the detection of 511 keV gamma rays, which are prominent in the active background when neutrons are being produced. Alternatives, such as 79m Br produced by inelastic neutron scattering on 79 Br, are limited by short half-life, low-energy gamma emission, and inability to selectively measure D–T neutrons. This work describes a novel alternative approach to measure ≳10 MeV neutron fields based on self-activation analysis of a LaBr 3 :Ce detector. The activity of 78 Br, the activation product of the 79 Br(n,2n) 78 Br reaction, is used to determine the neutron flux and infer the neutron production rate. We experimentally demonstrate the method with a cylindrical LaBr 3 :Ce crystal with a diameter and height of 3.81 cm that was placed at an ~18 cm distance from the neutron production point, at a 90° angle with respect to the deuteron beam in a D–T neutron generator. Operating voltage and current of the generator were adjusted to evaluate the technique’s performance over a nominal generator output range of approximately (1 - 9) x 10 7 n/s. The neutron output obtained from LaBr 3 :Ce activation agrees to within three standard deviations of the output obtained using copper activation. The self-activation technique can be conveniently employed in a variety of applications to simplify measurements of fast neutrons produced in D–T fusion reactions.

Active interrogationLaBr3↗

Complementary Measurements of Residual Stresses Before and After Base Plate Removal in an Intricate Additively-Manufactured Stainless-Steel Valve Housing

Residual stress measurements using neutron diffraction and the contour method were performed on a valve housing made from 316 L stainless steel powder with intricate three-dimensional internal features using laser powder-bed fusion additive manufacturing. The measurements captured the evolution of the residual stress fields from a state where the valve housing was attached to the base plate to a state where the housing was cut free from the base plate. Making use of this cut, thus making it a non-destructive measurement in this application, the contour method mapped the residual stress component normal to the cut plane (this stress field is completely relieved by cutting) over the whole cut plane, as well as the change in all stresses in the entire housing due to the cut. The non-destructive nature of the neutron diffraction measurements enabled measurements of residual stress at various points in the build prior to cutting and again after cutting. Good agreement was observed between the two measurement techniques, which showed large, tensile build-direction residual stresses in the outer regions of the housing. The contour results showed large changes in multiple stress components upon removal of the build from the base plate in two distinct regions: near the plane where the build was cut free from the base plate and near the internal features that act as stress concentrators. These observations should be useful in understanding the driving mechanisms for builds cracking near the base plate and to identify regions of concern for structural integrity. Neutron diffraction measurements were also used to show the shear stresses near the base plate were significantly lower than normal stresses, an important assumption for the contour method because of the asymmetric cut.

36 MATERIALS SCIENCE↗

Quantifying experimental edge plasma evolution via multidimensional adaptive Gaussian process regression

The edge density and temperature of tokamak plasmas are strongly correlated with energy and particle confinement and their quantification is fundamental to understanding edge dynamics. These quantities exhibit behaviours ranging from sharp plasma gradients and fast transient phenomena (e.g. transitions between low and high confinement regimes) to nominal stationary phases. Analysis of experimental edge measurements therefore require robust fitting techniques to capture potentially stiff spatiotemporal evolution. Additionally, fusion plasma diagnostics inevitably involve measurement errors and data analysis requires a statistical framework to accurately quantify uncertainties. This paper outlines a generalized multidimensional adaptive Gaussian process routine capable of automatically handling noisy data and spatiotemporal correlations. We focus on the edge-pedestal region in order to underline advancements in quantifying time-dependent plasma profiles including transport barrier formation on the Alcator C-Mod tokamak.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

First measurement of the 10B(α,n)13N reaction in an inertial confinement fusion implosion at the National Ignition Facility: Initial steps toward the development of a radiochemistry mix diagnostic

We report the first measurement of the 10B(α,n)13N reaction in a polar-direct-drive exploding pusher (PDXP) at the National Ignition Facility (NIF). This work is motivated by the need to develop alternative mix diagnostics, radiochemistry being the focus here. The target is composed of a 65/35 at. % deuterium–tritium (DT) fill surrounded by a roughly 30 μm thick beryllium ablator. The inner portion of the beryllium ablator is doped with 10 at. % of 10B. Radiation-hydrodynamics calculations were performed in 1D to optimize both the remaining boron rho-R and the DT neutron yield. A charged-particle transport post-processor has been developed to study α-induced reactions on the ablator material. Results indicate a large 13N production from α-induced reactions on 10B, measurable by the radiochemical analysis of a gaseous samples system at the NIF. The PDXP target N201115-001 was successfully fielded on the NIF, and nitrogen from the 10B(α,n)13N reaction was measured. The 13N production yield, as well as the DT neutron yield, was, however, lower than expected. Some of the reduced yields can be explained by the oblate shape, but the ratios of the various radiochemical signals are not commensurate with expectations based on a simple reduction of the 1D results. Preliminary 2D radiation-hydrodynamics computations are consistent with the experimental measurements, and work is ongoing to extend the radiochemistry analysis into higher dimensions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Microcalorimeter measurement of x-ray spectra from a high-temperature magnetically confined plasma

A NASA-built x-ray microcalorimeter spectrometer has been installed on the MST facility at the Wisconsin Plasma Physics Laboratory and has recorded x-ray photons emitted by impurity ions of aluminum in a majority deuterium plasma. Much of the x-ray microcalorimeter development has been driven by the needs of astrophysics missions, where imaging arrays with few-eV spectral resolution are required. The goal of our project is to adapt these single-photon-counting microcalorimeters for magnetic fusion energy research and demonstrate the value of such measurements for fusion science. Microcalorimeter spectrometers combine the best characteristics of the x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer and the broadband coverage of an x-ray pulse height analysis system. Fusion experiments are increasingly employing high-Z plasma-facing components and require measurement of the concentration of all impurity ion species in the plasma. This diagnostic has the capability to satisfy this need for multi-species impurity ion data and will also contribute to measurements of impurity ion temperature and flow velocity, Zeff, and electron density. Here, we introduce x-ray microcalorimeter detectors and discuss the diagnostic capability for magnetic fusion energy experiments. We describe our experimental setup and spectrometer operation approach at MST, and we present the results from an initial measurement campaign.

Eckart, M. E. (ORCID:0000000338945889)↗

Yield requirements for studies of double (n,2n) reactions on dopant material in NIF Capsules

At the National Ignition Facility (NIF) inertial confinement fusion (ICF) ignition has been achieved, involving yields up to about 5 MJ. In addition, it has been suggested that proposed advances at the NIF facility, involving moderate upgrades in laser energy, could further increase fusions gains to yields in the 30-40 MJ range. Such high yield would open the possibility for new studies in many areas of science. In the present analysis, we focus on how these possible future fusion yields would make measurements of multi-step nuclear reactions on dopant material practical. For this, we consider the scenario of loading dopant material into the ablator of cryogenic capsules and estimate yields needed to measure previously inaccessible double (n,2n) reactions under ICF conditions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Point containment algorithms for constructive solid geometry with unbounded primitives

Here, we present several algorithms for evaluating point containment in constructive solid geometry (CSG) trees with unbounded primitives. Three algorithms are presented based on postfix, prefix, and infix notations of the CSG binary expression tree. We show that prefix and infix notations enable short-circuiting logic, which reduces the number of primitives that must be checked during point containment. To evaluate the performance of the algorithms, each algorithm was implemented in the OpenMC Monte Carlo particle transport code, which relies on CSG to represent solid bodies through which subatomic particles travel. Two sets of tests were carried out. First, the execution time to generate a rasterized image of a 2D slice of three CSG models of varying complexity was measured. Use of both prefix and infix notations offered significant speedup over the postfix notation that has traditionally been used in particle transport codes, with infix resulting in a 6 x reduction in execution time relative to postfix for a model of a tokamak fusion device. We then measured the execution time of neutron transport simulations of the same three models using each of the algorithms. The results and performance improvements reveal the same trends as for the rasterization test, with a 5.52 x overall speedup using the infix notation relative to the original postfix notation in OpenMC for the tokamak model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Doppler-Free Saturation Spectroscopy (DFSS) for Magnetic and Electric Field Measurements in an FRC plasma

The Cooperative Research and Development Agreement (CRADA) between Oak Ridge National Laboratory (ORNL) and TAE Technologies (TAE) focused on a feasibility study of Doppler-free saturation spectroscopy (DFSS) as a diagnostic strategy to measure the internal magnetic and electric fields of TAE’s C-2W field-reversed configuration (FRC) fusion device. Non-perturbative measurements of magnetic and electric fields in the core of FRCs can provide critical information needed to constrain equilibrium models and for the continued advancement of the fusion research program at TAE. DFSS is a well-established, laser-based absorption spectroscopy technique used to suppress inhomogeneous (Doppler) broadening linewidths and reveal Zeeman/Stark split hyperfine structures of target emission lines. The collaboration involved the use of ORNL’s suite of experimentally validated quantum mechanical modelling codes to simulate theoretical atomic spectra for a variety of machine parameters and laser beam configurations. TAE’s expertise was heavily used to provide measurements/models of the required machine parameters needed as inputs to the simulations. Results of an all-encompassing error analysis demonstrate the favourable capabilities of the diagnostic and provided invaluable insights as to how DFSS can be best used to accurately measure internal B/E-fields in C-2W.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Primary yields of protons measured using CR-39 in laser-induced deuteron–deuteron fusion reactions

The deuteron and deuteron (DD) fusion reactions is the most crucial reaction in the nuclear astrophysics and inertial confinement fusion, which is responsible for synthesizing chemical elements, and for powering the stellar evolution. The plasma environment generated by the high-intensity laser used in a DD fusion reaction is similar to the early stage of the Big Bang. Therefore, it is important to study laser-induced DD fusion reactions. Recently, we experimentally investigated such reactions using eight laser beams with the third harmonic impact on a deuterated polyethylene target at the ShenGuang-II Upgrade facility. In this work, we focused on the application of range-filter spectrometers assembled from a 70 μm aluminum filter and two CR-39 nuclear track detectors to measure the yields of primary DD protons. Based on the track diameter calibration results of 3 MeV protons used to diagnose the tracks on the range- filter spectrometers, we measured a primary DD proton yield of approximately (8:52 ± 1:7) × 10 6 , consistent with the yields from similar laser facilities worldwide, indicating that range-filter spectrometry is an effective way to measure primary DD protons. Furthermore, using other accurate detectors may help to measure the rare events of D 3 He protons.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Magnetic Field Vector Measurements Using Doppler-Free Saturation Spectroscopy

Experimentally measuring the magnetic-field vector topology in low-cost fusion-concept devices will provide critical information required to optimize and accelerate the development of those concepts for economical fusion energy. The magnetic-field measurement techniques most commonly used in modern tokamaks, i.e., beam-emission spectroscopy and optical emission spectroscopy, are not suitable below 5000 Gauss due to a lack of sensitivity. To diagnose magnetic field relevant to some of the ARPA-E-supported fusion concepts (50 to 1000 Gauss), a novel approach is required. Doppler-free saturation spectroscopy (DFSS) is a pump/probe laser-based technique and has been successfully demonstrated in the laboratory to provide a magnetic-field measurement accuracy of <10 Gauss. DFSS is implemented to obtain ultra-high-resolution spectra at a localized point in space. The spectral data is then fit to the Schrödinger equation for determination of the magnetic-field vector. To obtain the magnetic-field topology, the measurement spatial location is scanned over the desired region of space by steering the DFSS pump laser beam. The goal of this project is to assemble and commission a portable DFSS diagnostic capable of 2-D magnetic-field vector measurements utilizing the Hα spectral line profile.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Tomographic Reconstruction of the Neutron Time-Energy Spectrum from a Dense Plasma Focus

A dense plasma focus (DPF) nuclear fusion device is an attractive pulsed neutron source in many applications, due to its relatively large neutron yield, produced in a short time duration. In order to design a DPF that generates neutrons within a specified time profile, or generates a neutron energy spectrum with specific properties, it is necessary to be able to characterize and model the results of the fusion process releasing the neutrons. The time-energy spectrum of fusion neutrons is an ideal quantity to use to validate multiphysics codes that simulate the pinch and fusion processes, because it is a quantity that requires the physics of each of the stages leading up to and ending in the fusion reaction to be simulated correctly. In particular, since DPF fusion neutrons are not monoenergetic -- and there can often be several fusion pinches creating neutrons -- a computer simulation matching high quality neutron spectrum measurements provides great confidence in the fidelity of the simulation. In order to make such a comparison, it is first necessary to have quality measurements from which to infer the spectrum. In this work we pose neutron spectroscopy as the classical tomographic inverse problem from neutron time of flight data at multiple distances, but enhanced by using an additional measurement of the time profile of the fusion pinch near its source and by using detector pairs set up in a geometry that allows for scatter background subtraction. The detector pairs enhance the quality of the time of flight measurements, and the additional constraint posed by the measured time profile allows for reconstructions discretized as finely as the time measurements and in energy as finely as 100 keV, without the problem being underdetermined. We present results from a Deuterium-fueled DPF at the U.S. Department of Energy's Nevada National Security Site and show that we can infer the time-energy spectrum from our measurements for both single and multi-pinch fusion reactions with equal fidelity.

Catenacci, Jared↗

X-ray continuum spectroscopy of inertial confinement fusion implosions at the National Ignition Facility

A methodology for measuring x-ray continuum spectra of inertial confinement fusion (ICF) implosions is described. The method relies on the use of ConSpec, a high-throughput spectrometer using a highly annealed pyrolytic graphite crystal , which measures the spectra in the ≃20–30 keV range. Due to its conical shape, the crystal is sagittally focusing a Bragg-reflected x-ray spectrum into a line, which enhances the recorded x-ray emission signal above the high neutron-induced background accompanying ICF implosions at the National Ignition Facility. To improve the overall measurement accuracy, the sensitivity of the spectrometer measured in an off-line x-ray laboratory setting was revised. The error analysis was expanded to include the accuracy of the off-line measurements, the effect of the neutron-induced background, as well as the influence of possible errors in alignment of the instrument to the ICF target. Here we demonstrate how the improved methodology is applied in the analysis of ConSpec data with examples of a relatively low-neutron-yield implosion using a tritium–hydrogen–deuterium mix as a fuel and a high-yield deuterium–tritium (DT) implosion producing high level of the background. In both cases, the shape of the measured spectrum agrees with the exponentially decaying spectral shape of bremsstrahlung emission to within ±10%. In the case of the high-yield DT experiment, non-monotonic deviations slightly exceeding the measurement uncertainties are observed and discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Temperature distributions and gradients in laser-heated plasmas relevant to magnetized liner inertial fusion

We present two-dimensional temperature measurements of magnetized and unmagnetized plasma experiments performed at Z relevant to the preheat stage in magnetized liner inertial fusion. The deuterium gas fill was doped with a trace amount of argon for spectroscopy purposes, and time-integrated spatially resolved spectra and narrow-band images were collected in both experiments. The spectrum and image data were included in two separate multiobjective analysis methods to extract the electron temperature spatial distribution T e ( r , z ) . The results indicate that the magnetic field increases T e , the axial extent of the laser heating, and the magnitude of the radial temperature gradients. Comparisons with simulations reveal that the simulations overpredict the extent of the laser heating and underpredict the temperature. Temperature gradient scale lengths extracted from the measurements also permit an assessment of the importance of nonlocal heat transport.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗