Inertial Confinement Fusion Experiments Research Articles

Characterization of strongly coupled plasmas produced in argon supercritical fluids

Strongly coupled plasmas (SCPs) exist in various places throughout the Universe, examples of which are inertial confinement fusion experiments, Jovian planets’ cores, neutron stars, and white dwarf stars. In recent decades, theoretical and numerical studies have been pursued to characterize the equation of states and thermodynamic properties of SCPs, which are fundamentally different from those of weakly coupled plasmas. One of the essential research topics is energy transport by radiation or opacity. In particular, in a subcritical medium at a low temperature, condensation renders the medium inhomogeneous, which significantly affects the radiation transport or opacity. However, no study has been conducted for opacity in inhomogeneous supercritical fluids (SCFs). A recent study reveals that an inhomogeneous SCF with nanometer-sized clusters and micrometer-sized droplets can be prepared. Here, we experimentally demonstrate that the emission timescale of an SCP in an inhomogeneous SCF is extended by up to 50% compared to that in a homogeneous SCF. This implies that the inhomogeneity of the SCF significantly enhances the photon confinement. This result is expected to draw interest in the investigation of radiation transport or opacity in the inhomogeneous SCF. A better understanding will lead to a method for increasing the brightness and light emission time from a dense plasma.

Ultrasonic manipulation for precise positioning and equidistant transfer of inertial confinement fusion microspheres

As the thermonuclear fuel container, the inertial confinement fusion (ICF) microspheres must be detected before the ICF experiment to maximize the profits of fusion reactions. However, in the current detection method, the ICF microsphere is in direct contact with the measurement platform, resulting in the ICF microsphere surface being easily damaged during the detection process. In this paper, an ultrasonic manipulation method is proposed, realizing non-destructive, high-precision, and high-efficient manipulation of the ICF microsphere by switching the two acoustic fields produced in the liquid. When detecting the ICF microsphere, the first acoustic field (1st AF) accurately traps the microsphere in the acoustic field center to achieve its precise positioning. And when the ICF microsphere is failed to pass the detection, it is transferred out of the microscope measurement area by switching to the second acoustic field (2nd AF). Two solid vibration modes, their corresponding acoustic fields, and the two acoustic streaming fields are first computed by the finite element method. Then, the manipulation experiments indicated that the ICF microspheres can be first driven to the center of the 1st AF and then be positioned here with a minimum positional fluctuation of 1.1 μm. By changing to the 2nd AF, the positioned microsphere can be transferred nearly 11 mm to the nearest antinode from the acoustic field center. Finally, based on the proposed ultrasonic manipulation method, the detection experiments of the ICF microsphere were carried out, illustrating that the positioning of the 1st AF meets the requirements of the morphology detection and the radius measurement of the ICF microsphere. The proposed method holds the advantages of non-destructive, high-precision, simple control scheme and meets the practical application needs of the microsphere fixed-point detection, presenting the potential promise for the field of microsphere detection.

High-resolution elliptical Kirkpatrick-Baez microscope for implosion higher-mode instability diagnosis.

High-resolution X-ray imaging diagnosis is a critical method for measuring Rayleigh-Taylor instability growth and hot spot interface morphology in inertial confinement fusion experiments. In this study, we develop a quasi-monochromatic elliptical Kirkpatrick-Baez microscope based on aberration theory, breaking the aberration limit of conventional Kirkpatrick-Baez microscopes. The microscope was characterized in the laboratory for spatial resolution performance and modulation transfer function before being implemented in cavity experiments at the SG-III prototype laser facility. The results demonstrate that the edge-based method achieves a spatial resolution of <2 µm in the central field of view and modulation of 800 lp/mm spatial frequency of >20%.

Hydroscaling indirect-drive implosions on the National Ignition Facility

A goal of the laser-based National Ignition Facility (NIF) is to increase the liberated fusion energy “yield” in inertial confinement fusion experiments well past the ignition threshold and the input laser energy. One method of increasing the yield, hydrodynamic scaling of current experiments, does not rely on improving compression or implosion velocity, but rather increases the scale of the implosion to increase hotspot areal density and confinement time. Indirect-drive (Hohlraum driven) implosions carried out at two target sizes, 12.5% apart, have validated hydroscaling expectations. Moreover, extending comparisons to the best-performing implosions at five different capsule sizes shows that their performance also agrees well with hydroscaling expectations even though not direct hydroscales of one another. In the future, by switching to a reduced loss Hohlraum geometry, simulations indicate that we can drive 20% larger-scale implosions within the current power and energy limitations on the NIF. At the demonstrated compression and velocity of these smaller-scale implosions, these 1.2× hydroscaled implosions should put us well past the ignition threshold.

Neutron backscatter edges as a diagnostic of burn propagation

High gain in hotspot-ignition inertial confinement fusion (ICF) implosions requires the propagation of thermonuclear burn from a central hotspot to the surrounding cold dense fuel. As ICF experiments enter the burning plasma regime, diagnostic signatures of burn propagation must be identified. In previous work [A. J. Crilly et al., Phys. Plasmas 27(1), 012701 (2020)], it has been shown that the spectral shape of the neutron backscatter edges is sensitive to the dense fuel hydrodynamic conditions. The backscatter edges are prominent features in the ICF neutron spectrum produced by the 180° scattering of primary deuterium–tritium fusion neutrons from ions. In this work, synthetic neutron spectra from radiation-hydrodynamics simulations of burning ICF implosions are used to assess the backscatter edge analysis in a propagating burn regime. Significant changes to the edge's spectral shape are observed as the degree of burn increases, and a simplified analysis is developed to infer scatter-averaged fluid velocity and temperature. The backscatter analysis offers direct measurement of the increased dense fuel temperatures that result from burn propagation.

Measurements of the temperature and velocity of the dense fuel layer in inertial confinement fusion experiments

The apparent ion temperature and mean velocity of the dense deuterium tritium fuel layer of an inertial confinement fusion target near peak compression have been measured using backscatter neutron spectroscopy. The average isotropic residual kinetic energy of the dense deuterium tritium fuel is estimated using the mean velocity measurement to be ∼103J across an ensemble of experiments. The apparent ion-temperature measurements from high-implosion velocity experiments are larger than expected from radiation-hydrodynamic simulations and are consistent with enhanced levels of shell decompression. These results suggest that high-mode instabilities may saturate the scaling of implosion performance with the implosion velocity for laser-direct-drive implosions.

Surface morphology and chemical microstructure of glow discharge polymer films prepared by plasma enhanced chemical vapor deposition at various Ar/H2 ratios

Glow discharge polymer were selected as ablators for inertial confinement fusion experiments. In order to improve the surface smoothness, Ar was added to the T2B/H2 plasma to prepare the glow discharge polymer film. The effects of Ar/H2 ratio on plasma properties, chemical bond structure and surface morphology of film were investigated. The results indicated that the density of hydrocarbon ions containing multi-carbon decreased gradually with the increase of Ar/H2 ratio. The degree of carbon-carbon crosslinking first increased and then decreased with the increase of Ar/H2 ratio. Within a certain range, an increase in Ar/H2 ratio led to a better surface morphology and fewer cauliflower-like defects. When too much Ar participated in the discharge, the surface morphology deteriorated again. These results indicated that the mechanism of Ar on the surface morphology of the glow discharge polymer films could be attributed to two physical effects: the aggregation of hydrocarbon groups in the plasma gas phase and the bombardment of the bonded hydrocarbon clusters in the films by high-energy ions and atoms during the process of film deposition.

Characterization of the solid deuterium layer in the inertial confinement fusion cryogenic target without the requirement of cryocooler deactivation

For the preparation of cryogenic fuel layer in inertial confinement fusion experiments, the vibration amplitude of cryogenic target must be reduced to the order of 1 μm or less to ensure the fuel layer to be distinguished by the X-ray phase contrast imaging system. An efficient vibration-reduction system with the Gifford-McMahon cryocooler mounted at the horizontal direction is designed, which is based on a two-stage floating thermal shielding and flexible connection. The obtained lowest temperature of the sample mount is less than 10 K, the temperature stability is within ±1 mK@18 K, the cooling power is greater than 500 mW, and the corresponding vibration amplitude is less than 1 μm. With this proposed method, the X-ray phase contrast imaging of cryogenic deuterium layer can be performed successively without requirement of cryocooler deactivation. Finally, the achieved resolution of the X-ray phase contrast imaging is about 1.5 μm while the contrast of solid-gas interface is larger than 5%, which is verified by the in-situ characterization of solidification of deuterium in the cryogenic target.

Suprathermal electrons from the anti-Stokes Langmuir decay instability cascade.

The study of parametric instabilities has played a crucial role in understanding energy transfer to plasma and, with that, the development of key applications such as inertial confinement fusion. When the densities are between 0.11n_{c}≲n_{e}≲0.14n_{c} and the electron temperature is in inertial confinement fusion-relevant temperatures, anomalous hot electrons with kinetic energies above 100keV are generated. Here a new electron acceleration mechanism-the anti-Stokes Langmuir decay instability cascade of forward stimulated Raman scattering-is investigated. This mechanism potentially explains anomalous energetic electron generation in indirectly driven inertial confinement fusion experiments, it also provides a new way of accelerating electrons to higher energy for applications such as novel x-ray sources.

Neural network surrogate models for absorptivity and emissivity spectra of multiple elements

The emissivity and absorptivity of radiation by matter are fundamental physical parameters required to adequately simulate inertial confinement fusion experiments. We present novel neural network models for predicting these interaction coefficients. This research extends the methods used by Kluth et al. (2020) that made similar predictions for a single chemical element, krypton. Our models are based on fully-connected or convolutional autoencoders coupled with a deep jointly-informed neural network (DJINN) to predict the emissivity and absorptivity of a given element and temperature/radiative field. We show that the previous work could not be directly extended to lower atomic number elements due to a thresholding effect on small values caused by a log10 transformation. Thus, in order to create a multi-element model, or even a single-element model, with low atomic number elements a different transformation is necessary. Utilization of a cube-root transform enables the creation of a multi-element model can achieve mean relative errors between 1% and 2%. Our work demonstrates that a single neural network can predict the results of atomic physics calculations, but additional work is necessary to consider mixtures of elements or a wider range of elements than those used in our study.

Cross-beam energy transfer in direct-drive ICF. I. Nonlinear and kinetic effects

Results are presented from a series of simulations examining the susceptibility of the cross-beam energy transfer (CBET) instability to nonlinear processes in the context of direct-drive inertial confinement fusion experiments on the OMEGA laser facility. These form the basis for the second paper of this series [A. G. Seaton, L. Yin, R. Follett, B. J. Albright, and A. Le, “Cross-beam energy transfer in direct-drive ICF. II. Theory and simulation of mitigation through increased laser bandwidth,” Phys. Plasmas 29, 042707 (2022)], where we examine the efficacy of increases in laser bandwidth at suppressing CBET. We choose laser and plasma conditions for the simulations that are favorable to CBET and promote nonlinearity. Through a comparison of outputs from the particle-in-cell code vector particle in cell (VPIC) and the linearized fluid code laser-plasma simulation environment (LPSE), a series of nonlinear effects have been identified in the kinetic simulations that include particle trapping, the two-ion wave decay, and ion-acoustic wave self-focusing. These effects produce time-dependent energy transfer, in contrast to the linearized fluid simulations in which a steady state is reached after an initial transient. Ion trapping is shown to allow for increased energy transfer relative to fluid simulations, with the remaining nonlinear processes acting to reduce the energy transfer. Nonlinear dynamics is contrasted for low- and high-intensity beams as well as between speckled and planar beams. For the parameters under consideration, beam profile has a significant effect on nonlinear dynamics, though the greatest sensitivity is to beam intensity.

Suppressing simulation bias in multi-modal data using transfer learning

Many problems in science and engineering require making predictions based on few observations. To build a robust predictive model, these sparse data may need to be augmented with simulated data, especially when the design space is multi-dimensional. Simulations, however, often suffer from an inherent bias. Estimation of this bias may be poorly constrained not only because of data sparsity, but also because traditional predictive models fit only one type of observed outputs, such as scalars or images, instead of all available output data modalities, which might have been acquired and simulated at great cost. To break this limitation and open up the path for multi-modal calibration, we propose to combine a novel, transfer learning technique for suppressing the bias with recent developments in deep learning, which allow building predictive models with multi-modal outputs. First, we train an initial neural network model on simulated data to learn important correlations between different output modalities and between simulation inputs and outputs. Then, the model is partially retrained, or transfer learned, to fit the experiments; a method that has never been implemented in this type of architecture. Using fewer than 10 inertial confinement fusion experiments for training, transfer learning systematically improves the simulation predictions while a simple output calibration, which we design as a baseline, makes the predictions worse. We also offer extensive cross-validation with real and carefully designed synthetic data. The method described in this paper can be applied to a wide range of problems that require transferring knowledge from simulations to the domain of experiments.

Species separation in polystyrene shock release evidenced by molecular-dynamics simulations and laser-drive experiments

Material shock release generally happens in the targets of high-energy-density (HED) and inertial confinement fusion (ICF) experiments but has been challenging to study experimentally, theoretically, or computationally. Here, we report extensive studies of polystyrene (CH) shock release by employing large-scale nonequilibrium molecular dynamics and laser-drive experiments at various shock strengths. Our experimental design prevents radiation preheating of the sample and employs a witness foil to investigate the release of shocked CH across a vacuum gap. We observe earlier acceleration of the foil by the release of CH under stronger shocks as well as reflectivity changes in the interferometry data before the foil moves, which is strong evidence of hydrogen streaming ahead of carbon at the release front, consistent with findings from our simulations. Furthermore, our calculations show that lighter species or hydrogen isotopes can carry more mass by one to two orders of magnitude to farther distances during the release and that only less than 0.1 times thermal expansion as predicted by hydrodynamics is needed to explain the high velocities and large scale lengths of low-density plasmas observed in radiation-preheated CH release experiments. These results highlight the significant role of species separation in the shock release of compounds. This process shall be considered, and its potential effects shall be clarified, in the design, interpretation, and analysis of future HED and ICF experiments.

Design, simulation, and experimental investigation on a novel multi-mode piezoelectric acoustofluidic device for ICF target manipulation

The Inertial Confinement Fusion (ICF) targets are hollow glass microspheres with strong viscosity, easy agglomeration, small diameter, and fragile structure. The size and morphology of the ICF target are crucial to the success of ICF experiment. To obtain qualified targets, the manual detection method and automatic detection systems are mainly employed. However, hard contact existed between the targets and the manipulation platform in both methods, which may cause target damage. To solve this issue, a novel multi-mode piezoelectric acoustofluidic manipulation device is proposed to achieve the non-contact manipulation of sub-millimeter size ICF targets during detection process. The proposed device mainly consists of a disk-shaped container and a four-transducer array. Standing and traveling vibration modes can be separately stimulated in the container when the four-transducer array is excited with a specific signal sequence. The modal simulation is first conducted to determine the dimensional parameters and required vibration modes. Furthermore, the acoustic streaming field simulation is used to verify the effectiveness of the modal simulation and interpret the manipulation mechanism. Then, the correctness of the simulation results is demonstrated through the experiments. In the experiments, the influences of the driving frequency, target diameter, and excitation voltage on the linear manipulation are investigated through an image recognition program, respectively. The target can be linearly manipulated, and has a maximum speed of 19.10 mm s−1 at 21.5 kHz. Furthermore, with the increase of the target diameter and excitation voltage, the speed of the target increases. Finally, the rotational manipulation of the targets are conducted, and the target can effectively rotate in the container at the driving frequency of 24.6 kHz. The proposed acoustofluidic manipulation device holds the merits of simple structure, low-frequency, multi-mode, and large particle manipulation ability, which may provide technical support for the detection and filter of ICF targets.

Novel designfor a polarizing DUV spectrometer using a Wollaston prism and its application as a diagnostic for measuring Thomson scattering data in the presence of strong self-emission backgrounds.

We present a novel designfor an optical spectrometer for use in ultraviolet Thomson scattering measurements of plasma parameters in high energy density (HED) inertial confinement fusion experiments on large-scale high-energy laser facilities. In experiments investigating high-Z plasmas, the fidelity of measurements is commonly limited by signal/background ratios approaching or exceeding unity. An alpha barium borate Wollaston prism can provide both spectral dispersion and polarization channel separation, allowing simultaneous measurement of both the Thomson scattering signal and plasma self-emission along a single line of sight and in a single experiment, which should greatly improve data quality and reduce the opportunity cost of taking high quality measurements. We present a basic discussion of the designand a worked example of an instrument designed to take fourth harmonic electron plasma wave measurements in HED experiments at the OMEGA laser facility.

Reduction of Cross-Beam Energy Transfer by a Speckle Pattern.

In this Letter, we show that cross-beam energy transfer (CBET), ubiquitous in inertial confinement fusion (ICF) experiments, may be strongly modified by the speckle pattern of the beams. This is demonstrated by the means of two-dimensional particle in cell simulations, supported by a linear model. In particular, we show that, although they would be the same in a plane wave model, the exchange rates of energy may be significantly different whether there is a plasma flow, or a wavelength shift, especially when the waves are weakly damped. When the crossed laser beams have different frequencies, the energy exchange rate is substantially reduced compared with the predictions of the plane wave model, widely used in the hydrodynamic codes that model and interpret ICF experiments. Such effects can partly explain the disagreement of the CBET predictions compared with experimental results.

Re-construction of a HPGe detector precise modeling for efficiency calibration

High purity germanium (HPGe) detector is a preferred choice for determining the activity of the radioactive samples for nuclear diagnostics of Inertial Confinement Fusion (ICF) experiments. Although the amounts of the radiochemical sample are limited, activity measurement at a close distance between the detector window and the radioactive source is a feasible method. Efficiency calibration of gamma rays at a close distance from the surface of a HPGe detector is crucial. Considering the problem of the coincidence at a close distance, the alternative way is to construct a precise model of the HPGe detector and then to calculate efficiency accurately at an arbitrary distance from the surface of the HPGe detector using Monte Carlo method. In this paper, internal geometry and structure except for dead layers of the HPGe detector is obtained by X-ray radiography and 3D reconstruction. The optimal dead layers of the germanium crystal are determined by tracing the minimal sum squared residual (SSR) of gamma-ray efficiencies between calculations and measurements for standard planar sources. Nonhomogeneous distribution of the dead layer is supposed according to the inner structure on the Ge crystal. The final results show that the corrected model improves the accuracy of the calculated efficiencies.

Coupling 1D xRAGE simulations with machine learning for graded inner shell design optimization in double shell capsules

Advances in machine learning provide the ability to leverage data from expensive simulations of high-energy-density experiments to significantly cut down on computational time and costs associated with the search for optimal target designs. This study presents an application of cutting-edge Bayesian optimization methods to the one-dimensional (1D) design optimization of double shell graded layer targets for inertial confinement fusion experiments. This investigation attempts to reduce hydrodynamic instabilities while retaining high yields for future NIF experiments. Machine learning methods can use predictive physics simulations to identify graded layer designs from within the vast design space that demonstrate high predicted performance, including novel designs with high uncertainty in performance that may hold unexpected promise. By applying machine learning tools to the simulation design, we map the trade-off between 1D yield and instability, specifically isolating parameter ranges, which maintain high performance while showing significantly improved Rayleigh–Taylor stability over the point design. The groundwork laid in this study will be a useful design tool for future NIF experiments with graded layer targets.

Microfluidic preparation of monodisperse hollow polyacrylonitrile microspheres for ICF

Hollow polyacrylonitrile (PAN) microspheres can be used as fuel carriers in inertial confinement fusion (ICF) experiments due to their high tensile strength, low permeability to hydrogen isotopes, and resistance to tritium radiation corrosion. Monodisperse oil in water in oil (O1/W/O2) compound droplets were fabricated by the microfluidic technique and then monodisperse hollow polyacrylonitrile microspheres were prepared by the solvent evaporation method. The stability of O1/W/O2 compound droplets was improved by the addition of the surfactant (Dow Corning 749), which reduced the interfacial tension between the O2 phase and the W phase as well as the Laplace pressure on the surface of the compound droplets. The sphericity and the surface quality of the microspheres were also improved by the introduction of DMF droplets into the O2 phase to avoid emulsification, decreasing the DMF mass transfer rate. Therefore, fine monodisperse PAN microspheres with the diameter range of 700–800 µm were successfully prepared. Moreover, the size relationship between the compound droplets and the resulting microspheres was also evaluated, which can be used to predict the sizes of the PAN microspheres. The results provide a method for controllable preparation of monodisperse hollow PAN microspheres for ICF experiments.

Collective stimulated Brillouin scattering with shared ion acoustic wave under the action of two overlapping laser beams

The overlapping of multiple beams is common in inertial confinement fusion (ICF), making the collective stimulated Brillouin scattering (SBS) with shared ion acoustic wave (IAW) potentially important because of the effectively larger laser intensities to drive the instability. In this work, based on a linear kinetic model, an exact analytic solution for the convective amplification of collective SBS with shared IAW stimulated by two overlapped beams is presented. From this solution, effects of the wavelength difference, crossing angle, polarization states, and finite beam overlapping volume of the two laser beams on the collective SBS modes with shared IAW are studied. It is found that a wavelength difference of several nanometers between the laser beams has negligible effects, except for a very small crossing angle about one degree. However, the crossing angle, beam polarization states, and finite beam overlapping volume can have significant influences. Furthermore, the out-of-plane modes, in which the wavevectors of daughter waves lie in different planes from the two overlapped beams, are found to be important for certain polarization states and crossing angles of the laser beams. This work is helpful to comprehend and estimate the collective SBS with shared IAW in ICF experiments.