Inertial Confinement Fusion Research Research Articles

Data-driven prediction of scaling and ignition of inertial confinement fusion experiments

Recent advances in inertial confinement fusion (ICF) at the National Ignition Facility (NIF), including ignition and energy gain, are enabled by a close coupling between experiments and high-fidelity simulations. Neither simulations nor experiments can fully constrain the behavior of ICF implosions on their own, meaning pre- and postshot simulation studies must incorporate experimental data to be reliable. Linking past data with simulations to make predictions for upcoming designs and quantifying the uncertainty in those predictions has been an ongoing challenge in ICF research. We have developed a data-driven approach to prediction and uncertainty quantification that combines large ensembles of simulations with Bayesian inference and deep learning. The approach builds a predictive model for the statistical distribution of key performance parameters, which is jointly informed by past experiments and physics simulations. The prediction distribution captures the impact of experimental uncertainty, expert priors, design changes, and shot-to-shot variations. We have used this new capability to predict a 10× increase in ignition probability between Hybrid-E shots driven with 2.05 MJ compared to 1.9 MJ, and validated our predictions against subsequent experiments. We describe our new Bayesian postshot and prediction capabilities, discuss their application to NIF ignition and validate the results, and finally investigate the impact of data sparsity on our prediction results.

Transient absorption of warm dense matter created by an X-ray free-electron laser

AbstractWarm dense matter is at the boundary between a plasma and a condensed phase and plays a role in astrophysics, planetary science and inertial confinement fusion research. However, its electronic structure and ionic structure upon irradiation with strong laser pulses remain poorly understood. Here, we use an intense and ultrafast X-ray free-electron laser pulse to simultaneously create and characterize warm dense copper using L-edge X-ray absorption spectroscopy over a large irradiation intensity range. Below a pulse intensity of 1015 W cm−2, an absorption peak below the L edge appears, originating from transient depletion of the 3d band. This peak shifts to lower energy with increasing intensity, indicating the movement of the 3d band upon strong X-ray excitation. At higher intensities, substantial ionization and collisions lead to the transition from reverse saturable absorption to saturable absorption of the X-ray free-electron laser pulse, two nonlinear effects that hold promise for X-ray pulse-shaping. We employ theoretical calculations that combine a model based on kinetic Boltzmann equations with finite-temperature real-space density-functional theory to interpret these observations. The results can be used to benchmark non-equilibrium models of electronic structure in warm dense matter.

High-brightness betatron emission from the interaction of a sub picosecond laser pulse with pre-ionized low-density polymer foam for ICF research

Direct laser acceleration (DLA) of electrons in plasmas of near-critical density (NCD) is a very advancing platform for high-energy PW-class lasers of moderate relativistic intensity supporting Inertial Confinement Fusion research. Experiments conducted at the PHELIX sub-PW Nd:glass laser demonstrated application-promising characteristics of DLA-based radiation and particle sources, such as ultra-high number, high directionality and high conversion efficiency. In this context, the bright synchrotron-like (betatron) radiation of DLA electrons, which arises from the interaction of a sub-ps PHELIX laser pulse with an intensity of 1019 W/cm2 with pre-ionized low-density polymer foam, was studied. The experimental results show that the betatron radiation produced by DLA electrons in NCD plasma is well directed with a half-angle of 100–200 mrad, yielding (3.4 ± 0.4)·1010 photons/keV/sr at 10 keV photon energy. The experimental photon fluence and the brilliance agree well with the particle-in-cell simulations. These results pave the way for innovative applications of the DLA regime using low-density pre-ionized foams in high energy density research.

Charged particle transport coefficient challenges in high energy density plasmas

High energy density physics (HEDP) and inertial confinement fusion (ICF) research typically relies on computational modeling using radiation-hydrodynamics codes in order to design experiments and understand their results. These tools, in turn, rely on numerous charged particle transport and relaxation coefficients to account for laser energy absorption, viscous dissipation, mass transport, thermal conduction, electrical conduction, non-local ion (including charged fusion product) transport, non-local electron transport, magnetohydrodynamics, multi-ion-species thermalization, and electron-ion equilibration. In many situations, these coefficients couple to other physics, such as imposed or self-generated magnetic fields. Furthermore, how these coefficients combine are sensitive to plasma conditions as well as how materials are distributed within a computational cell. Uncertainties in these coefficients and how they couple to other physics could explain many of the discrepancies between simulation predictions and experimental results that persist in even the most detailed calculations. This paper reviews the challenges faced by radiation-hydrodynamics in predicting the results of HEDP and ICF experiments with regard to these and other physics models typically included in simulation codes.

Ab initio path integral Monte Carlo simulations of warm dense two-component systems without fixed nodes: Structural properties.

We present extensive new abinitio path integral Monte Carlo (PIMC) results for a variety of structural properties of warm dense hydrogen and beryllium. To deal with the fermion signproblem-an exponential computational bottleneck due to the antisymmetry of the electronic thermal density matrix-we employ the recently proposed [Y. Xiong and H. Xiong, J. Chem. Phys. 157, 094112 (2022); T. Dornheim et al., J. Chem. Phys. 159, 164113 (2023)] ξ-extrapolation method and find excellent agreement with the exact direct PIMC reference data where available. This opens up the intriguing possibility of studying a gamut of properties of light elements and potentially material mixtures over a substantial part of the warm dense matter regime, with direct relevance for astrophysics, material science, and inertial confinement fusion research.

Theoretical and experimental study on spatial characteristics of a large imaging area streak camera developed for ICF precise diagnostics

The Modulation Transfer Function (MTF) offers an objective measure to assess the imaging performance of streak cameras. This study aims to enhance the diagnostic precision of streak camera used in laser-driven inertial confinement fusion research. We introduce a novel method for evaluating the MTF of a recently developed large imaging area streak tube. This method integrates the electron optical MTF theory and the production of a reticle plate-type photocathode. Using ultraviolet and femtosecond lasers, we established two calibration platforms for the streak camera in both static and dynamic modes. We measured the radius of the electrons reaching fluorescent screen and obtained MTFs. Our findings reveal that the ultimate spatial resolutions at off-axis distances of 1.2, 4.7, 8.2, and 11.7mm are 28.5, 27.2, 26.4, 24.9 lp/mm (static) and 25.0, 22.8, 23.0, 21.2 lp/mm (dynamic). Notably, we successfully charted the ultimate spatial resolution distributions across the streak camera's full imaging area in both modes.

Bound-State Breaking and the Importance of Thermal Exchange-Correlation Effects in Warm Dense Hydrogen.

Hydrogen at extreme temperatures and pressures is of key relevance for cutting-edge technological applications, with inertial confinement fusion research being a prime example. In addition, it is ubiquitous throughout our universe and naturally occurs in a variety of astrophysical objects. In the present work, we present exact ab initio path integral Monte Carlo (PIMC) results for the electronic density of warm dense hydrogen along a line of constant degeneracy across a broad range of densities. Using the well-known concept of reduced density gradients, we develop a new framework to identify the breaking of bound states due to pressure ionization in bulk hydrogen. Moreover, we use our PIMC results as a reference to rigorously assess the accuracy of a variety of exchange-correlation (XC) functionals in density functional theory calculations for different density regions. Here, a key finding is the importance of thermal XC effects for the accurate description of density gradients in high-energy-density systems. Our exact PIMC test set is freely available online and can be used to guide the development of new methodologies for the simulation of warm dense matter and beyond.

Hybrid optimization of laser-driven fusion targets and laser profiles

Quasi-isentropic compression is an effective method to achieve high-density and high-temperature implosion in laser-driven inertial confinement fusion (ICF). However, it requires precise matching between the laser profile and the target structure. Designing the optimal laser profile and the corresponding target for ICF is a challenge due to the large number of parameters involved. In this paper, we present a novel method that combines random walk and Bayesian optimization. The basic sampling data for Bayesian optimization are a series of laser pulse profiles and target structures that can produce relatively high areal densities obtained by the random walk method. This approach reduces the number of samples required for Bayesian optimization and mitigates low efficiency in the latter stages of the random walk method. The method also reduces the randomness in the optimization process and enhances the optimization efficiency. It should have important applications in ICF research.

Surface morphology and oxidation evolution of glow discharge polymer capsules during tumble polishing

Glow discharge polymer (GDP) capsules are often used as the ablation layer in laser inertial confinement fusion (ICF) research, because of its excellent properties in low X-ray opacity, high tensile strength, and high thermal conductivity. The isolated surface defects and distribution of oxygen content on the surface of GDP capsules can lead to the increase of hydrodynamic instability and the decrease of compression efficiency. In this work, by using media spheres and silica nanoparticles, a tumble polishing device for GDP capsule polishing is built to improve the surface morphology. The effects of polishing parameters on the surface roughness and oxygen content of GDP were investigated. The test results show that tumble polishing can effectively remove convex defects, only 24 identified defects with a height below 231 nm were found after 144 h polishing, reduce the surface roughness (∼10 nm) and improve the surface quality of GDP capsules. Moreover, the oxygen content distribution on the surface of GDP is uniform after tumble polishing. And the depth analysis results show that the tumble polishing has no obvious effect on the change of oxygen content in GDP shell under the surface layer.

Research into CUP-VISAR velocity reconstruction based on weighted DRUNet and total variation joint optimization.

This Letter proposes a CUP-VISAR data reconstruction algorithm for laser-driven inertial confinement fusion (ICF) research. The algorithm combines weighted deep residual U-Net (DRUNet) and joint optimization with total variation (TV) to improve shockwave velocity fringe image reconstruction. The simulation results demonstrate that the proposed algorithm outperforms the ADMM-TV and enhanced 3D total variation (E-3DTV) algorithms, enhancing the quality of the reconstructed images and thereby improving the accuracy of velocity field calculations. Furthermore, it addresses the challenges of the high compression ratio caused by the diagnostic requirements of the larger number of sampling frames in the CUP-VISAR system and the issues of aliasing within a large encoding aperture. The proposed algorithm demonstrates good robustness to noise, ensuring reliable reconstruction even under Gaussian noise with a relative intensity of 0.05. This algorithm contributes to ICF diagnostics in complex environmental conditions and has theoretical significance and practical application value for achieving controlled thermonuclear fusion.

Prediction of superionic state in LiH2 at conditions enroute to nuclear fusion

Hydrogen and lithium, along with their compounds, are crucial materials for nuclear fusion research. High-pressure studies have revealed intricate structural transitions in all these materials. However, research on lithium hydrides beyond LiH has mostly focused on the low-temperature regime. Here, we use density functional theory and ab initio molecular dynamics simulations to investigate the behavior of LiH2, a hydrogen-rich compound, near its melting point. Our study is particularly relevant to the low-pressure region of the compression pathway of lithium hydrides toward fusion. We discovered a premelting superionic phase transition in LiH2 that has significant implications for its mass transportation, elastic properties, and sound velocity. The theoretical boundary for the superionic transition and melting temperature was then determined. In contrast, we also found that the primary compound of lithium hydrides, LiH, does not exhibit a superionic transition. These findings have important implications for optimizing the compression path to achieve the ignition condition in inertial confinement fusion research, especially when lithium tritium-deuteride (LiTD) is used as the fuel.

Focused Energy, A New Approach Towards Inertial Fusion Energy

Focused Energy is a new startup company with the goal of developing laser-driven inertial fusion energy for electrical power production. The company combines the results from decades of fundamental research in inertial confinement fusion at universities and national laboratories with the flexibility and the speed of a startup company. Focused Energy has chosen the direct-drive, proton fast ignition approach to reach ignition, burn and high gain as the most promising approach. Located in Austin/US and Darmstadt/Germany, supported by the science community and private investment Focused Energy is paving the way to inertial fusion energy combining the best skill set and state-of-the-art technology from both sides of the Atlantic Ocean. In this paper we discuss the details and reasoning for the approach and the technical directions we have chosen. We will outline our roadmap for getting to a fusion pilot plant in the mid to late 2030s.

Polishing and Local Planarization High-Precision HDC Capsules by Four-Cup-Type Technology for Inertial Confinement Fusion

In inertial confinement fusion (ICF) experiments, high-density carbon (HDC) is being evaluated as an alternative to the current point-design ablator material (glow discharge plasma) due to its high density and optimal opacity, which leads to a higher energy efficiency and implosion stability. Chemical vapor deposition–coated HDC capsules have a near-perfect surface figure but a microscopically rough surface, so polishing is needed to achieve the required nanometer surface finish. Herein, HDC capsule polishing is investigated with modified four-cup-type polishing technology. The surface morphology, microstructures, and wall thicknesses of the polished capsules were examined by multiple techniques, such as an optical microscope, scanning electron microscope, X-ray radiography, and so on. The results show that the HDC capsules can be polished to a surface roughness less than 15 nm and a wall thickness nonuniformity of about 0.5 μm. The Raman spectra indicated that four-cup polishing had no obvious influence on the original surface crystallinity and phase composition of the HDC capsules. The crystallographic of the HDC capsules with different four-cup polishing times had no deterioration. This work plays an important role for the application of HDC capsules in ICF research.

Gamma-ray measurements for inertial confinement fusion applications

High-energy γ rays generated from inertial confinement fusion (ICF) experiments have become an important signature for studying the dynamics of implosion processes. Due to their high-energy and penetrating nature, γ rays are the most unperturbed fusion products, which can preserve the original birth information of the fusion process. Fusion γ rays provide a direct measure of nuclear reaction rates (unlike x rays) without being compromised by Doppler spreading (unlike neutrons). However, unambiguous γ-ray measurements for ICF study further required a decade-long period of technological development, which included a deepening understanding of fusion γ-ray characteristics and innovations in instrument performance. This review article introduces the production mechanism of the prompt and secondary γ rays and various ICF performance parameters (e.g., bang time and burn width), which can be derived from γ-ray measurement. A technical overview will be followed by summarizing γ-ray detectors fielded or proposed, especially for high-yield ICF experiments at the Omega Laser Facility and National Ignition Facility. Over the past few years, γ-ray diagnostic technologies have been extended beyond ICF research. A few examples of non-ICF applications of γ-ray detectors are introduced at the end of this article.

Suppression of stimulated Brillouin scattering by multicolor alternating-polarization bundle light in inertial confinement fusion

This study proposes a novel method to mitigate stimulated Brillouin scattering (SBS) using multicolor alternating-polarization bundle light. The bundle light combines multiwavelength, spike trains of uneven duration and delay for a single beam to multicolor alternating polarization for bundle beams. SBS suppression is verified using a three-dimensional large-scale laser plasma code. The numerical results show that the SBS reflectivity can be decreased by nearly two orders in low density plasma. The proposed method can extend the repetition time of a single beam from several picoseconds to tens of picoseconds. Moreover, it has potential applications in inertial confinement fusion research.

Invited article: X-ray phase contrast imaging in inertial confinement fusion and high energy density research.

X-ray phase contrast imaging (XPCI) provides enhanced image contrast beyond absorption-based x-ray imaging alone due to refraction and diffraction from gradients in the object material density. It is sensitive to small variations in density, such as internal voids, cracks, grains, defects, and material flow, as well as to stronger density variations such as from a shock wave. Beyond its initial use in biology and materials science, XPCI is now routinely used in inertial confinement fusion (ICF) and high energy density (HED) research, first to characterize ICF capsules and targets, and later applied in dynamic experiments, where coherent x-ray sources, ultrafast x-ray pulses, and high temporal and spatial resolution are required. In this Review article, XPCI image formation theory is presented, its diverse use in ICF and HED research is discussed, the unique requirements for ultrafast XPCI imaging are given, as well as current challenges and issues in its use.

Imaging velocity interferometer system for any reflector (VISAR) diagnostics for high energy density sciences.

Two variants of optical imaging velocimetry, specifically the one-dimensional streaked line-imaging and the two-dimensional time-resolved area-imaging versions of the Velocity Interferometer System for Any Reflector (VISAR), have become important diagnostics in high energy density sciences, including inertial confinement fusion and dynamic compression of condensed matter. Here, we give a brief review of the historical development of these techniques, then describe the current implementations at major high energy density (HED) facilities worldwide, including the OMEGA Laser Facility and the National Ignition Facility. We illustrate the versatility and power of these techniques by reviewing diverse applications of imaging VISARs for gas-gun and laser-driven dynamic compression experiments for materials science, shock physics, condensed matter physics, chemical physics, plasma physics, planetary science and astronomy, as well as a broad range of HED experiments and laser-driven inertial confinement fusion research.

Microforce Sensing and Flexible Assembly Method for Key Parts of ICF Microtargets

Microassembly is one of the key techniques in various advanced industrial applications. Meanwhile, high success rates for axial hole assembly of thin-walled deep-cavity-type items remain a challenging issue. Hence, the flexible assembly approach of thin-walled deep-cavity parts is investigated in this study using the assembly of the key components, the microtarget component TMP (thermomechanical package) and the hohlraum in ICF (inertial confinement fusion) research, as examples. A clamping force-assembly force mapping model based on multisource microforce sensors was developed to overcome the incapacity of microscopic vision to properly identify the condition of components after contact. The ICF microtarget flexible assembly system, which integrates multisource microforce sensing and a six degrees of freedom micromotion sliding table, is presented to address the constraint that the standard microassembly approach is difficult to operate once the parts contact. This method can detect contact force down to the mN level, modify deviation of the component posture efficiently, and achieve nondestructive ICF microtarget assembly at the end.

Demonstration of a spherical plasma mirror for the counter-propagating kilojoule-class petawatt LFEX laser system.

A counter-propagating laser-beam platform using a spherical plasma mirror was developed for the kilojoule-class petawatt LFEX laser. The temporal and spatial overlaps of the incoming and redirected beams were measured with an optical interferometer and an x-ray pinhole camera. The plasma mirror performance was evaluated by measuring fast electrons, ions, and neutrons generated in the counter-propagating laser interaction with a Cu-doped deuterated film on both sides. The reflectivity and peak intensity were estimated as ∼50% and ∼5 × 1018 W/cm2, respectively. The platform could enable studies of counter-streaming charged particles in high-energy-density plasmas for fundamental and inertial confinement fusion research.

Evidence for Dissociation and Ionization in Shock Compressed Nitrogen to 800GPa.

Triple bonding in the nitrogen molecule (N_{2}) is among the strongest chemical bonds with a dissociation enthalpy of 9.8 eV/molecule. Nitrogen is therefore an excellent test bed for theoretical and numerical methods aimed at understanding how bonding evolves under the influence of the extreme pressures and temperatures of the warm dense matter regime. Here, we report laser-driven shock experiments on fluid molecular nitrogen up to 800GPa and 4.0 g/cm^{3}. Line-imaging velocimetry measurements and impedance matching method with a quartz reference yield shock equation of state data of initially precompressed nitrogen. Comparison with numerical simulations using path integral MonteCarlo and density functional theory molecular dynamics reveals clear signatures of chemical dissociation and the onset of L-shell ionization. Combining data along multiple shock Hugoniot curves starting from densities between 0.76 and 1.29 g/cm^{3}, our study documents how pressure and density affect these changes in chemical bonding and provides benchmarks for future theoretical developments in this regime, with applications for planetary interior modeling, high energy density science, and inertial confinement fusion research.