Inertial Confinement Fusion Experiments Research Articles

Comparison of chamber beam geometry robustness to mispointing, imbalance and target offset for direct-drive laser fusion facilities

Abstract This study focuses on the optimization of beam chamber geometry designs for future direct-drive laser facilities. It provides a review of leading target chamber geometries, with a particular emphasis on random errors. Through comprehensive solid-sphere illuminations and analysis, we identify an optimized beam geometry design, highlighting its robustness and performance under realistic experimental conditions. Three major sources of random errors are evaluated, closely linked to experimental evaluations at OMEGA. The findings underscore the importance of optimizing the irradiation system alongside beam pattern considerations to enhance the efficiency and reliability of inertial confinement fusion experiments. We conclude that for a desired illumination uniformity of 1% in the presence of system errors, the split icosahedron design is the most robust. However, for a 0.3% uniformity goal, the charged-particle, icosahedron, and t-sphere methods exhibit similar performance.

Precision Polishing of Ablator Capsules via in situ Process Monitoring and Machine Learning–Based Optimization

In inertial confinement fusion (ICF) experiments seeking output gains of unity and beyond, the quality of the ablator capsule is paramount for minimizing the hydrodynamic mix that quenches the central hot spot. Defects in the form of foreign particles or missing mass on the surface and within the wall of the capsule are primary offenders. High-density carbon capsules made for ICF experiments at the National Ignition Facility are precision polished to achieve surface smoothness on the order of a few nanometers as well as to minimize isolated defects in the form of pits. Given the critical role of this process, we are developing smart manufacturing techniques with the goal of elevating the efficiency of this process. Our approach is to use MEMS (micro-electromechanical systems)–based sensors to capture the fine vibration signals generated during the polishing process and combine them with synchronized visual feedback as needed. Beyond using these sensors for process monitoring, we use specific deep learning methods to analyze the data and extract correlations with both the process parameters and the final performance of the polishing run. Here, we describe the multiple fronts we have explored in this regard and the results we have gotten so far. This approach promises to have the potential to ultimately provide real-time feedback that can be used to ensure the progress of the run as well as a means for faster optimization.

Time Lens Photon Doppler Velocimetry (TL-PDV) for extreme measurements

Capturing extreme surface velocities with >50 km/s dynamic range, which arise in shock physics such as inertial confinement fusion (ICF), is beyond the reach of conventional photon Doppler velocimetry (PDV) systems due to the need for extremely large electrical bandwidth under such conditions. The recent ignition in ICF calls for new velocimetry that can measure velocities exceeding 100 km/s. Time lens PDV (TL-PDV) is a solution where the high frequency beat signal from a conventional PDV system is periodically temporally magnified in the optical domain using a time lens. Here we experimentally demonstrate TL-PDV for the first time, validate the performance over a 74 km/s velocity range with high accuracy using a temporal magnification factor of 7.6, and verify excellent agreement with conventional PDV for laser driven micro-flyer experiments. TL-PDV currently provides the largest velocity dynamic range among PDV systems and is scalable to even higher velocities, which makes it an ideal candidate for material characterization under the most extreme conditions such as optimizing fuel efficiency in ICF experiments.

Ex situ calibration of the scattered-light time-history diagnostic on the National Ignition Facility.

The scattered-light time-history diagnostic (SLTD) suite measures time-resolved scattered light in three wavelength bands: stimulated Brillouin scattering (350-352nm), stimulated Raman scattering (430-760nm), and plasma emission at half the laser frequency (695-735nm), at 15 locations around the National Ignition Facility (NIF) target chamber. The SLTD, along with the full-aperture backscatter station (FABS), collects scattered light from direct- and indirect-drive inertial confinement fusion experiments. The SLTD calibration was revisited after a discrepancy between FABS and SLTD measurements was observed on NIF polar direct-drive [Skupsky etal., Phys. Plasmas 11, 2763 (2004)] experiments. An integrated calibration of the SLTD was performed for the first time, and individual components were also calibrated for the wavelengths of 351, 527, and 532nm. The optical transmission of the instrument was measured to be (1.12 ± 0.04) × 10-7 and (1.96 ± 0.11) × 10-7 for the wavelengths of 351 and 532nm, respectively. The revised calibration at 351nm brings the SLTD measured scattered energy in agreement with the FABS measured scattered energy after additionally accounting for the degradation of an optical element in FABS. This decreased the inferred absorption by 7% for a representative experiment. However, discrepancies remain between FABS and SLTD measurements in the SRS band (532nm).

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.

Splitting of double-core solid-in-water-in-oil droplet in a microfluidic Y-junction

Solid particle-encapsulated droplets have significant applications in biochemistry, advanced materials, and inertial confinement fusion (ICF) experiments. However, there is a problem of encapsulating two solid cores in a single droplet during the preparation of single-core droplets, which reduces the utilization efficiency. In this study, an effective microfluidic approach for continuous splitting of solid-in-water-in-oil droplets encapsulating double solid cores is developed. Visualization experiments are conducted to analyze the movements of solid cores and evolution of liquid–liquid interface during the splitting. The results show that the squeezing stage during the splitting process is shortened due to the presence of solid cores. The splitting mechanisms were also revealed by analyzing the interaction forces between the solid cores and aqueous phase. The force analysis of the aqueous phase showed that sum of squeezing and shear force could overcome the interfacial tension, ensuring the successful splitting of the double-core droplets. The force analysis of the solid cores revealed that the motion of the core could be divided into three typical stages: deceleration, hitting and separation. The combined effect of the aqueous phase, channel wall, and interfacial forces ensured the stable separation of the two solid cores. The length distribution of the daughter droplets exhibited excellent monodispersity. The microfluidic method proposed in this work would effectively improve the controlled preparation efficiency of solid-in-water-in-oil droplets.

Cu Aerogel with Low Density, High Purity, and Oxidation Resistance for Bright X‐Ray Backlight

AbstractCu aerogel is an ideal material as the X‐ray backlight target in laser diagnosis experiments owing to its porous structure and K‐shell X‐ray emission spectrum. However, because of their poor formability and oxidation resistance, the preparation of pure Cu aerogels with nanoscale particles and pores is still difficult. In this study, a Cu aerogel is fabricated by reducing nanoporous Cu‐based aerogels and assembling Cu nanoparticles. Numerous pores ranging from 1 to 10 nm are present in the Cu aerogel, resulting in a spongy Cu aerogel for integral heating. Meanwhile, a metal passivation process is integrated into the fabrication process to prevent the oxidation of the Cu aerogel. The prepared Cu aerogel is then applied to the Shenguang‐II facility, improving integral heating when irradiated under high‐energy laser pulse irradiation. Compared with the high‐purity Cu foil reference target, the Cu aerogel target emitted nearly 18% more X‐ray energy on the K‐shell, and the noise in the L‐shell spectrum of Cu aerogel is largely restrained. These results indicate that the Cu aerogel is beneficial for the laser diagnostic experiments and essential for the X‐ray radiography in inertial confinement fusion experiments.

Characterization of the upgraded single-line-of-sight time-resolved x-ray imager using short-pulse visible and UV lasers.

The single-line-of-sight time-resolved x-ray imager (SLOS-TRXI), a fast-gated x-ray imager used for capturing x-ray self-emission in inertial confinement fusion experiments on OMEGA, has been upgraded and characterized. SLOS-TRXI combines an electron-dilation imager and a hybrid complementary metal-oxide-semiconductor (hCMOS) sensor to capture multiple gated frames on a single line of sight with a temporal resolution of ∼40ps and a spatial resolution of 10 µm. The original hCMOS sensor with four frames was replaced with a newer-generation hCMOS sensor having eight frames. Gate characterizations of both the sensor and the entire SLOS-TRXI diagnostic were performed using ∼10-ps FWHM visible (2ω) and UV (4ω) short-pulse lasers, respectively. A stepped echelon was used to generate a train of five UV laser pulses having an interpulse separation of 30 ± 3ps. Characterization results of the hCMOS gating (2.28 ± 0.02-ns FWHM on average) and a temporal resolution of the upgraded SLOS-TRXI (34 ± 4-ps FWHM on average) are presented. A temporal magnification for the electron-dilation imager between 40 and 60 was inferred from the characterization results. The spatial resolution of the upgraded SLOS-TRXI remains the same in light of this work.

A compact and portable gamma-ray spectrometer (GRASP) for inertial confinement fusion and basic science experiments.

A compact and portable gamma-ray spectrometer has been designed to diagnose different components of the inertial confinement fusion-relevant γ-ray spectrum with energies between ∼3.7-17.9MeV. The system is designed to be as compact as possible for convenient transportation and fielding in diagnostic ports on the OMEGA laser, the National Ignition Facility, and other photon-source facilities. The system consists of a conversion foil for Compton scattering in front of four magnetic spectrometer "arms," each covering a different energy range and constructed out of cylindrical permanent magnet Halbach arrays. Monte Carlo simulations have been used to optimize and assess the performance of the conversion foil, and COSY INFINITY ion-optical simulations have been used to optimize the spectrometer magnets. The performance of the design is assessed for a simulated direct-drive γ-ray spectrum. Spanning its total γ-ray energy bandwidth and using a 1.7mm thick boron conversion foil, the system's total energy resolution and efficiency are ∼15.8%-4.5% and 5.4 × 10-7-3.7 × 10-7e-/γ, respectively, with room for improvement. Spectral γ-ray measurements will provide guidance to the inertial confinement fusion program toward achieving high-energy gain relevant to inertial fusion energy and enable new measurement capabilities for basic discovery science.

Time-of-flight vs time-of-arrival in neutron spectroscopic measurements for high energy density plasmas.

The neutron time-of-flight (nToF) diagnostic technique has a lengthy history in Inertial Confinement Fusion (ICF) and High Energy Density (HED) Science experiments. Its initial utility resulted from the simple relationship between the full width half maximum of the fusion peak signal in a distant detector and the burn averaged conditions of an ideal plasma producing the flux [Lehner and Pohl, Z. Phys. 207, 83-104 (1967)]. More recent precision measurements [Gatu-Johnson etal., Phys. Rev. E 94(8), 021202 (2016)] and theoretical studies [Munro, Nucl. Fusion 56, 035001 (2016)] have shown the spectrum to be more subtle and complicated, driving the desire for an absolute calibration of the spectrum to disambiguate plasma dynamics from the conditions producing thermonuclear reactions. In experiments where the neutron production history is not well measured, but the neutron signal is preceded by a concomitant flux of photons, the spectrum can be in situ calibrated using a set of collinear detectors to obtain a true "time-of-flight" measurement. This article presents the motivation and overview of this technique along with estimates of the experimental precision needed to make useful measurements in existing and future nToF systems such as the pulsed power Z-machine located in Albuquerque, NM, at Sandia National Laboratories.

Understanding the deficiency in inertial confinement fusion hohlraum x-ray flux predictions using experiments at the National Ignition Facility.

The predicted implosion performance of deuterium-tritium fuel capsules in indirect-drive inertial confinement fusion experiments relies on precise calculations of the x-ray drive in laser-heated cavities (hohlraums). This requires accurate, spectrally dependent simulations of laser to x-ray conversion efficiencies and x-ray absorption losses to the hohlraum wall. A set of National Ignition Facility experiments have identified a cause for the long-standing hohlraum "drive deficit" as the overprediction of gold emission at ∼2.5 keV in nonlocal thermodynamic equilibrium coronal plasma regions within the hohlraum. Reducing the emission and absorption opacity in this spectral region by ∼20% brings simulations into agreement with measured x-ray fluxes and spectra.

Explosively driven Richtmyer–Meshkov instability jet suppression and enhancement via coupling machine learning and additive manufacturing

The ability to control the behavior of fluid instabilities at material interfaces, such as the shock-driven Richtmyer–Meshkov instability, is a grand technological challenge with a broad number of applications ranging from inertial confinement fusion experiments to explosively driven shaped charges. In this work, we use a linear-geometry shaped charge as a means of studying methods for controlling material jetting that results from the Richtmyer–Meshkov instability. A shaped charge produces a high-velocity jet by focusing the energy from the detonation of high explosives. The interaction of the resulting detonation wave with a hollowed cavity lined with a thin metal layer produces the unstable jetting effect. By modifying the characteristics of the detonation wave prior to striking the lined cavity, the kinetic energy of the jet can be enhanced or reduced. Modifying the geometry of the liner material can also be used to alter jetting properties. We apply optimization methods to investigate several design parameterizations for both enhancing or suppressing the shaped-charge jet. This is accomplished using 2D and 3D hydrodynamic simulations to investigate the design space that we consider. We also apply new additive manufacturing methods for producing the shaped-charge assemblies, which allow for the experimental testing of complicated design geometries obtained through computational optimization. We present a direct comparison of our optimized designs with experimental results carried out at the High Explosives Application Facility at Lawrence Livermore National Laboratory.

Development of a full aperture backscatter system for the Orion laser.

A full aperture backscatter system (FABS) is currently in development on the Orion laser at AWE to measure scattered light from the stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) processes. The light is to be collected through the full aperture of the final optic assembly and traverse back down the beam path, with fractions of this light being directed to an optics table. By measuring the energy of this backscattered light, it is possible to gain insight into some of the laser-plasma instabilities that are present on the laser system and should give an indication of some of the scattered light losses due to the SBS and SRS processes. The uncontrolled scattered light can lead to hotter electrons, which then preheat the target causing a degradation in compression and can inhibit ignition in inertial confinement fusion experiments, as well as secondary instabilities whereby the scattered light may in itself cause further LPIs, such as two-ion decay or the Langmuir decay instability. The FABS diagnostic on Orion is planned to enhance the plasma diagnostics suite available and provide quantitative guidance on increasing the energy coupling. Current progress includes the characterization of filters and, hence, a broadband xenon lamp to be used in measuring the transmission efficiency of the optics chain, desktop alignment of the backscatter optics, and characterization of the streak cameras.

Evaluation of an x-ray multilayer mirror for radiography at sub-1-keV photon energies.

An x-ray multilayer mirror on a spherical substrate designed for near-normal incidence with a photon energy of ∼738eV (F Heα) was procured and tested. This device is intended to be used for in-flight radiography of the shell in inertial confinement fusion experiments with cryogenic targets on the OMEGA laser at the Laboratory for Laser Energetics. Experiments in self-emission on a small (∼10J) laser system showed that the reflectivity of the mirror is high enough to record an image at laser energies as low as 0.1J. A second set of tests in backlighting geometry on a larger (kJ)-scale, short-pulse laser yielded usable radiographs with laser energies as low as 40J with a spatial resolution of ∼10μm.

Novel deuterium-tritium (DT) neutron activation diagnostics for increased neutron yield operations at Sandia's Z-facility (invited).

As inertial confinement fusion experiments at Sandia National Laboratories' Z-facility begin using deuterium-tritium (DT) fuels, the expected DT-neutron yields are expected to increase many orders of magnitude. This brings about the need for diagnostics that have a significantly higher dynamic range of operation and ability to obtain yields where there will be significant radiation holds. Using the current activation diagnostic infrastructure at the Z-facility, a pair of activation diagnostics are being developed. The first is fluorine activation, which will be used in a similar manner to current copper activation diagnostics that are fielded in a rabbit for quick removal from the experiments without opening the vacuum experiment containment. The second is a prompt DT-neutron detector that exploits the activation of oxygen with similar operation characteristics as the current Be-probes fielded at Z. We will discuss the design of this pair of instruments, along with calibration work performed at Sandia's Ion Beam Laboratory in support of the development and anticipated instrument performance.

Mitigation strategies for time-resolved x-ray diagnostic data degradation due to high neutron yield of ICF experiments at the NIF.

Inertial confinement fusion experiments taking place at the National Ignition Facility are generating ever increasing amounts of fusion energy, with the deuterium tritium fusion neutron yield growing a hundredfold over the past ten years. Strategies must be developed to mitigate this harsh environment's deleterious effects on the operation and the performance of the time-resolved x-ray imagers deployed in the National Ignition Facility target bay to record the dynamics of the implosions. We review the evolution of these imagers in recent years and detail some of the past and present efforts undertaken to maintain or improve the quality of the experimental data collected on high neutron yield experiments. These include the use of a dump-and-read electronic backend, the selection of photographic film with a low background sensitivity, and the optical filtering of Cherenkov radiation.

PM2D: A parallel GPU-based code for the kinetic simulation of laser plasma instabilities at large scales

Laser plasma instabilities (LPIs) have significant influences on the laser energy deposition efficiency and therefore are important processes in inertial confined fusion (ICF). Numerical simulations play important roles in revealing the complex physics of LPIs. Since LPIs are typically a three wave coupling process, the precise simulations of LPIs with kinetic effects require to resolve the laser period (around one femtosecond) and laser wavelength (less than one micron). In the typical ICF experiments, however, LPIs are involved in a spatial scale of several millimeters and a temporal scale of several nanoseconds. Therefore, the precise kinetic simulations of LPIs in such scales require huge computational resources and are hard to be carried out by present kinetic codes like particle-in-cell (PIC) codes. In this paper, a full wave fluid model of LPIs is constructed and numerically solved by the particle-mesh method, where the plasma is described by macro particles that can move across the mesh grids freely. Based upon this model, a two-dimensional (2D) GPU code named PM2D is developed. The PM2D code can simulate the kinetic effects of LPIs self-consistently as normal PIC codes. Moreover, as the physical model adopted in the PM2D code is specifically constructed for LPIs, the required macro particles per grid in the simulations can be largely reduced and thus overall simulation cost is considerably reduced comparing with PIC codes. More importantly, the numerical noise in the PM2D code is much lower, which makes it more robust than PIC codes in the simulation of LPIs for the long-time scale above 10 picoseconds. After the distributed computing is realized, our PM2D code is able to run on GPU clusters with a total mesh grids up to several billions, which meets the requirements for the simulations of LPIs at ICF experimental scale with reasonable cost. Program summaryProgram Title: PM2DCPC Library link to program files:https://doi.org/10.17632/xscj6vnkkw.1Licensing provisions: GNU General Public License v3.0.Programming language: C++, CUDA.Nature of problem: Although the large scale simulations of laser plasma instabilities (LPIs) is of great significance for the inertial confinement fusion (ICF), there is still no suitable code to simulate these problems. PM2D code based on a GPU platform provides an effective method to simulate these large scale problems in ICF.Solution method: A fluid model for LPIs is established firstly, which contains wave equations that describe the laser propagating process, electron and ion fluid equations that describe the plasma motions, and a Poisson's equation that describes the electrostatic field induced by charge separation. The wave equation is solved on a rectangular region using absorption boundary conditions on all of four boundaries. The absorption boundary condition on the left boundary is further extended to allow the incidence of driven lasers and absorption of scattering lasers simultaneously. The fluid equations in the physical model are solved by the particle-mesh method, in which the macro particles are driven to move by fluid forces. Since macro particles can move freely within the fixed fluid grids, the PM2D code can capture the kinetic effects self-consistently. The Poisson's equation for the electrostatic field is solved by a Fourier decomposition method in the y direction, which helps to decrease the simulation cost greatly. The PM2D code is developed on a GPU platform base on CUDA toolkit, which largely increase the computational speed.

Bayesian batch optimization for molybdenum versus tungsten inertial confinement fusion double shell target design

AbstractAccess to reliable, clean energy sources is a major concern for national security. Much research is focused on the “grand challenge” of producing energy via controlled fusion reactions in a laboratory setting. For fusion experiments, specifically inertial confinement fusion (ICF), to produce sufficient energy, the fusion reactions in the ICF fuel need to become self‐sustaining and burn deuterium‐tritium (DT) fuel efficiently. The recent record‐breaking NIF ignition shot was able to achieve this goal as well as produce more energy than used to drive the experiment. This achievement brings self‐sustaining fusion‐based power systems closer than ever before, capable of providing humans with access to secure, renewable energy. In order to further progress toward the actualization of such power systems, more ICF experiments need to be conducted at large laser facilities such as the United States's National Ignition Facility (NIF) or France's Laser Mega‐Joule. The high cost per shot and limited number of shots that are possible per year make it prohibitive to perform large numbers of experiments. As such, experimental design relies heavily on complex predictive physics simulations for high‐fidelity “preshot” analysis. These multidimensional, multi‐physics, high‐fidelity simulations have to account for a variety of input parameters as well as modeling the extreme conditions (pressures and densities) present at ignition. Such simulations (especially in 3D) can become computationally prohibitive to turn around for each ICF experiment. In this work, we explore using Bayesian optimization with Gaussian processes (GPs) to find optimal designs for ICF double shell targets, while keeping computational costs to manageable levels. These double shell targets have an inner shell that grades from beryllium on the outer surface to the higher Z material molybdenum, as opposed to the nominally used tungsten, on the inside in order to trade off between the high performance associated with high density inner shells and capsule stability. We describe our results for “capsule‐only” xRAGE simulations to study the physics between different capsule designs, inner shell materials, and potential for future experiments.

Time-dependent density-functional theory study on nonlocal electron stopping for inertial confinement fusion

Understanding laser–target coupling is of the utmost importance for achieving high performance in laser-direct-drive (LDD) inertial confinement fusion (ICF) experiments. Thus, accurate modeling of electron transport and deposition through ICF-relevant materials and conditions is necessary to quantify the total thermal conduction and ablation. The stopping range is a key transport quantity used in thermal conduction models; in this work, we review the overall role that the electron mean free path (MFP) plays in thermal conduction and hydrodynamic simulations. The currently used modified Lee–More model employs various physics approximations. We discuss a recent model that uses time-dependent density functional theory (TD-DFT) to eliminate these approximations in both the calculation of the electron stopping power and corresponding MFP in conduction zone polystyrene (CH) plasma. In general, the TD-DFT calculations showed a larger MFP (lower stopping power) than the standard modified Lee–More model. Using the TD-DFT results, an analytical model for the electron deposition range, λTD−DFT(ρ,T,K), was devised for CH plasmas between ρ=[0.05−1.05] g/cm3, kBT=[100−1000] eV. We implemented this model into LILAC, for simulations of a National Ignition Facility-scale LDD implosion and compared key physics quantities to ones obtained by simulations using the standard model. The implications of the obtained results and the path moving forward to calculate this same quantity in conduction-zone deuterium–tritium plasmas are further discussed, to hopefully close the understanding gap for laser target coupling in LDD-ICF simulations.

Direct multiple monochromatic x-ray imaging with a pinhole array and a laterally graded multilayer mirror.

Multiple monochromatic x-ray imaging (MMI) is a technique for diagnosing the emission spectra of tracer elements in laser-driven inertial confinement fusion experiments. This study proposes an MMI method that combines a simple pinhole array with a laterally graded multilayer mirror. The method directly obtains multiple monochromatic x-ray images by regulating the multilayer thickness in different mirror positions to compensate for the energy-broadening effect. This paper presents a comprehensive design scheme, the multilayer fabrication and experimental verification of the gradient MMI imaging performance. The experimental results show that the method achieves monochromatic imaging with a spectral resolution of ∼70-90eV in several keV energy regions. This paper presents a practical diagnostic approach for directly and synchronously capturing the spatial, temporal, and spectral information of laser plasma x rays.