Inertial Fusion Research Articles

Generative modeling for high-shot-rate laser systems relevant to inertial fusion energy

Generative modeling for high-shot-rate laser systems relevant to inertial fusion energy

Microstructure evolution of AlSi10Mg alloy in RAP process

Abstract With its computer-aided layer-by-layer production approach, additive manufacturing (AM) and powder bed laser fusion (PBLF) paved the way to produce metallic parts more precisely than any other manufacturing technique. However, the combinability and the interaction of this relatively new manufacturing technique with the other near-net shape production techniques is still a mystery. In this study, the recrystallization and partial melting (RAP) behavior, which is a feedstock production approach for semisolid forming methods, investigated on AlSi10Mg parts produced by PBLF and conventional casting were compared in terms of microstructural and hardness evaluations. After the reheating process, the globalization of Si particles and the breakdown of the Si network around the melt pools were displayed with light microscope and scanning electron microscope (SEM) images. The hardness values of the PBLF as-fabricated specimens were found to be significantly higher than the as-cast specimens; however, the values were almost equaled after the RAP treatment and even got lower on the bottom and top regions of the PBLF samples after 20 min of reheating because of the enlarged shrinkage porosities and the coarsened morphology.

Dilation framing camera with the dual-pulse excitation technique

In an inertial confinement fusion (ICF) ultrafast diagnostic system that is based on electron beam time-dilation, an ultrafast electrical pulse is used to excite a microstrip photocathode (PC), which generates a varying PC voltage to obtain a photoelectron velocity that varies with emission time. The photoelectron beam achieves time-dilation through the drift process and is then detected by a time-resolved sensor, thereby increasing the temporal resolution of the diagnostic system. A pulse time-dilation diagnostic system is simulated, while the sensor is a gated microchannel plate (MCP) detector with a temporal resolution of 100 ps and an excitation pulse on a PC with a slope of 3 V/ps; the diagnostic system achieves a temporal resolution of 11.12 ps. However, the excitation pulse creates a voltage difference across the PC. A voltage difference of 900 V can be acquired for a PC length of 60 mm, which yields a nonuniform spatial resolution ranging from 30.4 µm to approximately 3000 µm. Furthermore, the voltage difference across the PC also limits the frame size to 2.2 mm along the pulse propagation direction according to the simulation results. To achieve a uniform spatial resolution and a larger frame size, a dual-pulse excitation technique on a PC is presented, which is the technique to symmetrically apply voltage pulses at both ends of the PC microstrip. The theoretical results show that this technique will improve the uniformity of the PC voltage spatial distribution. When the PC pulse slope is 3 V/ps and the dual-pulse excitation technique is employed, the diagnostic system has a temporal resolution of 5.91 ps and a uniform spatial resolution of 30.4 µm. Furthermore, the frame size along the pulse propagation direction is improved to the effective length of the microstrip PC.

High Energy Density Radiative Transfer in the Diffusion Regime with Fourier Neural Operators

AbstractRadiative heat transfer is a fundamental process in high energy density physics and inertial fusion. Accurately predicting the behavior of Marshak waves across a wide range of material properties and drive conditions is crucial for design and analysis of these systems. Conventional numerical solvers and analytical approximations often face challenges in terms of accuracy and computational efficiency. In this work, we propose a novel approach to model Marshak waves using Fourier Neural Operators (FNO). We develop two FNO-based models: (1) a base model that learns the mapping between the drive condition and material properties to a solution approximation based on the widely used analytic model by Hammer & Rosen (2003), and (2) a model that corrects the inaccuracies of the analytic approximation by learning the mapping to a more accurate numerical solution. Our results demonstrate the strong generalization capabilities of the FNOs and show significant improvements in prediction accuracy compared to the base analytic model.

A First-Principles Study of the Structural and Thermo-Mechanical Properties of Tungsten-Based Plasma-Facing Materials

Tungsten (W) and tungsten alloys are being considered as leading candidates for structural and functional materials in future fusion energy devices. The most attractive properties of tungsten for the design of magnetic and inertial fusion energy reactors are its high melting point, high thermal conductivity, low sputtering yield, and low long-term disposal radioactive footprint. Despite these relevant features, there is a lack of understanding of how the structural and mechanical properties of W-based alloys are affected by the temperature in fusion power plants. In this work, we present a study on the thermo-mechanical properties of five W-based plasma-facing materials. First-principles density functional theory (DFT) calculations are combined with the quasi-harmonic approximation (QHA) theory to investigate the electronic, structural, mechanical, and thermal properties of these W-based alloys as a function of temperature. The coefficient of thermal expansion, temperature-dependent elastic constants, and several elastic parameters, including bulk and Young’s modulus, are calculated. Our work advances the understanding of the structural and thermo-mechanical behavior of W-based materials, thus providing insights into the design and selection of candidate plasma-facing materials in fusion energy devices.

Autonomous Positioning for Mobile Vehicles Based on Visual‐Inertial Fusion in Challenging Dark Roadway Scenes

ABSTRACTAccurate positioning for autonomous driven underground mining vehicles (UMVs) and coal mine robots (CMRs) is indeed one of the cores in the intelligentization of coal mining. Completely different from positioning on the ground and in parking scenes, there will be great difficulties in realizing the accurate active positioning for UMVs shuttled in dark and narrow roadways. We propose an effective visual and inertial fusion positioning method for autonomous CMRs and roadheaders in challenging roadway scenarios based on the odometer‐aided inertial navigation system and visual pose estimation system. Velocity information of the odometer is adapted to restrain the error accumulation of inertial positioning based on a Kalman filter. The hybrid visual feature detection algorithm is put forward to improve the accuracy and robustness of visual observation information in a dark environment. Autonomous experiments for CMRs and roadheaders are separately performed in the narrow roadway and dark passageway to demonstrate the applicability of our localization method. The proposed approach outperforms the subsystems and existing methods in accuracy and has outstanding stability.

Understanding the impact of an applied axial magnetic field on efficient current coupling on the Z machine

Magnetized liner inertial fusion (MagLIF) is an attractive concept for producing thermonuclear fusion reactions. The MagLIF platform involves the operation of Helmholtz coils to apply a 15 Tesla axial magnetic field to the load region, where a cylindrical, fuel-filled metal liner is imploded by a ∼20 MA current pulse. The fringe field from these coils extends into the transmission line that delivers the current to the target. We investigated the extent to which this applied field disturbs the nominal power flow within that transmission line. A simplified model of the geometry shows that adding the applied magnetic field results in magnetic field lines that connect the cathode to the anode, suggesting electrons may not be magnetically insulated in this region. Particle-in-cell simulations indicated the addition of the applied magnetic field would not significantly impact the current delivery to the load. Velocimetry was used to experimentally assess the current delivery with and without the applied magnetic field. We find no measurable effects of the applied field on current delivery in the configuration investigated in this study. Published by the American Physical Society 2024

Position and orientation estimation method based on 3D digital morphology contour registration

Abstract Accurately and quickly obtaining the positions and orientations of mechanical parts based on the digital morphologies of mechanical parts are the key to achieving efficient and accurate assembly of mechanical parts. However, due to poor robustness and compactness in extracting digital morphology contours of mechanical parts, the accuracy of assembly positions and orientations obtained by using digital morphology contours cannot meet the requirements of high-precision assembly. Therefore, this paper proposes a position and orientation estimation method based on 3D digital morphology contour registration. This method extracts and optimizes digital morphology contours of mechanical parts and obtains the assembly positions and orientations by using an improved iterative closest point method to register the extracted digital morphology contours with those of the mechanical parts in assembly targets with desired positions and orientations. Experiments are conducted using mechanical parts from the ABC dataset and inertial confinement fusion micro-target. From the experimental results, when using the assembly positions and orientations obtained through the method proposed in this paper to assemble mechanical parts, it can achieve translation absolute errors of 8 μm, 5 μm, and 9 μm along the X-, Y-, and Z-axes, respectively. Similarly, the angular absolute errors in rotations around the Z-, Y-, and X-axes can be less than or equal to 0.16°, 0.15°, and 0.11°, respectively. The results prove that the proposed method in this paper exhibits high computational efficiency and accuracy, providing an effective approach for digital assembly of mechanical parts.

Applications of a Rayleigh-Taylor model to direct-drive laser fusion

Applications of a Rayleigh-Taylor model to direct-drive laser fusion

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.

Study of ablation and shock generation across three orders of magnitude of laser intensity with 100 ps laser pulses

The laser ablation and subsequent shock generation in solid targets plays an important role in a variety of research topics from equation of state models for materials to inertial confinement fusion. One of the long-standing issues is the knowledge of ablation depth in the picosecond time regime. We report on a direct technique for determining the ablation depth in aluminum using x-ray diffraction data from Linac Coherent Light Source at the Stanford Linear Accelerator Center. This technique gives a direct measurement of the shock wave propagation in the bulk target, enabling an ability to discern early timescale physics from late timescale effects not available in postmortem analysis. We find that the ablation depths only vary by 0.2 μm across three orders of magnitude of laser intensity, while the pressure increased by a factor of 10 following a square root dependence on laser pulse energy. We further observe that the ablation depth in this intensity range (1011–1013 W/cm2 in intensity, corresponding to 0.8–80 J/cm2 in fluence) cannot be modeled by a universal scaling law, given the complexity of the mechanisms governing laser ablation in this intensity regime.

Numerical Studies of Influencing Factors on the Homogeneity of the Powder Mixture during the Powder Spreading Process of Powder Bed Fusion–Laser Beam/Metal

Recent studies have focused on the alloying of nitrogen (N) in high‐alloy stainless steels by powder bed fusion–laser beam/metal (PBF‐LB/M). However, N‐induced pore formation remains a challenge. To overcome this, a combined PBF‐LB/M and hot isostatic pressing (HIP) process is developed. AISI 304L stainless steel powder was mixed with silicon nitride (Si3N4) powder and processed by PBF‐LB/M, allowing partial retention of Si3N4. This helps to maintain sufficient N content while reducing pore formation. The microstructure is then homogenised by HIP. A major challenge of this process is to maintain the homogeneity of the deposited powder mixture on the powder bed to ensure a consistent N content in the final products. This study combines experimental and numerical methods to address this issue: scanning electron microscopy is used to characterize the microstructure and map the local N content, while various numerical studies based on the discrete element method are carried out to investigate the effects of layer thickness, substrate surface roughness, and powder properties on the intermediate product. The numerical approach effectively predicts the N content distribution and homogeneity on the powder bed. The models are validated with experimental data and optimal process conditions for PBF‐LB/M are identified.

Simulating Nonlinear Radiation Diffusion Through Quantum Computing

The recent success of the fusion ignition experiment at Lawrence-Livermore National Laboratory has renewed excitement for inertial confinement fusion. Designing such experiments relies on computational modeling using radiation hydrodynamic codes run on massively parallel supercomputers which simulate hydrodynamic flows, radiation diffusion, and thermonuclear burn. Constructing a quantum algorithm for radiation hydrodynamics that provides a quantum speedup is of great interest. A recent quantum algorithm that solves the Navier-Stokes equations of hydrodynamics with a quadratic speedup marks a first step towards this goal. Here we take the next step and present a quantum algorithm for nonlinear radiation diffusion that also has a quadratic speedup. To verify the algorithm we consider radiation striking a cold, optically thick target that generates a Marshak wave in the target. Results of numerical simulation of the quantum algorithm are compared with those of a standard partial differential equation solver and excellent agreement is found.

Key Issues and Application Prospects in High-Temperature Plasma Physics

High-temperature plasma physics is the core area of research into nuclear fusion energy, and it also has broad application potential in industry and astrophysics. The paper systematically reviews the fundamental definition, the formation mechanism, and research history of high-temperature plasma; analyzes focal problems including energy confinement, plasma instability, radiation cooling, material tolerance, turbulent dynamics; and discusses the technical progress and challenges of magnetic confinement and inertial confinement experimental devices. The comparison of tokamak against laser inertial confinement gives further illustration of the pros and disadvantages of the two fusion routes. Nuclear fusion energy, from an application viewpoint, represents a very ideal energy source for future sustainable development, and high-temperature plasma technology is also very promising in industrial processing, astrophysical experiments, and other fields. In conjunction with relevant recent research, suggestions are also given in this paper regarding high-temperature plasma in the future development direction. All this is done as an effort to offer reference and guidance for future developments in that field.

Influence of crystal dimension on performance of spherical crystal self-emission imager

Abstract The spherical crystal imaging system, noted for its high energy spectral resolution (monochromaticity) and spatial resolution, is extensively applied in high energy density physics and inertial confinement fusion research. This system supports studies on fast electron transport, hydrodynamic instabilities, and implosion dynamics. The X-ray source, produced through laser-plasma interaction, emits a limited number of photons within short time scales, resulting in predominantly photon-starved images. Through ray-tracing simulations, we investigated the impact of varying crystal dimensions on the performance of a spherical crystal self-emission imager. We observed that increasing the crystal dimension leads to higher imaging efficiency but at the expense of monochromaticity, causing broader spectral acceptance and reduced spatial resolution. Furthermore, we presented a theoretical model to estimate the spatial resolution of the imaging system within a specific energy spectrum range, detailing the expressions for the effective size of the crystal. The spatial resolution derived from the model closely matches the numerical simulations.

Spectral line broadening of the Raman scattered waves in laser plasmas

Parametric instabilities are of a great concern especially in the context of laser fusion. Electromagnetic waves scattered by these instabilities can reach high amplitudes, thereby diverting a significant portion of the energy from the incident laser beam, which is essential for compressing and heating the fuel capsule. In addition, the emerging electrostatic wave can trap and accelerate electrons, which, upon impact, can preheat the target and thus prevent its effective compression. Here, we focus on the stimulated Raman scattering and the associated processes. Spectral broadening of the backward Raman scattering daughter wave is theoretically described process, which is caused by so-called trapped particle instability. The existence of trapped and freely moving electrons in plasma influenced by high amplitude electron plasma wave leads also to the anomalous dispersion of this wave. It is reflected in the shifts in the electrostatic spectrum which, on the other hand, must be visible also in the electromagnetic spectrum via the feed-back loop of the instability. In the present paper, we discuss scattered wave modes spectral broadening using results of numerical simulations.

Role of nonlocal heat transport on the laser ablative Rayleigh-Taylor instability

Abstract Ablative Rayleigh–Taylor instability (ARTI) and nonlocal heat transport are the critical problems in laser-driven inertial confinement fusion, while their coupling with each other is not completely understood yet. Here the ARTI in the presence of nonlocal heat transport is studied self-consistently for the first time theoretically and by using radiation hydrodynamic simulations. It is found that the nonlocal heat flux generated by the hot electron transport tends to attenuate the growth of instability, especially for short wavelength perturbations. A linear theory of the ARTI coupled with the nonlocal heat flux is developed, and a prominent stabilization of the ablation front via the nonlocal heat flux is found, in good agreement with numerical simulations. This effect becomes more significant as the laser intensity increases. Our results should have important references for the target designing for inertial confinement fusion.

Advances in and Future Perspectives on High-Power Ceramic Lasers

Advancements in laser glass compositions and manufacturing techniques has allowed the development of a new category of high-energy and high-power laser systems which are being used in various applications, such as for fundamental research, material processing and inertial confinement fusion (ICF) technologies research. A ceramic laser is a remarkable revolution in solid state lasers. It exhibits crystalline properties, high yields, better thermal conductivity, a uniformly broadened emission cross-section, and a higher mechanical constant. Polycrystalline ceramic lasers combine the properties of glasses and crystals, which offer the unique advantages of high thermal stability, excellent optical transparency, and the ability to incorporate active laser ions homogeneously. They are less expensive and have a similar fabrication process to glass lasers. Recent developments in these classes of lasers have led to improvements in their efficiency, beam quality, and wavelength versatility, making them suitable for a broad range of applications, such as scientific research requiring ultra-fast laser pulses, medical procedures like laser surgery and high-precision cutting and welding in industrial manufacturing. The future of ceramic lasers looks promising, with ongoing research focused on enhancing their performance, developing new doping materials and expanding their functional wavelengths. The ongoing progress in high-power ceramic lasers is continuously expanding the limits of laser technology, therefore allowing the development of more powerful and efficient systems for a wide range of advanced and complex applications. In this paper, we review the advances, limitations and future perspectives of ceramic lasers.

Reducing the total stimulated brillouin scattering of two-color lasers through two-ion decay

Abstract A mechanism coupling the stimulated Brillouin scattering (SBS) of two-color lasers with wavelengths of 527 and 351 nm via the two-ion decay 
instability is proposed. When the SBS reflectivities of both lasers exceed 10%, the ion-acoustic wave excited by the 527 nm laser seeds the decay process of the ion-acoustic wave excited by the 351 nm laser, thereby promoting the decay of the latter into the former. This results in a significant reduction in the SBS reflectivity of the 351 nm laser, while the SBS of the 527 nm laser exhibits minimal variation, consequently reducing the total SBS reflectivity. The total SBS reflectivity initially decreases and then increases as the intensity fraction α of the 527 nm laser rises. When α ∽(20%-30%), the laser energy reflections from both lasers become approximately equal, achieving the minimum total reflectivity. Through this mechanism, the incidence of the two-color lasers can achieve lower reflected energy compared to monochromatic lasers with the same total intensity. These results demonstrate the significant potential of replacing a small fraction of high-frequency light with low-frequency light in enhancing the laser-target coupling efficiency for inertial fusion energy.

A linearized hydrodynamic code for laser ablation and its application to laser pulse shaping for direct-drive fusion

One of the most harmful processes in inertial confinement fusion is Rayleigh–Taylor instability (RTI), and an efficient way to mitigate it is pulse shaping. However, because shaped laser pulses lead to unsteady ablation, it is insufficient to evaluate RTI based solely on the instability growth rate. Here, for better prediction of RTI during linear growth, hydrodynamic equations for laser ablation (including both balance and linearized perturbation equations) are solved numerically and used to optimize the laser pulse shape for direct-drive inertial confinement fusion. For given target conditions and laser energy, simulations show that a picket pulse before the main laser pulse can reduce RTI significantly, and it is clear that the reduction comes from two aspects: (i) the lower RTI seed due to rarefaction at the descending edge of the picket in the imprint stage and (ii) the smaller growth rate due to enhanced ablation velocity at the main pulse in the acceleration stage. It is found that the perturbed laser deposition in an underdense plasma also has a profound influence on RTI seeds in the imprint stage.