Ignition Target Research Articles

Investigating the impact of intermediate-mode perturbations on diagnosing plasma conditions in DT cryogenic implosions via synthetic x-ray Thomson scattering

Abstract The pursuit of inertial confinement fusion ignition target designs requires precise experimental validation of the conditions within imploding capsules, in particular the density and temperature of the compressed shell. Previous work has identified x-ray Thomson scattering (XRTS) as a viable diagnostic tool for inferring the in-flight compressed deuterium-tritium shell conditions during capsule implosions (Poole et al 2022 Phys. Plasmas 29 072703). However, this study focused on one-dimensional simulations, which do not account for the growth of hydrodynamic instabilities. In this work, two-dimensional DRACO simulations incorporating intermediate-mode perturbations up to Legendre mode ℓ = 50 were used to generate synthetic XRTS spectra with the SPECT3D code. The analysis employed Markov-Chain Monte Carlo techniques to infer plasma conditions from these spectra. The results demonstrate that the XRTS diagnostic platform can effectively discern the in-flight compressed shell conditions for targets with varying adiabats, even in the presence of intermediate-mode perturbations. This work underscores the potential of XRTS for realistic inertial confinement fusion experiments, providing a robust method for probing the complex dynamics of fusion implosions.

The long road to ignition: An eyewitness account

This paper reviews the many twists and turns in the long journey that culminated in ignition in late 2022 using the laser heated indirect-drive approach to imploding DT filled targets at the National Ignition Facility (NIF), located at the Lawrence Livermore National Laboratory (LLNL). We describe the early origins of the Laser Program at LLNL and key developments such as the paradigm shifting birth of high energy density physics (HEDP) studies with lasers, changes in choice of laser wavelength, and the development of key diagnostics and computer codes. Fulfilling the requirements of the multi-faceted Nova Technical Contract was a necessary condition for the approval of the NIF, but more importantly, the end of the Cold War and the cessation of nuclear testing were key catalysts in that approval, along with the ready-and-waiting field of HEDP. The inherent flexibility of the field of laser driven inertial confinement fusion played a fundamental role in achieving success at the NIF. We describe how the ultimately successful ignition target design evolved from the original “point design” target, through the lessons of experiment. All key aspects of that original design changed: The capsule's materials and size were changed; the hohlraum's materials, size, laser entrance hole size, and gas fills were also all changed, as were the laser pulse shapes that go along with all those changes. The philosophy to globally optimize performance for stability (by raising the adiabat and thus lowering the implosion convergence) was also key, as was progress in target fabrication, and in increasing NIF's energy output. The persistence of the research staff and the steadfast backing of our supporters were also necessary elements in this success. We gratefully acknowledge seven decades of researcher endeavors and four decades of the dedicated efforts of many hundreds of personnel across the globe who have participated in NIF construction, operation, target fabrication, diagnostic, and theoretical advances that have culminated in ignition.

Design of first experiment to achieve fusion target gain > 1

A decades-long quest to achieve fusion energy target gain and ignition in a controlled laboratory experiment, dating back to 1962, has been realized at the National Ignition Facility (NIF) on December 5, 2022 [Abu-Shawareb et al., Phys. Rev. Lett. 132, 065102 (2024)] where an imploded pellet of deuterium and tritium (DT) fuel generated more fusion energy (3.15 MJ) than laser energy incident on the target (2.05 MJ). In these experiments, laser beams incident on the inside of a cylindrical can (Hohlraum) generate an intense ∼3 × 106 million degree x-ray radiation bath that is used to spherically implode ∼2 mm diameter pellets containing frozen deuterium and tritium. The maximum fusion energy produced in this configuration to date is 3.88 MJ using 2.05 MJ of incident laser energy and 5.2 MJ using 2.2 MJ of incident laser energy, producing a new record target gain of ∼2.4×. This paper describes the physics (target and laser) design of this platform and follow-on experiments that show increased performance. We show robust megajoule fusion energy output using this design as well as explore design modification using radiation hydrodynamic simulations benchmarked against experimental data, which can further improve the performance of this platform.

The Role of Target Melting in Particle Impact Ignition with Inert Particulate

The high gas temperatures and oxygen pressures in the turbine of oxygen-rich turbomachinery put conventional engineering alloys such as IN718 at risk of particle impact ignition, i.e., metal fires initiated when particulate strikes a solid surface. The standard model of particle impact ignition assumes that the impacting particle must first ignite in order to kindle to the target material. Here, we invalidate this belief through particle impact ignition experiments which show that IN718 can ignite when struck by inert Al2O3 particles with supersonic impact velocities. Through post-mortem analysis of non-ignited samples, we find that subsonic particle impact causes minimal crater damage whereas supersonic particle impact leaves extensive crater plasticity and pileup, with evidence of molten ejecta near the impact site. Complementary finite element simulations of supersonic impact events confirm extreme adiabatic heating and localized melting. These findings demonstrate that particle impact can drive target ignition even in the absence of particle burning provided the thermal excursion at impact exceeds the melting point of the target material.

Cascaded solenoid acceleration of vortex laser-driven collimated proton beam

Efficient cascaded proton acceleration driven by an intense Laguerre–Gaussian (LG) laser is realized in combined three-dimensional particle-in-cell simulations and CST STUDIO SUITE (CST) simulations. CST simulations show that there is no divergent force component in the transverse direction in the coil center. Therefore, the collimated proton beam driven by the LG laser in the first stage benefits from the uniform beam acceleration in the second stage. By contrast, the proton beam with larger divergence disperses to the outside of the coil because of the divergent force near the coil wire in the Gaussian laser case. Finally, a quasi-monoenergetic proton beam with a higher flux is generated by the LG laser, which is much better than the Gaussian laser case. The obtained proton beam can potentially be used in some special applications, such as proton radiography, fast ignition of fusion targets, biomedical applications, and production of warm dense matter.

Study of thedouble-layerr high-density carbon-polystyrene ablator in the ignition of inertial fusion targets

لایه کَندگی کربن‌­چگالی‌­بالا یکی از گزینه‌­های امیدبخش در افروزش گرماهسته­‌ای در هدف­‌های گداخت محصورشدگی لختی است. به منظور کاهش ناپایداری‌­های هیدرودینامیکی و هم‌­چنین حفظ سوخت از پدیده پیش‌­گرمایش، از طراحی لایه کَندگی دوتایی با لایه بیرونی کربن­‌چگالی­‌بالا که اخیراً توسط محققین مورد توجه قرار گرفته است، استفاده شده است. از این‌­رو در این پژوهش، بهینه‌­سازی لایه کَندگی دوتایی یک هدف کروی دلخواه با تک لایه کَندگی پلی‌­استایرن را با استفاده از کد هیدرودینامیکی MULTI-IFE مورد بررسی قرار دادیم. این هدف، تحت تابش باریکه‌­های متقارن لیزر با طول پالس ns۷/۲۲، طول موج µm۲۵/۰ و انرژی کل MJ 7/1 قرار گرفت. محاسبات ما نشان می‌­دهد که ضخامت بهینه کربن­‌چگالی‌­بالا حدود mm ۶/۵ است. استفاده از لایه کَندگی دوتایی سبب می‌­شود که انرژی لیزر جذب شده در سطح هدف حدود %­۸ افزایش یابد. افزایش انرژی جذب شده منجر به افزایش حدود %­5 توان آلفای تولیدی شده و در نتیجه کسر مصرف سوخت حدود %­۵/۱ افزایش می‌­یابد. در نهایت بهره سوخت حدود %­۱۲ افزایش یافت.

Toward more robust ignition of inertial fusion targets

Following the 3.15 MJ fusion milestone at the National Ignition Facility, the further development of inertial confinement fusion, both as a source for future electricity generation and for high-energy-density physics applications, requires the development of more robust ignition concepts at current laser facility energy scales. This can potentially be achieved by auxiliary heating the hotspot of low convergence wetted foam implosions where hydrodynamic and parametric instabilities are minimized. This paper presents the first multi-dimensional Vlasov–Maxwell and particle-in-cell simulations to model this collisionless interaction, only recently made possible by access to the largest modern supercomputers. The key parameter of interest is the maximum fraction of energy that can be extracted from the electron beams into the hotspot plasma. The simulations indicate that significant coupling efficiencies are achieved over a wide range of beam parameters and spatial configurations. The implications for experimental tests on the National Ignition Facility are discussed.

Big Data-aided Study of the Physical Process of Volume Ignition

Volume ignition is a method of igniting a fuel as a whole by simultaneously achieving ignition conditions throughout the fuel zone. The basic criterion for ignition is that the thermonuclear energy is greater than the energy leakage at the fuel boundary, resulting in self-sustaining heating and deep combustion. Deuterium-tritium fuels are wrapped in medium to high Z media to reduce radiative leakage and achieve lower-temperature holistic ignition and non-equilibrium combustion, ultimately allowing the fuel to achieve high combustion efficiency. Volume ignition is the use of energy balance relations under the local thermodynamic equilibrium approximation to establish the energy balance equation for thermonuclear systems, and the system ignition threshold is obtained by solving this equation. By understanding the physical process, we believe that the non-equilibrium process is universal to the volume ignition process. Changes in external factors (density, boundaries, albedo, etc.) at the moment of ignition can have a significant impact on the development direction of the system, and an ignition system with a large surface density can nevertheless withstand a large amount of reverse work and continue to burn. The design of the ignition target tries to avoid these factors through margin design, but conversely, the rational use of these laws can further improve the design margin of the capsule. With the aid of big data, the volume ignition method is easier to calculate and has a shorter iteration time. The traditional way is to propose a model, set the material, and then perform the calculation while using big data can set any model and material for calculation. In this paper, a simple comparison will be made to find out that the efficiency of the physical design of a volume ignition target will be effectively improved with the aid of big data. Volume ignition targets can be used not only in Z-pinch-driven systems but also for laser-driven volume ignition.

Simulation of Direct Drive Target Compression and Ignition Taking into Account Hot Electrons Generation

Simulation of Direct Drive Target Compression and Ignition Taking into Account Hot Electrons Generation

Determination of laser entrance hole size for ignition-scale octahedral spherical hohlraums

A recently proposed octahedral spherical hohlraum with six laser entrance holes (LEHs) is an attractive concept for an upgraded laser facility aiming at a predictable and reproducible fusion gain with a simple target design. However, with the laser energies available at present, LEH size can be a critical issue. Owing to the uncertainties in simulation results, the LEH size should be determined on the basis of experimental evidence. However, determination of LEH size of an ignition target at a small-scale laser facility poses difficulties. In this paper, we propose to use the prepulse of an ignition pulse to determine the LEH size for ignition-scale hohlraums via LEH closure behavior, and we present convincing evidence from multiple diagnostics at the SGIII facility with ignition-scale hohlraum, laser prepulse, and laser beam size. The LEH closure observed in our experiment is in agreement with data from the National Ignition Facility. The total LEH area of the octahedral hohlraum is found to be very close to that of a cylindrical hohlraum, thus successfully demonstrating the feasibility of the octahedral hohlraum in terms of laser energy, which is crucially important for sizing an ignition-scale octahedrally configured laser system. This work provides a novel way to determine the LEH size of an ignition target at a small-scale laser facility, and it can be applied to other hohlraum configurations for the indirect drive approach.

Study of the radiation temperature on ablator by using the shock wave technique

A new diagnostic platform for more accurate diagnosis of the peak radiation temperature on ablator has been proposed. A nearly constant radiation temperature was obtained by two laser entrance holes spherical hohlraum. The peak radiation temperature on ablator was determined by the shock wave technique. A high-quality burn-through image of a two-step-shaped Au ablator was obtained in the experiment. The simulated mass ablation rate agrees well with the experimental result, while the peak radiation temperature measured by flat-response x-ray detectors outside the hohlraum was of ∼20 eV’s lower than that obtained by the shock wave technique. This deviation results in ∼20%’s decrease in the mass ablation rate in the simulation. Thus, the new diagnostic platform can provide more accurate peak radiation temperature diagnosis. This can greatly support the inertial confinement fusion ignition target design.

A hierarchical assembly knowledge representation framework and microdevice assembly ontology

Microdevice assembly knowledge is dispersed in different product development phases, such as assembly design, assembly simulation and assembly process, and a lot of essential knowledge is implicit and heterogeneous. It is difficult for researchers and computer-aided systems to share and reuse different assembly knowledge quickly and accurately, leading to inefficient and inaccurate assembly process planning. To integrate and structurally represent the assembly design knowledge, assembly simulation knowledge and assembly process knowledge of microdevice, this paper proposes a hierarchical assembly knowledge representation framework and develops a microdevice assembly ontology. There are four layers in the framework, including the organizational structure, the structural relationship, the assembly accuracy, and the process characteristics. The assembly design knowledge that is integrated involves the basic properties of the assembly object as well as the spatial, mating, and assembly relationship, etc. Assembly simulation knowledge refers to the permissible range of assembly force and contact force. Knowledge of assembly processes comprises assembly sequence and operating method of the part. The microdevice assembly ontology is developed based on METHONTOLOGY, and implemented with Protégé. The corresponding SWRL rules have been established to inference the implicit knowledge in assembly design. An ignition target assembly knowledge model based on the microdevice assembly ontology is constructed. In the assembly task of the ignition target, engineers can quickly and accurately access the required assembly knowledge from the ignition target assembly knowledge model, thus verifying the integrity and validity of the microdevice assembly ontology.

Machine learning on the ignition threshold for inertial confinement fusion

In inertial confinement fusion, the ignition threshold factor (ITF), defined as the ratio of the available shell kinetic energy to the minimum ignition energy, is an important metric for quantifying how far an implosion is from its performance cliff. Traditional ITF research is based on analytical theories with explicit scaling laws and parameters obtained by numerically fitting simulation data. This present study uses machine learning (ML) methods to train implicit but more reliable ITF expressions. One-dimensional numerical simulations are used to develop a dataset with 20 000 targets, in which alpha particle heating magnifies the fusion yield by a factor of 6.5. These targets are defined as marginal ignition targets whose ITF equals unity. ML models such as neural networks, support vector machines, and Gaussian processes are trained to connect the minimum ignition velocity vigt with other implosion parameters, yielding an ML-based ITF of (vimp/vigt)7.5, where vimp represents the implosion velocity. Then, these ML models are used to obtain curves of the ignition probability vs the ITF and improved ignition cliffs that show considerably better accuracy than traditional scaling laws, which are observed. The results demonstrate that ML methods have promising application prospects for quantifying ignition margins and can be useful in optimizing ignition target designs and practical implosion experiments.

A case study of using x-ray Thomson scattering to diagnose the in-flight plasma conditions of DT cryogenic implosions

The design of inertial confinement fusion ignition targets requires radiation-hydrodynamics simulations with accurate models of the fundamental material properties (i.e., equation of state, opacity, and conductivity). Validation of these models is required via experimentation. A feasibility study of using spatially integrated, spectrally resolved, x-ray Thomson scattering measurements to diagnose the temperature, density, and ionization of the compressed DT shell of a cryogenic DT implosion at two-thirds convergence was conducted. Synthetic scattering spectra were generated using 1D implosion simulations from the LILAC code that were post processed with the x-ray scattering model, which is incorporated within SPECT3D. Analysis of two extreme adiabat capsule conditions showed that the plasma conditions for both compressed DT shells could be resolved.

Behavior of Gas Injected Fast Ignition Targets

Behavior of Gas Injected Fast Ignition Targets

Preliminary Cryogenic Layering by the Infrared Heating Method Modified with Cone Temperature Control for the Polystyrene Shell FIREX Target

The infrared (IR) heating method for a central ignition target with spherical symmetry is modified for the axisymmetric Fast Ignition Realization EXperiment (FIREX) target. The challenge is that the FIREX target pretends to be a thermally spherical shell. Our previous simulation studies (A. Iwamoto et al., Fusion Sci. Technol. 56, 427 (2009), A. Iwamoto et al., J. Phys.: Conf. Ser. 244, 032039 (2010)) have shown that the combination of volumetric heating in a fuel and cone temperature control has the potential to finish a uniform fuel layer. We have developed the IR heating system, dedicated to the FIREX target, with exclusive cone temperature control. The ability of solid fuel layering was examined by using an 826 µm polystyrene (PS) shell with a gold cone of 1.2 mm in length instead of the 500 µm FIREX target for easy observation. The system could control the profile of a solid fuel layer in the PS shell target. Eventually, the solid layer with the best sphericity of 92% was formed, and the RMS roughness of the inner surface was 44 - 49 µm in modes 1 to 100 and 14 - 26 µm in modes 5 to 100.

Improvement of ignition and burning target design for fast ignition scheme

For the fast ignition scheme of laser fusion, a highly compressed fuel core is necessary using a cone-inserted spherical solid target. We design an optimal compression method with a multistep laser pulse shape using a 2D radiation hydrodynamics simulation. In the design, the maximum areal density is ρR max = 0.46 g cm−2 with 8 kJ of implosion laser. Considering the scaling law of hydrodynamic, we obtained 82 kJ and 1.3 MJ for the ignition-scale-target (ρR max = 1.1 g cm−2) and burning-scale-target (ρR max = 2.5 g cm−2), respectively. In a preliminary study of hydrodynamic instability in the optimized cone-inserted implosion, there is no significant growth of the instabilities that affect the implosion performance. A parameter search to investigate the feasibility and robustness of a high areal density design for changes in the laser pulse shape is performed. As a result, a delay of several tens of pico-second of the rising laser pulse is acceptable.

Optimization of tungsten-doped high density carbon target in inertial confinement fusion

A tungsten (W)-doped high density carbon (HDC) target is a promising design for an ignition target in inertial confinement fusion (ICF). The influence of silicon (Si) and W on the transmission of M-band x-ray has been studied by experiment. With a radiation temperature of ∼200 eV, the transmitted M-band x-ray (1.6–4.4 keV) flux and spectrum of Si or W-buried HDC sample were measured by M-XRDs and TGS, respectively. The thin layer of Si or W was buried at two different depths (2.1 μm and 11 μm). Results show that M-band transmission flux of Si-buried HDC sample decreases with buried depth (bd). However, bd does not influence that of the W-buried HDC sample. The one-dimensional simulation result is consistent with the experimental result. In the 1D simulation, the Au M-band (2–5 keV) transmission flux of Si-buried HDC also decreases with bd. However, the Au M-band transmission flux of W-buried HDC increases at first and then decreases. This is mainly due to the different characteristics of Si and W opacity. Especially as the peak radiation temperature reaches 260 eV, W can still absorb the M-band x-ray efficiently as it is buried near the radiation source. Based on these studies, an optimized W-doped HDC target with low doped fraction and mass has been proposed in this paper.

An extended scaling for the ignition threshold through statistical modeling

In laser-driven fusion, determining the ignition margin is an important prerequisite for evaluating the ignition robustness of a target design. The ignition threshold factor (ITF), defined as the shell kinetic energy at the time of maximum implosion velocity divided by the minimum ignition energy of the capsule, is widely adopted as a specific metric of the ignition margin. In this paper, in contrast to previous definitions of ITF, an additional quantity, i.e., the shell aspect ratio (Ar) at the maximum implosion velocity time, is found to have an important influence on the ignition margin. With including the quantity of Ar, we have obtained an extended ITF through the statistical modeling of following two steps with the help of a free available MULTI-IFE code and the PyMC3 Bayesian inference package: first, the sensitivity of the ignition cliff on implosion physical quantities at the maximum implosion velocity time is evaluated and the importance of Ar is revealed; second, an extended ITF that is proportional to Arα(α≈−1.6) is obtained. Our simulations on fusion yields identify a definitive ignition cliff when the extended ITF equals unity. We then conclude that the shell aspect ratio Ar is an important quantity in our extended ITF expression that will be helpful for evaluating and optimizing the ignition target designs and practical implosion experiments.

Optimal design of shock ignition target in order to stop generated hot electrons within the cold fuel

AbstractGeneration of hot electrons (HEs) within ignitor pulse interaction with pre‐compressed fuel is an important challenge in the shock ignition approach. Target optimization in order to prevent the destructive effects of HE is the main goal of the present work. In the first stage, the spectrum of electron energy generated during the interaction of ignitor pulse at different widths with the HiPER pre‐compressed target has been estimated by applying particle simulation tool. Then, by changing the thickness of the cold fuel in the range of 185–225 μm, the corresponding areal densities are calculated using 1D hydrodynamic simulations. Finally, in order to assess the energy fusion yield, the iso‐gain curves are obtained for different ignitor time windows as well as target thicknesses. Simulation results indicate that by decreasing the baseline, target thickness leads to a 17–70% increase in the fuel areal density. Subsequently, it has been demonstrated that by properly adjusting the parameters of ignitor pulse launch time and its width and employing a target with areal density high enough to stop the HEs, energy gain above 140 can be achieved. Optimal areas for shell thickness and ignitor time window are identified.