Exploration of polar direct drive wetted foam concepts for neutron sources on the National Ignition Facility laser
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Design options for achieving burning plasma and gain for next-generation laser direct-drive facilities
The development of next-generation laser technology opens up the potential to build a direct-drive inertial confinement fusion facility that is capable of reaching burning plasma or ignition conditions at moderate laser energies in the hundreds of kilojoules. The addition of bandwidth to the laser source and the use of focal-spot zooming are predicted to significantly increase the ablative drive pressure, while a larger number of beam-ports (∼100) will keep irradiation nonuniformities below an acceptable limit. This study outlines target design options for reaching burning plasma and ignition using laser direct-drive (LDD). It is shown that such designs are predicted to robustly reach ignition at a laser drive energy of 250 kJ. Two-dimensional (2D) simulations are used to investigate the scaling of target performance vs power imbalance, mispointing and imprint. The simulations confirm that 250 kJ is sufficient to maintain gain above unity when perturbed with the amount of power imbalance, mispointing and laser imprint achievable in current direct-drive experiments on OMEGA.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Physics of laser direct-drive liquid deuterium–tritium wetted foam capsules for inertial confinement fusion
Laser direct-drive liquid deuterium–tritium (DT) wetted foam capsules hold substantial promise for future advancements in inertial confinement fusion (ICF). For this new class of ICF capsules, additive manufacturing (AM) techniques are used to create low aspect ratio spherical shells of a low-density, foam-like CH lattice, which is wetted with cryogenic liquid DT. In the present paper, we discuss key physics issues intrinsic to ignition and burn propagation in laser direct drive wetted foam capsules. These include requirements on laser energy and power, implosion velocity, fuel adiabat, and initial density and dimensions of the CH lattice shell.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
The role of thermal conduction on the size scaling of laser direct-drive implosions
Ablation pressure is a key parameter controlling laser drive performance in direct-drive inertial confinement fusion implosions. In this work, we develop a theoretical framework to clarify the role of thermal conduction in determining how ablation pressure scales with laser intensity, wavelength, and target size. We focus on a steady-state, spherically expanding plasma produced when an over-dense pellet is irradiated by a high-intensity laser. Scaling laws for ablation pressure and the mass ablation rate are derived with respect to laser intensity, wavelength, and target size. As a result, the theoretical predictions are found to be in good agreement with one-dimensional hydrodynamics simulations in spherical geometry.
Ablation↗
Progress toward hydro-equivalent ignition in OMEGA direct-drive DT-layered implosions
Considerable progress has been made in deuterium-tritium-layered implosion experiments on the OMEGA Laser System, bringing the prospects for thermonuclear ignition in direct-drive configurations with megajoule-class lasers closer to reality. Doing so has required navigating the balance between improved 1D performance and multidimensional stability. Using statistical modeling based on over 350 cryogenic implosions to identify various degradation mechanisms, and combined with multidimensional simulations and experimental techniques such as target offsets to combat residual flows, core conditions have repeatably been achieved that extrapolate to the burning-plasma state when scaled to 2.15 MJ of symmetric laser illumination. Using high implosion velocities (> 450 km/s) and moderately high adiabats (~5), these experiments produced record-high scaled Lawson parameters in direct drive equal to 89 ± 2% of that required for ignition with expected yields of up to 1.5 ± 0.2 MJ. To improve these results still further, focused physics studies are performed to improve physics understanding and identify routes to even greater performance. Recent studies include investigations into the impact of mounting features, laser imprint, reduced fuel temperatures, and greater on-target intensities through subscale experiments. This manuscript gives a summary of the cryogenic direct-drive program on the OMEGA laser, including routes taken to achieve the current best performance, the status of recent focused physics investigations, and future designs—such as target solutions to laser imprint and reducing vapor density to increase convergence—that are expected lead to the demonstration of hydro-equivalent ignition on OMEGA.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Approaching hydro-equivalent ignition in laser direct-drive via target design optimization using novel statistical modeling
Laser direct-drive offers significant advantages in terms of target simplicity, improved energy coupling, and large fuel masses over indirect drive. However, performance degradations from hydrodynamic and laser-plasma instabilities seeded and driven by the direct illumination pose limitations on the parameter space available for achieving ignition. In this paper, new design improvements are identified to forge a path forward for a hydro-equivalent ignition demonstration. The first is related to a new formulation of the statistical model (SM) used to accurately predict target performance directly from input parameters such as laser pulse shape and target specifications. This new SM formulation provides direct guidance on target dimensions and laser beam-to-target radius to achieve the highest fusion yield on the OMEGA laser. The second improvement comes from cooling the deuterium–tritium (DT) ice layer below the triple point right before shot time leading to lower DT vapor densities and higher convergence. Guided by these design improvements, a Bayesian optimization algorithm was used to design an implosion that is predicted to closely approach a Lawson triple product that hydrodynamically scales to ignition if equivalent laser–target coupling is achieved at laser energies typical of the National Ignition Facility.
Deuterium↗
Low-mode nonuniformity in direct-drive ICF implosions due to laser smoothing techniques employed on OMEGA
For successful laser-direct-drive inertial confinement fusion implosions, the laser irradiation must be highly uniform over the target surface. On OMEGA, multiple laser beams are used to illuminate targets quasi-uniformly. High-mode-number nonuniformities due to laser speckle on each individual beam are reduced by splitting each beam into two orthogonal polarizations (i.e., polarization smoothing, or PS) and a range of wavelengths (i.e., smoothing by spectral dispersion) that are dispersed at the target plane. However, cross-beam energy transfer (CBET) is sensitive to both the polarizations and wavelengths of the interacting beams, so the interplay between CBET and the laser-smoothing schemes results in unique intensity variation across each beam profile, which is a systematic source of low-mode drive nonuniformity on OMEGA. Here, we model these effects and find that the predicted ℓ = 1 mode in the laser-absorption distribution is consistent with the systematic core-flow direction that has been determined from the OMEGA implosion database. We also observe good agreement with the measured core-flow directions for two specific sets of implosions (one with PS, the other without PS) when we also account for the measured beam mispointing and the beam power imbalance.
Crossed beam scattering↗
Hybrid direct drive with a two-sided ultraviolet laser
This paper presents a “hybrid” approach to direct drive inertial confinement fusion that can exploit a high-energy gas laser with two opposed beams. The target and driver are asymmetric, much like experiments performed on the National Ignition Facility, but have been designed to benefit from scale and their particular compatibility with a fusion power plant. The imploded masses (and areal densities) are increased by a factor of 12 (3) and provide a path to high-gain implosions that robustly ignite. The design also mitigates common concerns such as laser imprint and cross-beam energy transfer. We discuss the rationales for a hybrid target, the methods used to control implosion symmetry, and the implication(s) for inertial fusion energy.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Mitigation of hot-electron preheat from the two-plasmon-decay instability using silicon-doped plastic shells in direct-drive implosions on OMEGA
Hot electrons generated by the two-plasmon-decay (TPD) instability in laser-direct-drive implosions preheat the fuel and degrade performance. The mitigation of preheat using silicon-doped ablators (i.e., preheat is reduced by a factor of 1.40 ± 0.04) while decreasing the ratio of the laser spot diameter to the target diameter (R b /R t ) to mitigate cross-beam energy transfer has been demonstrated on the OMEGA laser for quarter-critical laser intensities of 4.1 x 10 14 W/cm 2 , equivalent to an incident laser intensity of 8.3 x 10 14 W/cm 2 . The silicon dopant increases the electron temperature of the ablation plasma, which raises the intensity threshold for the onset of the TPD instability. These results show that implosion designs utilizing higher drive intensities can be used to achieve higher shell velocities, which are currently inaccessible with plastic ablators due to excessive preheat.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Wetted-foam target stability for laser-direct-drive inertial confinement fusion
Using highly resolved three-dimensional radiation-hydrodynamic simulations, the effects of wetted-foam (WF) ablators in direct-drive inertial confinement fusion cryogenic implosions on the OMEGA laser were investigated. The performance of these implosions was compared with that of implosions using targets with solid-plastic ablators. Simulations considering WF ablators as a homogeneous material show no mitigation of the effects of laser imprint, where the absence of this mitigation can be attributed to the use of picket pulses. Heterogeneous random-structure WF ablators introduce large short-wavelength perturbations, which are seeded by WF inhomogeneities. These perturbations exceed the perturbations seeded by the imprint and cause an additional reduction in implosion performance when the average pore size in the WF exceeds ~0.2 μm. Foam-seeded perturbations are expected to be less damaging in more stable high-adiabat ( ≳ 5) implosions. Here, the perturbations introduced by the WF ablators can be substantially reduced by using a foam matrix with a layered structure instead of a random one. Further study of the effects of WFs in ignition designs of the MJ laser energy scale is required in the perspective of future inertial fusion energy mass production.
Igumenshchev, I. V. [University of Rochester, NY (↗
Correlating hot-electron preheat from the two-plasmon decay instability to $ρR$ degradation for direct-drive implosions on OMEGA
Focused experiments were performed with the goal of isolating the impact of hot-electron preheat on compression in deuterium–tritium (DT) layered, direct-drive implosions on OMEGA. Preheat from the hot-electrons generated by two-plasmon–decay (TPD) activity was varied through the incident laser intensity, with the total preheat energy deposited into unablated DT fuel being inferred using hard x-ray measurements. Results show a clear correlation between the inferred preheat levels and $ρR$ degradation that is consistent with a derived, 1D preheat model. A fit to the results also provides an estimated range of $ρR$ degradation from non-preheat sources for the adiabat ≈5 implosions used in this study. A follow-up study using targets containing Si-doped (5.5%) CH ablators (CHSi) demonstrated results consistent with mid-Z dopants mitigating TPD activity by showing these targets had almost no $ρR$ degradation as the laser intensity was increased up to levels previously considered damaging.
Doping↗
Size scaling of acceleration phase energetics and its effects on direct-drive DT-layered implosions
A fundamental question in inertial confinement fusion is how implosion performance, and therefore ignition thresholds and fusion gain, evolve with target size. In laser-driven direct drive fusion, the scaling of laser-drive performance with size is critical to this evolution and to extrapolating results from the 30-kJ OMEGA laser-fusion experiments to ignition-class facilities such as the National Ignition Facility. Beyond the well-known adverse effects of cross-beam energy transfer (CBET) on drive performance, here we demonstrate that effects related to the non-scaling physics of thermal conduction and electron–ion energy equilibration exert an influence on drive behavior with scale that equals or surpasses that of CBET. We find that a significant portion of the lost implosion performance with increasing scale is due to the loss of shell implosion velocity. Here, we show that while modest modifications to hydro-scaled designs can recover most of the lost implosion velocity, a full hydro-equivalent performance extrapolation is difficult to achieve without CBET mitigation or subcooling below the triple point of DT.
Patel, D. [University of Rochester, NY (United Sta↗
Enhancements in Laser-Direct-Drive Nuclear Performance with Target Radius
Inertial confinement fusion is likely to require significant improvements in technology and design for large targets to implode at high driver energies. Here, to assess the potential for benefits, we report on nuclear performance as a function of target radius 𝑅 t in direct-drive cryogenic implosions as performed on the OMEGA laser. The neutron yield and areal density are found to increase as 𝑅 5.0±0.2 t and 𝑅 1.8±0.2 t , respectively, and exhibit a much stronger dependence on target radius than previous studies have assumed. As this paper demonstrates, these types of sensitivities should be expected when implosions are unstable, and have been degraded by a range of multidimensional effects in 3D. If the imperfections in a target and laser system are fixed in magnitude, it follows that larger implosions have higher relative quality, and approach criteria to ignite much more quickly than commonly appreciated.
inertial confinement fusion↗
Simulation of Direct-Drive Hybrid Using Two Opposed Beams for Inertial Fusion Energy
Xcimer Energy is working to deliver high levels of laser light at the costs required for applications in fusion energy. The architecture of their integrated laser system is unique, in that it naturally supports a target which is illuminated by two opposed beams, and in a manner that can readily complement a reactor. This INFUSE proposal was submitted to investigate a target concept termed the Direct-Drive Hybrid or DDH, and make special use of radiation hydrodynamic simulations and expertise available to the Laboratory for Laser Energetics. The associated capabilities are unique to the field, and take great advantage of investments and advances made by the DOE and DOD over many decades. The primary goals were to stand up the DDH design in calculations for the first time, study and refine the concept, and provide the scientific basis for understanding and projecting proposals by Xcimer Energy. The main accomplishments have been summarized below.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Automated and highly parallelized Bayesian optimization scheme for direct drive fusion experiments on OMEGA
Finding the optimal implosion design on existing experimental facilities for inertial confinement fusion requires an exhaustive search of the vast design parameter space. This is infeasible both with experiments and with simulations. Consequently, a large fraction of the experimentally realizable design space remains unexplored, and new design schemes are challenging to optimize in a reasonable time frame. On the OMEGA laser facility, predictive machine learning models have been developed to accurately forecast the result of an experiment using only inexpensive simulations and the large dataset of prior experimental data. However, the full design space remains vast enough to be unassailable with simple optimization techniques. Here we develop an automated and optimally parallel Bayesian optimization algorithm that can entirely optimize the target and pulse shape of a direct-drive ICF implosion under a given design paradigm. We use this algorithm to find a markedly improved design for the performance implosions on OMEGA that is predicted to hydroequivalently scale to ignition at 2.15 MJ.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Performance enhancement of direct-drive shock-augmented ignition inertial fusion implosions through shock timing optimization
Shock-augmented ignition (SAI) [R. H. H. Scott et al., Phys. Rev. Lett. 129, 195001 (2022)] is an alternative inertial confinement fusion concept that is designed to achieve high energy gain by combining improved resilience to instabilities with enhanced fuel compression. In SAI, lower implosion velocities can improve hydrodynamic stability and limited laser intensities reduce the excitation of detrimental laser-plasma instabilities, enabling greater areal density accumulation. Here we report an experimental investigation of SAI using a series of warm D 2 implosions. By timing the augmenting shock to arrive at the implosion center immediately prior to peak compression, the areal density and hot-spot pressure increase by 50% and 112%, respectively, relative to unoptimized designs. Ignition-scale simulations reproduce the observed timing dependence and demonstrate the potential of SAI for inertial fusion energy.
Direct drive↗
Distortions in charged-particle images of laser direct-drive inertial confinement fusion implosions
Energetic charged particles generated by inertial confinement fusion (ICF) implosions encode information about the spatial morphology of the hotspot and dense fuel during the time of peak fusion reactions. The knock-on deuteron imager (KoDI) was developed at the Omega Laser Facility to image these particles in order to diagnose low-mode asymmetries in the hotspot and dense fuel layer of cryogenic deuterium–tritium ICF implosions. However, the images collected are distorted in several ways that prevent reconstruction of the deuteron source. In this paper, we describe these distortions and a series of attempts to mitigate or compensate for them. We present several potential mechanisms for the distortions, including a new model for scattering of charged particles in filamentary electric or magnetic fields surrounding the implosion. Particle-tracing is used to create synthetic KoDI data based on the filamentary field model that reproduces the main experimentally observed image distortions. We conclude that the filamentary scattering model best matches the observed image distortions. Finally, we discuss potential impacts of filamentary fields on other charged-particle diagnostics.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Inferring performance metrics for laser direct drive experiments on OMEGA
Quantifying performance improvements on the OMEGA laser facility requires robust inference of established no-alpha performance metrics, which requires, at minimum, a model to infer the shocked fuel mass and pressure of the confined fusion plasma. In this work, we describe the methodology used to infer performance metrics on OMEGA and present the current state-of-the art model used to infer these metrics from OMEGA experiments. In particular, since neutron images of cryogenic implosions are not available on OMEGA at present, we present how x-ray sizes are determined on OMEGA using a Gaussian Process regression model and how the neutron production region's size is inferred from them. As a result, we end by benchmarking the model using synthetic data and 1-D LILAC simulations and test its experimental self-consistency across available x-ray diagnostic channels.
Gopalaswamy, V. [Laboratory for Laser Energetics, ↗