Fusion Ignition Experiment Research Articles

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.

Ionization by XFEL radiation produces distinct structure in liquid water

In the warm dense matter (WDM) regime, where condensed, gas, and plasma phases coexist, matter frequently exhibits unusual properties that cannot be described by contemporary theory. Experiments reporting phenomena in WDM are therefore of interest to advance our physical understanding of this regime, which is found in dwarf stars, giant planets, and fusion ignition experiments. Using 7.1 keV X-ray free electron laser radiation (nominally 5×105 J/cm2), we produced and probed transient WDM in liquid water. Wide-angle X-ray scattering (WAXS) from the probe reveals a new ~9 Å structure that forms within 75 fs. By 100 fs, the WAXS peak corresponding to this new structure is of comparable magnitude to the ambient water peak, which is attenuated. Simulations suggest that the experiment probes a superposition of two regimes. In the first, fluences expected at the focus severely ionize the water, which becomes effectively transparent to the probe. In the second, out-of-focus pump radiation produces O1+ and O2+ ions, which rearrange due to Coulombic repulsion over 10 s of fs. Our simulations account for a decrease in ambient water signal and an increase in low-angle X-ray scattering but not the experimentally observed 9 Å feature, presenting a new challenge for theory.

Hybrid CMOS detectors for high-speed X-ray imaging.

Hybrid CMOS (hCMOS) x-ray framing cameras are a new and powerful detector option for experiments in the fields of Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP). These digital cameras capture multiple images along a single line-of-sight with a time resolution as short as 1.5ns and with high quantum efficiency. To manage the high data rate, an image sequence is acquired in a short burst of time and subsequently read out on a much longer time scale. The technology is well suited for operating in high radiation environments, including fusion ignition experiments. Diagnostics using hCMOS cameras are now deployed in experiments on major laser and pulsed-power ICF facilities around the world. Continued advances in microelectronics technologies will enable faster and more capable detectors well into the future. This paper reviews this detector technology with a focus on application to ICF and HEDP experiments.

Optimization of Backscatter and Symmetry for Laser Fusion Experiments Using Multiple Tunable Wavelengths

A new additional wavelength-tuning capability has been implemented on the National Ignition Facility (NIF) laser allowing for unprecedented control of crossed-beam energy transfer (CBET) between all groups of beams for better performance of indirect-drive inertial confinement fusion (ICF) ignition experiments. In particular, this advance allows for negation and reversal of flow-induced CBET and the rebalancing of the intensity of different groups of beams within an indirect-drive (ICF) hohlraum. Experiments conducted at the NIF using a 1.1 MJ laser pulse with peak power of 390 TW demonstrate this high level of control through measurements of the precise changes to stimulated Brillouin scattering, typically driven by flow-induced CBET at the end of the pulse. Additionally, this new capability is shown to be able to predictably control gold-wall plasma expansion in the target driven by early-time flow-induced CBET. Estimates of early-time CBET from the wall expansion are shown to be consistent with simulation expectations. This new additional capability to control symmetry and backscatter expands the design space of current experiments and provides extra margin for increased laser energy in near-future experiments.

Analysis of design principles of the experiments on the National Ignition Facility since 2010

<sec>Since completion of the National Ignition Facility (NIF) in 2010, more than 1030 experiments were carried out to achieve ignition. Though the experiments were unsuccessful in the first 8 years, the NIF has improved the experimental designs and achieved fusion yields from 55kJ, 170kJ to 1.35MJ since 2019, approaching to the ignition milestone. The designs are based on the experimental database, which has been widely used for optimization design, yield prediction, corrected simulation, etc. However, so far the published experimental data is very limited. Also, it is difficult to obtain a completion data matrix for analyzing and understanding the experimental designs of NIF experiments at each stage and to know how the NIF sets strategic priorities for each phase.</sec><sec>In this paper, we proposed an optimization method, which combines the PMM algorithm and trust region algorithm, to restore the missing NIF experimental data. Based on the completed data, the design principles of experiments on the NIF were analyzed, and the hot spot pressure was predicted by machine learning algorithms. The results may be helpful for the designs of laser fusion ignition experiments in China.</sec>

2-D Numerical Investigation of the Formation of Z-Pinch-Driven Dynamic Hohlraum at 8-MA Current Level

The newly developed code MULTI2D-Z is applied to simulate the formation process of a dynamic hohlraum driven by tungsten wire-array Z-pinch at 8-MA current level. In the initial phase, the foam is expanding due to the radiation heating from the imploding wire-array plasma. At the same time, this causes a compression-wave spreading inward in the foam, and in turn a challenge for the safety of capsule embedded in the center of the foam in the inertial confinement fusion ignition experiment. There is instability development during the collision of the expanding foam plasma and the imploding wire-array plasma. However, fortunately, this instability was then spontaneously restrained due to the subsequent snow-plow implosion. The formed wall-like high-Z plasma (shock wave), in which the radiation mean free path is about 10 $\mu \text{m}$ , compresses the foam, and plays a well role in trapping the radiation in the foam. When the foam has been compressed a little and the high-Z plasma temperature decreases, the dynamic hohlraum begins to form. While the thermal radiation wave in the foam is spreading toward the center axis, the radiation temperature profile becomes comparatively uniform, and almost equates to the plasma (matter) temperature except at the place of the shock wave. The radiation temperature in the hohlraum is above 100 eV. The simulated results also suggested that the radiation energy loss to the electrodes would be one of the main reasons for the nonuniformity in the radiation field and implosion instability development during hohlraum formation.

Progress in detailed modelling of low foot and high foot implosion experiments on the National Ignition Facility

Several dozen high convergence inertial confinement fusion ignition experiments have now been completed on the National Ignition Facility (NIF). These include both “low foot” experiments from the National Ignition Campaign (NIC) and more recent “high foot” experiments. At the time of the NIC, there were large discrepancies between simulated implosion performance and experimental data. In particular, simulations over predicted neutron yields by up to an order of magnitude, and some experiments showed clear evidence of mixing of ablator material deep into the hot spot that could not be explained at the time. While the agreement between data and simulation improved for high foot implosion experiments, discrepancies nevertheless remain. This paper describes the state of detailed modelling of both low foot and high foot implosions using 1-D, 2-D, and 3-D radiation hydrodynamics simulations with HYDRA. The simulations include a range of effects, in particular, the impact of the plastic membrane used to support the capsule in the hohlraum, as well as low-mode radiation asymmetries tuned to match radiography measurements. The same simulation methodology is applied to low foot NIC implosion experiments and high foot implosions, and shows a qualitatively similar level of agreement for both types of implosions. While comparison with the experimental data remains imperfect, a reasonable level of agreement is emerging and shows a growing understanding of the high-convergence implosions being performed on NIF.

Ignition conditions relaxation for central hot-spot ignition with an ion-electron non-equilibrium model

Fusion ignition experiments on the National Ignition Facility have demonstrated >5 keV hot spot with ρRh lower than 0.3 g/cm2 [Döppner et al., Phys. Rev. Lett. 115, 055001 (2015)]. We present an ion-electron non-equilibrium model, in which the hot-spot ion temperature is higher than its electron temperature so that the hot-spot nuclear reactions are enhanced while energy leaks are considerably reduced. Theoretical analysis shows that the ignition region would be significantly enlarged in the hot-spot ρR-T space as compared with the commonly used equilibrium model. Simulations show that shocks could be utilized to create and maintain non-equilibrium conditions within the hot spot, and the hot-spot ρR requirement is remarkably reduced for achieving self-heating.

Radiation hydrodynamics modeling of the highest compression inertial confinement fusion ignition experiment from the National Ignition Campaign

The recently completed National Ignition Campaign (NIC) on the National Ignition Facility (NIF) showed significant discrepancies between post-shot simulations of implosion performance and experimentally measured performance, particularly in thermonuclear yield. This discrepancy between simulation and observation persisted despite concerted efforts to include all of the known sources of performance degradation within a reasonable two-dimensional (2-D), and even three-dimensional (3-D), simulation model, e.g., using measured surface imperfections and radiation drives adjusted to reproduce observed implosion trajectories [Clark et al., Phys. Plasmas 20, 056318 (2013)]. Since the completion of the NIC, several effects have been identified that could explain these discrepancies and that were omitted in previous simulations. In particular, there is now clear evidence for larger than anticipated long-wavelength radiation drive asymmetries and a larger than expected perturbation seeded by the capsule support tent. This paper describes an updated suite of one-dimensional (1-D), 2-D, and 3-D simulations that include the current best understanding of these effects identified since the NIC, as applied to a specific NIC shot. The relative importance of each effect on the experimental observables is compared. In combination, these effects reduce the simulated-to-measured yield ratio from 125:1 in 1-D to 1.5:1 in 3-D, as compared to 15:1 in the best 2-D simulations published previously. While the agreement with the experimental data remains imperfect, the comparison to the data is significantly improved and suggests that the largest sources for the previous discrepancies between simulation and experiment are now being included.

A survey of pulse shape options for a revised plastic ablator ignition design

Recent experimental results using the “high foot” pulse shape for inertial confinement fusion ignition experiments on the National Ignition Facility (NIF) [Moses et al., Phys. Plasmas 16, 041006 (2009)] have shown encouraging progress compared to earlier “low foot” experiments. These results strongly suggest that controlling ablation front instability growth can significantly improve implosion performance even in the presence of persistent, large, low-mode distortions. Simultaneously, hydrodynamic growth radiography experiments have confirmed that ablation front instability growth is being modeled fairly well in NIF experiments. It is timely then to combine these two results and ask how current ignition pulse shapes could be modified to improve one-dimensional implosion performance while maintaining the stability properties demonstrated with the high foot. This paper presents such a survey of pulse shapes intermediate between the low and high foot extremes in search of an intermediate foot optimum. Of the design space surveyed, it is found that a higher picket version of the low foot pulse shape shows the most promise for improved compression without loss of stability.

Experimental investigation on sub-miliKelvin temperature control at liquid hydrogen temperatures

Temperature control around 20K with a tolerance of better than 1 mili-Kelvin is challenging but essential for applications like cooling deuterium/tritium target in fusion ignition experiments. To explore the practicability of cryogenic temperature control on this level with a cryocooler as the cold source instead of a cryogen bath, experimental investigations were conducted to approach the best performance by optimizing configurations of effective measures. A special heat sink was fabricated and attached to the second-stage cold head of a G-M cryocooler which could reach temperatures down to 2.5K and provide 13W cooling power at 18K. A thermal reservoir and/or temperature transmission damping were incorporated, as well as a manganin resistance wire electrical heater attached to the heat sink. Experimental results show that an optimal stability of the temperature of the shielded heat sink within±0.4 mK at 20K was achieved by choosing proper excitation for the sensors, control mode for the heater and capacity for the thermal reservoir.

Molecular dynamics with atomic transitions and nuclear reactions

We describe molecular dynamics particle simulations with particles that have internal structure and/or undergo reactions. These calculations give an atomic-scale description of hot plasma based on well-established microphysics and test kinetic theories used to calculate energy exchange and transport in plasma hydrodynamic simulations. The computer experiments can be given detailed diagnostics, without the usual limits of resolution of Laboratory equipment. Typical applications are non-equilibrium atomic kinetics for emission or absorption of X-ray laser radiation by solid targets and/or atomic-scale simulation of hot plasma with fusion reactions, as in inertial fusion ignition experiments.

Detailed implosion modeling of deuterium-tritium layered experiments on the National Ignition Facility

More than two dozen inertial confinement fusion ignition experiments with cryogenic deuterium-tritium layers have now been performed on the National Ignition Facility (NIF) [G. H. Miller et al., Opt. Eng. 443, 2841 (2004)]. Each of these yields a wealth of data including neutron yield, neutron down-scatter fraction, burn-averaged ion temperature, x-ray image shape and size, primary and down-scattered neutron image shape and size, etc. Compared to 2-D radiation-hydrodynamics simulations modeling both the hohlraum and the capsule implosion, however, the measured capsule yield is usually lower by a factor of 5 to 10, and the ion temperature varies from simulations, while most other observables are well matched between experiment and simulation. In an effort to understand this discrepancy, we perform detailed post-shot simulations of a subset of NIF implosion experiments. Using two-dimensional HYDRA simulations [M. M. Marinak, et al., Phys. Plasmas 8, 2275 (2001).] of the capsule only, these simulations represent as accurately as possible the conditions of a given experiment, including the as-shot capsule metrology, capsule surface roughness, and ice layer defects as seeds for the growth of hydrodynamic instabilities. The radiation drive used in these capsule-only simulations can be tuned to reproduce quite well the measured implosion timing, kinematics, and low-mode asymmetry. In order to simulate the experiments as accurately as possible, a limited number of fully three-dimensional implosion simulations are also being performed. Despite detailed efforts to incorporate all of the effects known and believed to be important in determining implosion performance, substantial yield discrepancies remain between experiment and simulation. Some possible alternate scenarios and effects that could resolve this discrepancy are discussed.

Experimental research on X-ray radiation and ablation of an Ag foil targets irradiated by high intensity 2ω0 laser light beam

The characteristics of radiation and ablation are investigated for an Ag foil irradiated with 2 ns, ～5×1014 W/cm2, 526.5 nm laser at SGII laser facility. The flight trajectory and velocity of the Ag foil are measured by X-ray streak camera. The experimental results show that they are in good agreement with simulations of one-dimensional radiation hydrodynamics code multi-1d using a flux-limited f=0.01. A rocket model is used to fit the experimental data, and the mass ablation rate and ablation pressure are obtained. The L-shell X-ray conversion and spectra of the laser-produced Ag plasma are measured with a Bragg crystal spectrometer and an array of X-ray diodes. The design for X-ray backlighting radiograph experiments will benefit from these experimental results. The result and method presented in this article are significant for the ablation research on the capsule shell and cavity wall material in laser fusion ignition experiment.

Ab initio calculations of the equation of state of hydrogen in a regime relevant for inertial fusion applications

We describe ab initio electronic structure calculations (density functional theory molecular dynamics and coupled electron-ion quantum Monte Carlo) of the equation of state (EOS) of hydrogen in a pressure–temperature regime relevant for simulating the initial phase of an inertial confinement fusion capsule implosion. We find the computed EOS to be quite close to that of the most recent SESAME table (constructed by G. Kerley, 2003). A simple density-dependent but temperature-independent correction brings the 2003-Kerley EOS into excellent agreement with ours in the chosen region of the hydrogen phase diagram. Simulations of fusion ignition experiments on the National Ignition Facility (NIF) with this modified 2003-Kerley table are shown to produce results nearly indistinguishable from those of the 2003-Kerley EOS, which was used to design the capsule. In this sense, we do not expect that further improvements to the hydrogen EOS in this particular regime will impact the capsule design.

Amplification of light in a plasma by stimulated ion acoustic waves driven by multiple crossing pump beams

Experiments demonstrate the amplification of 351 nm laser light in a hot dense plasma similar to those in inertial confinement fusion ignition experiments. A seed beam interacts with one or two counter-propagating pump beams, each with an intensity of 1.2×10(15) W/cm2 at 351 nm, crossing the seed at 24.8° at the position where the flow is Mach 1, allowing resonant stimulation of ion acoustic waves. Results show that the energy and power transferred to the seed are increased with two pumps beyond the level that occurs with a single pump, demonstrating that, under conditions similar to ignition experiments where each beam has a low gain exponent, the total scatter produced by the multiple beams can be significantly larger than that of the individual beams. It is further demonstrated that the amplification is greatly reduced when the pump polarization is orthogonal to the seed, as expected from models of stimulated scatter.

Plastic ablator ignition capsule design for the National Ignition Facility

The National Ignition Campaign, tasked with designing and fielding targets for fusion ignition experiments on the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, S228 (2004)], has carried forward three complementary target designs for the past several years: a beryllium ablator design, a plastic ablator design, and a high-density carbon or synthetic diamond design. This paper describes current simulations and design optimization to develop the plastic ablator capsule design as a candidate for the first ignition attempt on NIF. The trade-offs in capsule scale and laser energy that must be made to achieve a comparable ignition probability to that with beryllium are emphasized. Large numbers of one-dimensional simulations, meant to assess the statistical behavior of the target design, as well as two-dimensional simulations to assess the target’s susceptibility to Rayleigh–Taylor growth are presented.

Experimental D-T Ice-Layering Target Assembly

Inertial confinement fusion ignition experiments in the National Ignition Facility require a capsule containing deuterium-tritium fuel at cryogenic temperatures. To better understand how to produce and control the required uniform fuel ice layer, experimental layering targets are fabricated and assembled to be dimensionally similar to the ignition targets and vacuum leaktight at 18 K. Low production yield of these targets demanded a more quantitative understanding of the interfacial behavior of bonded joints and required the development of more deterministic assembly methods. Each sealing joint was individually analyzed, and target components, assembly processes, and tooling were modified as needed to make robust leaktight targets. The function, design, and assembly methods of experimental layering targets are described.

Design of the NIF Cryogenic Target System

The U.S. Department of Energy has embarked on a campaign to conduct credible fusion ignition experiments on the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in 2010. The target assembly specified for this campaign requires the formation of a deuterium-tritium fuel ice layer in a 2-mm-diam capsule at the center of a 9-mm-long × 5-mm-diam cylinder, called a hohlraum. The ice layer must be formed and maintained at temperatures below 20 K. At laser shot time, the target is positioned at the center of the NIF target chamber, aligned to the laser beams, and held stable to <7-μm root-mean-square. We have completed the final design of the cryogenic target system and are currently integrating the devices necessary to create, characterize, and position the cryogenic target for ignition experiments.

Developing depleted uranium and gold cocktail hohlraums for the National Ignition Facility

Fusion ignition experiments are planned to begin at the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] using the indirect drive configuration [J. D. Lindl, P. Amendt, R. L. Berger, S. G. Glendinning, S. H. Glenzer, S. W. Haan, R. L, Kauffman, O. L. Landen, and L. J. Suter, Phys. Plasmas 11, 339 (2004)]. Although the x-ray drive in this configuration is highly symmetric, energy is lost in the conversion process due to x-ray penetration into the hohlraum wall. To mitigate this loss, depleted uranium is incorporated into the traditional gold hohlraum to increase the efficiency of the laser to x-ray energy conversion by making the wall more opaque to the x rays [H. Nishumura, T. Endo, H. Shiraga, U. Kato, and S. Nakai, Appl. Phys. Lett. 62, 1344 (1993)]. Multilayered depleted uranium (DU) and gold hohlraums are deposited by sputtering by alternately rotating a hohlraum mold in front of separate DU and Au sources to build up multilayers to the desired wall thickness. This mold is removed to leave a freestanding hohlraum half; two halves are used to assemble the complete NIF hohlraum to the design specifications. In practice, exposed DU oxidizes in air and other chemicals necessary to hohlraum production, so research has focused on developing a fabrication process that protects the U from damaging environments. This paper reports on the most current depleted uranium and gold cocktail hohlraum fabrication techniques, including characterization by Auger electron spectroscopy, which is used to verify sample composition and the amount of oxygen uptake over time.