Overview: Development of the National Ignition Facility and the Transition to a User Facility for the Ignition Campaign and High Energy Density Scientific Research

Quantifying Uncertainty in Laser-Induced Damage Threshold for Cylindrical Gratings

The laser-induced damage threshold (LIDT) is a key measure of an optical component's resistance to laser damage, making its accurate determination crucial. Following the ISO 21254 standards, we studied the measurement strategy and uncertainty fitting method for laser damage, establishing a calculation model for uncertainty. Research indicates that precise LIDT measurement can be achieved by using a small energy level difference and conducting multiple measurements. The LIDT values for the cylindrical grating are 15.34 ± 0.00052 J/cm2 (95% confidence) and 15.34 ± 0.00078 J/cm2 (99% confidence), demonstrating low uncertainty and reliable results. This strategy effectively measures the LIDT and uncertainty of various grating surface shapes, offering reliable data for assessing their anti-laser-damage performance.

Improving fourth harmonic generation performance by elevating the operation temperature of ADP crystal

In current inertial confinement fusion (ICF) facilities, potassium dihydrogen phosphate (KH2PO4, KDP) type crystals are the only nonlinear optical (NLO) materials that can satisfy the aperture requirement of the ICF laser driver. Ammonium dihydrogen phosphate (NH4H2PO4, ADP) crystal is a typical isomer of KDP crystal, with a large nonlinear optical coefficient, high ultraviolet transmittance, and large growth sizes, which is an important deep ultraviolet (UV) NLO material. In this paper, we investigated the effect of ADP temperature on its fourth-harmonic-generation (FHG) performance. When the temperature of the ADP crystal was elevated to 48.9 °C, the 90° phase-matched FHG of the 1064 nm laser was realized. Compared with the 79° phase-matched FHG at room temperature (23.0 °C), the output energy at 266 nm, conversion efficiency, angular acceptance, and laser-induced damage threshold (LIDT) increased 113%, 71%, 623%, 19.6%, respectively. It shows that elevating ADP temperature is an efficient method to improve its deep UV frequency conversion properties, which may also be available to other NLO crystals. This discovery provides a very valuable technology for the future development of UV, deep UV lasers in ICF facilities.

A Novel Method for Precision Measurement and Result Optimization of Detuning Angle for KDP Crystals

In this paper, we investigate the theory of energy distribution when divergent light undergoes harmonic conversion in KDP crystals, and based on this theory, we design and construct a precision measuring instrument for the detuning angle of (KDP) Crystals (MIDC). The device can obtain the detuning angle of the crystal by a single measurement with an average measurement error of 72.78 urad. At the same time, it also has the function of scanning the full aperture of the crystals. Using the MIDC, it is possible to quickly measure the KDP crystal at a single point and quickly scan the crystal detuning angle at full aperture. In addition, we conduct a theoretical study on the variation of detuning angle caused by gravity-influencing factors under online conditions, propose an optimization formula for the offline measurement results of detuning angle, and calculate the optimized values of detuning angle for two kinds of crystals under 45° online conditions. We finally study the error source of the MIDC device, analyze the trend of the influence of positioning errors of the crystal and optical elements on the detuning angle measurement results, and provide theoretical support for the error monitoring and correction of MIDC.

Hydrodynamic cumulation mechanism caused by quantum shell effects

The computational and theoretical analysis carried out in this article demonstrates the existence of a nontrivial mechanism for the compression of a submicron-sized gas bubble formed by a gas of classical ions and a gas of degenerate electrons. This mechanism fundamentally differs from conventional compression mechanisms. It is shown that taking into account the quantum effect of a large spatial scale in the distribution of electrons qualitatively changes the character of cumulative processes. Because of a large-scale electric field caused by quantum shell effects, the compression process is characterized by the formation of multiple shock waves. The values of gas temperature and pressure achieved during compression occur higher by two orders of magnitude as compared with the classical adiabatic regime. The analysis is carried out within the framework of the following model: the dynamics of the electron subsystem is described by equations of a quantum electron fluid, while the hydrodynamic approximation is adopted for the ionic subsystem. The large-scale effect is taken into account by means of effective external field acting on electrons. The theoretical analysis carried out within this approach clarifies the nature of the cumulative process in the system under consideration; some quantitative characteristics obtained with numerical simulation are presented. The possibility of experimental observation of this cumulative mechanism is analyzed. It is suggested that the manifestation of the effect can be observed during laser compression of a system of submicron targets by measuring the neutron yield.

Dynamics and Power Balance of Near Unity Target Gain Inertial Confinement Fusion Implosions

The change in the power balance, temporal dynamics, emission weighted size, temperature, mass, and areal density of inertially confined fusion plasmas have been quantified for experiments that reach target gains up to 0.72. It is observed that as the target gain rises, increased rates of self-heating initially overcome expansion power losses. This leads to reacting plasmas that reach peak fusion production at later times with increased size, temperature, mass and with lower emission weighted areal densities. Analytic models are consistent with the observations and inferences for how these quantities evolve as the rate of fusion self-heating, fusion yield, and target gain increase. At peak fusion production, it is found that as temperatures and target gains rise, the expansion power loss increases to a near constant ratio of the fusion self-heating power. This is consistent with models that indicate that the expansion losses dominate the dynamics in this regime.

CNN-based neural network model for amplified laser pulse temporal shape prediction with dynamic requirement in high-power laser facility

The temporal shape of laser pulses is one of the essential performances in the inertial confinement fusion (ICF) facility. Due to the complexity and instability of the laser propagation system, it is hard to predict the pulse shapes precisely by pure analytic methods based on the physical model [Frantz-Nodvik (F-N) equation]. Here, we present a data-driven model based on a convolutional neural network (CNN) for precise prediction. The neural network model introduces sixteen parameters neglected in the F-N equation based models to expand the representation dimension. The sensitivity analysis of the experimental results confirms that these parameters have different degrees of influence on the temporal output shapes and cannot be ignored. The network characterizes the whole physical process with commonality and specificity features to improve the description ability. The prediction accuracy evaluated by a root mean square of the proposed model is 7.93%, which is better compared to three optimized physical models. This study explores a nonanalytic methodology of combining prior physical knowledge with data-driven models to map the complex physical process by numerical models, which has strong representation capability and great potential to model other measurable processes in physical science.

Design of multi neutron-to-gamma converter array for measuring time resolved ion temperature of inertial confinement fusion implosions

The ion temperature varying during inertial confinement fusion implosions changes the amount of Doppler broadening of the fusion products, creating subtle changes in the fusion neutron pulse as it moves away from the implosion. A diagnostic design to try to measure these subtle effects is introduced-leveraging the fast time resolution of gas Cherenkov detectors along with a multi-puck array that converts a small amount of the neutron pulse into gamma-rays, one can measure multiple snapshots of the neutron pulse at intermediate distances. Precise measurements of the propagating neutron pulse, specifically the variation in the peak location and the skew, could be used to infer time-evolved ion temperature evolved during peak compression.

Study on the Repair Technology of Laser Damage-Fused Silica Optics Based on the Neural Network Method

As a key component of a high-power laser device, fused silica optics needs to bear great laser energy, and laser damage is easily generated on the optical surface. In order to improve the service life and availability of optics, it is necessary to repair the damaged optics. In this work, the repair technique of damaged, fused silica optics was studied. The neural network method was mainly used to establish the correlation between the number of small-scale damage points and the repair depth. The prediction accuracy of the model is better than 90%. Based on the neural network model, the removal depth parameters were optimized with the suppression coefficient of the damage points. The processing effect of the optimized parameters was verified by magnetorheological polishing experiments. In this paper, a repair technique based on a neural network was proposed, which avoids the low efficiency caused by processing iterations in the repair process, and can accurately what was expected. The method proposed in this work has an important reference value in the repair process of fused silica optics.

First Indirect Drive Experiment Using a Six-Cylinder-Port Hohlraum

The new hohlraum experimental platform and the quasi-3D simulation model are developed to enable the study of the indirect drive experiment using the six-cylinder-port hohlraum for the first time. It is also the first implosion experiment for the six laser-entrance-hole hohlraum to effectively use all the laser beams of the laser facility that is primarily designed for the cylindrical hohlraum. The experiments performed at the 100 kJ Laser Facility produce a peak hohlraum radiation temperature of ∼222  eV for ∼80  kJ and 2 ns square laser pulse. The inferred x-ray conversion efficiency η∼87% is similar to the cylindrical hohlraum and higher than the octahedral spherical hohlraum at the same laser facility, while the low laser backscatter is similar to the outer cone of the cylindrical hohlraum. The hohlraum radiation temperature and M-band (>1.6  keV) flux can be well reproduced by the quasi-3D simulation. The variations of the yield-over-clean and the hot spot shape can also be semiquantitatively explained by the calculated major radiation asymmetry of the quasi-3D simulation. Our work demonstrates the capability for the study of the indirect drive with the six-cylinder-port hohlraum at the cylindrically configured laser facility, which is essential for numerically assessing the laser energy required by the ignition-scale six-cylinder-port hohlraum.

Design of Third-Order Dispersion Compensation for the SG PW Laser System Using a Birefringent Crystal

This study aims to update the existing SG PW laser system and improve the temporal contrast and shape fidelity of a compressed pulse with a 150 fs level for multi-PW (5–10 PW). The design of third-order dispersion (TOD) compensation via a birefringent crystal was studied through numerical simulations and experiments. The dispersions introduced by the birefringent crystal were calculated using the Jones matrix element by changing the in-plane rotation angle ϕ, thickness d, incident angle θ, and temperature T, while also considering the transmission spectral bandwidth. The group-velocity dispersion (GVD), TOD, and fourth-order dispersion (FOD) of the existing SG PW laser system and its influence on the compressed pulse with different pulse durations were analyzed. The results suggest that a TOD of 1.3×106 fs3 needs to compensate for the multi-PW design. The compensation scheme is designed using a quartz crystal of d = 6.5 mm, θ = 90°, ϕ = 17°, and T = 21 °C, corresponding to the thickness, inclination angle, in-plane rotation angle, and temperature, respectively. Furthermore, we show a principle-proof experiment offline and measure the GVD and TOD by the Wizzler, which is based on theoretical simulations. These results can be applied to independently and continuously control the TOD of short-pulse laser systems.

Laser energy prediction with ensemble neural networks for high-power laser facility

The energy accuracy of laser beams is an essential property of the inertial confinement fusion (ICF) facility. However, the energy gain is difficult to control precisely by traditional Frantz-Nodvik equations due to the dramatically-increasing complexity of the huge optical system. A novel method based on ensemble deep neural networks is proposed to predict the laser output energy of the main amplifier. The artificial neural network counts in 39 more related factors that the physical model neglected, and an ensemble method is exploited to obtain robust and stable predictions. The sensitivity of each factor is analyzed by saliency after training to find out the factors which should be controlled strictly. The identification of factor sensitivities reduces relatively unimportant factors, simplifying the neural network model with little effect on the prediction results. The predictive accuracy is benchmarked against the measured energy and the proposed method obtains a relative deviation of 1.59% in prediction, which has a 2.5 times improvement in accuracy over the conventional method.

Theoretical Comparison of Optothermal Absorption in Transmissive Metalenses Composed of Nanobricks and Nanoholes

Background: Optical components with high damage thresholds are very desirable in intense-light systems. Metalenses, being composed of phase-control nanostructures with peculiar properties, are one of the important component candidates in future optical systems. However, the optothermal mechanism in metalenses is still not investigated adequately. Methods: In this study, the optothermal absorption in transmissive metalenses made of silicon nanobricks and nanoholes is investigated comparatively to address this issue. Results: The geometrical dependencies of nanostructures’ transmittance, phase difference, and field distribution are calculated numerically via simulations. To demonstrate the optothermal mechanism in metalenses, the mean absorption efficiencies of the selected unit-cells, which would constitute metalenses, are analyzed. The results show that the electric field in the silicon zone would lead to an obvious thermal effect, and the enhancement of the localized electric field also results in the strong absorption of optical energy. Then, two typical metalenses are designed based on these nanobricks and nanoholes. The optothermal simulations show that the nanobrick-based metalens can handle a power density of 0.15 W/µm2, and the density of the nanohole-based design is 0.12 W/µm2. Conclusions: The study analyzes and compares the optothermal absorption in nanobricks and nanoholes, which shows that the electric-field distribution in absorbent materials and the localized-field enhancement are the two key effects that lead to optothermal absorption. This study provides an approach to improve the anti-damage potentials of transmissive metalenses for intense-light systems.

Forward-looking insights in laser-generated ultra-intense γ-ray and neutron sources for nuclear application and science

Ultra-intense MeV photon and neutron beams are indispensable tools in many research fields such as nuclear, atomic and material science as well as in medical and biophysical applications. For applications in laboratory nuclear astrophysics, neutron fluxes in excess of 10²¹ n/(cm² s) are required. Such ultra-high fluxes are unattainable with existing conventional reactor- and accelerator-based facilities. Currently discussed concepts for generating high-flux neutron beams are based on ultra-high power multi-petawatt lasers operating around 10²³ W/cm² intensities. Here, we present an efficient concept for generating γ and neutron beams based on enhanced production of direct laser-accelerated electrons in relativistic laser interactions with a long-scale near critical density plasma at 10¹⁹ W/cm² intensity. Experimental insights in the laser-driven generation of ultra-intense, well-directed multi-MeV beams of photons more than 10¹² ph/sr and an ultra-high intense neutron source with greater than 6 × 10¹⁰ neutrons per shot are presented. More than 1.4% laser-to-gamma conversion efficiency above 10 MeV and 0.05% laser-to-neutron conversion efficiency were recorded, already at moderate relativistic laser intensities and ps pulse duration. This approach promises a strong boost of the diagnostic potential of existing kJ PW laser systems used for Inertial Confinement Fusion (ICF) research.

Laser-plasma interaction can provide alternative platform over conventional method for particle and photon beam generation. Here the authors demonstrate generation of gamma ray and neutron beams from intense laser interaction with near critical density plasma.

High-Accuracy Surface Topography Manufacturing for Continuous Phase Plates Using an Atmospheric Pressure Plasma Jet

The continuous phase plate (CPP) is the vital diffractive optical element involved in laser beam shaping and smoothing in high-power laser systems. The high gradients, small spatial periods, and complex features make it difficult to achieve high accuracy when manufacturing such systems. A high-accuracy and high-efficiency surface topography manufacturing method for CPP is presented in this paper. The atmospheric pressure plasma jet (APPJ) system is presented and the removal characteristics are studied to obtain the optimal processing parameters. An optimized iterative algorithm based on the dwell point matrix and a fast Fourier transform (FFT) is proposed to improve the accuracy and efficiency in the dwell time calculation process. A 120 mm × 120 mm CPP surface topography with a 1326.2 nm peak-to-valley (PV) value is fabricated with four iteration steps after approximately 1.6 h of plasma processing. The residual figure error between the prescribed surface topography and plasma-processed surface topography is 28.08 nm root mean square (RMS). The far-field distribution characteristic of the plasma-fabricated surface is analyzed, for which the energy radius deviation is 11 μm at 90% encircled energy. The experimental results demonstrates the potential of the APPJ approach for the manufacturing of complex surface topographies.

Spatio-temporal focal spot characterization and modeling of the NIF ARC kilojoule picosecond laser

The advanced radiographic capability (ARC) laser system, part of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, is a short-pulse laser capability integrated into the NIF. The ARC is designed to provide adjustable pulse lengths of ∼1-38ps in four independent beamlets, each with energies up to 1 kJ (depending on pulse duration). A detailed model of the ARC lasers has been developed that predicts the time- and space-resolved focal spots on target for each shot. Measurements made to characterize static and dynamic wavefront characteristics of the ARC are important inputs to the code. Modeling has been validated with measurements of the time-integrated focal spot at the target chamber center (TCC) at low power, and the space-integrated pulse duration at high power, using currently available diagnostics. These simulations indicate that each of the four ARC beamlets achieves a peak intensity on target of up to a few 10¹⁸W/cm².

Ground fused silica processed by combined chemical etching and CO2 laser polishing with super-smooth surface and high damage resistance

Laser damage in fused silica, particularly ultraviolet laser damage, is still a key problem limiting the development of high-power laser systems. In this Letter, a combined process of chemical etching and CO₂ laser polishing was applied to ground fused silica. A super-smooth surface with a root-mean-square roughness of 0.25 nm was achieved through this combined process. Furthermore, the combined process can reduce the introduction of photoactive metal impurity elements, destructive defects, and chemical-structure defects, resulting in a 0% probability damage threshold nearly 33% higher than a conventional chemical mechanical polished sample for a 7.6 ns pulse at a wavelength of 355 nm.

Recent Advances in Laser-Induced Surface Damage of KH2PO4 Crystal

As a hard and brittle material, KDP crystal is easily damaged by the irradiation of laser in a laser-driven inertial confinement fusion device due to various factors, which will also affect the quality of subsequent incident laser. Thus, the mechanism of laser-induced damage is essentially helpful for increasing the laser-induced damage threshold and the value of optical crystal elements. The intrinsic damage mechanism of crystal materials under laser irradiation of different pulse duration is reviewed in detail. The process from the initiation to finalization of laser-induced damage has been divided into three stages (i.e., energy deposition, damage initiation, and damage forming) to ensure the understanding of laser-induced damage mechanism. It is clear that defects have a great impact on damage under short-pulse laser irradiation. The burst damage accounts for the majority of whole damage morphology, while the melting pit are more likely to appear under high-fluence laser. The three stages of damage are complementary and the multi-physics coupling technology needs to be fully applied to ensure the intuitive prediction of damage thresholds for various initial forms of KDP crystals. The improved laser-induced damage threshold prediction can provide support for improving the resistance of materials to various types of laser-induced damage.

Model Development for Nanosecond Laser-Induced Damage Caused by Manufacturing-Induced Defects on Potassium Dihydrogen Phosphate Crystals

Nanosecond laser-induced damage on (potassium dihydrogen phosphate) KDP crystals is a complex process, which involves coupled actions of multi-physics fields. However, the mechanisms governing the laser damage behaviors have not been fully understood and there have been no available models to accurately describe this complex process. In this work, based on the theories of electromagnetic, thermodynamic, and hydrodynamic fields, a coupled multi-physics model is developed to describe the transient behavior of laser-supported energy deposition and diffusion accompanied by the surface defect (e.g., surface cracks)-initiated laser damage process. It is found that the light intensification caused by the defects near the crystal surface plays a significant role in triggering the laser-induced damage, and a large amount of energy is quickly deposited via the light intensity-activated nonlinear excitation. Using the developed model, the maximum temperature of the crystal material irradiated by a 3 ns pulse laser is calculated, which agrees well with previously reported experimental results. Furthermore, the modeling results suggest that physical processes such as material melting, boiling, and flowing have effects on the evolution of the laser damage process. In addition, the experimentally measured morphology of laser damage sites exhibits damage features of boiling cores, molten regions, and fracture zones, which are direct evidence of bowl-shaped high-temperature expansion predicted by the model. These results well validate that the proposed coupled multi-physics model is competent to describe the dynamic behaviors of laser damage, which can serve as a powerful tool to understand the general mechanisms of laser interactions with KDP optical crystals in the presence of different defects.

Impact of Localized Radiative Loss on Inertial Confinement Fusion Implosions

The impact to fusion energy production due to the radiative loss from a localized mix in inertial confinement implosions using high density carbon capsule targets has been quantified. The radiative loss from the localized mix and local cooling of the reacting plasma conditions was quantified using neutron and x-ray images to reconstruct the hot spot conditions during thermonuclear burn. Such localized features arise from ablator material that is injected into the hot spot from the Rayleigh-Taylor growth of capsule surface perturbations, particularly the tube used to fill the capsule with deuterium and tritium fuel. Observations, consistent with analytic estimates, show the degradation to fusion energy production to be linearly proportional to the fraction of the total emission that is associated with injected ablator material and that this radiative loss has been the primary source of variations, of up to 1.6 times, in observed fusion energy production. Reducing the fill tube diameter has increased the ignition metric χ_{no α} from 0.49 to 0.72, 92% of that required to achieve a burning hot spot.

Dynamic behavior modeling of laser-induced damage initiated by surface defects on KDP crystals under nanosecond laser irradiation

The issue of laser-induced damage of transparent dielectric optics has severely limited the development of high-power laser systems. Exploring the transient dynamic behaviors of laser damage on KDP surface by developing multi-physics coupling dynamics model is an important way to reveal the mechanism of nanosecond laser damage. In this work, KDP crystals are taken as an example to explore the mechanism of laser-induced surface damage. Based on the theories of electromagnetic field, heat conduction and fluid dynamics, a multi-physics coupling dynamics model is established for describing the evolution of nanosecond damage processes. The dynamics of laser energy transmission, thermal field distribution and damage morphology during nanosecond laser irradiation are simulated with this model. It is found that the enhancement of light intensity caused by surface defect plays an important role in the initial energy deposition and damage initiation of the laser irradiation area. The evolution of temperature field and crater morphology during subsequent laser irradiation is helpful to understand the laser damage process. The feasibility of this model is verified by the morphology information of typical defect-induced laser damage. This work provides further insights in explaining the laser-induced damage by surface defects on KDP crystals. The model can be also applied to investigate the laser damage mechanisms of other transparent dielectric optics.

Injection laser system for Advanced Radiographic Capability using chirped pulse amplification on the National Ignition Facility

We report on the design, performance, and qualification of the injection laser system designed to deliver joule-level chirped pulse beamlets arranged in dual rectangular beam formats into two main laser amplifier beamlines of the National Ignition Facility. The system is designed to meet the requirements of the Advanced Radiographic Capability upgrade with features that deliver performance, adjustability, and long-term reliability.

Rayleigh-Taylor instability experiments on the LULI2000 laser in scaled conditions for young supernova remnants

We describe a platform developed on the LULI2000 laser facility to investigate the evolution of Rayleigh-Taylor instability (RTI) in scaled conditions relevant to young supernova remnants (SNRs) up to 200 years. An RT unstable interface is imaged with a short-pulse laser-driven (PICO2000) x-ray source, providing an unprecedented simultaneous high spatial (24μm) and temporal (10 ps) resolution. This experiment provides relevant data to compare with astrophysical codes, as observational data on the development of RTI at the early stage of the SNR expansion are missing. A comparison is also performed with FLASH radiative magnetohydrodynamic simulations.

Simultaneous measurement of the HT and DT fusion burn histories in inertial fusion implosions

Measuring the thermonuclear burn history is an important way to diagnose inertial fusion implosions. Using the gas Cherenkov detectors at the OMEGA laser facility, we measure the HT fusion burn in a H₂+T₂ gas-fueled implosion for the first time. Using multiple detectors with varied Cherenkov thresholds, we demonstrate a technique for simultaneously measuring both the HT and DT burn histories from an implosion where the total reaction yields are comparable. This new technique will be used to study material mixing and kinetic phenomena in implosions.

Low-noise time-resolved optical sensing of electromagnetic pulses from petawatt laser-matter interactions

We report on the development and deployment of an optical diagnostic for single-shot measurement of the electric-field components of electromagnetic pulses from high-intensity laser-matter interactions in a high-noise environment. The electro-optic Pockels effect in KDP crystals was used to measure transient electric fields using a geometry easily modifiable for magnetic field detection via Faraday rotation. Using dielectric sensors and an optical fibre-based readout ensures minimal field perturbations compared to conductive probes and greatly limits unwanted electrical pickup between probe and recording system. The device was tested at the Vulcan Petawatt facility with 1020 W cm-2 peak intensities, the first time such a diagnostic has been used in this regime. The probe crystals were located ~1.25 m from target and did not require direct view of the source plasma. The measured signals compare favourably with previously reported studies from Vulcan, in terms of the maximum measured intra-crystal field of 10.9 kV/m, signal duration and detected frequency content which was found to match the interaction chamber's horizontal-plane fundamental harmonics of 76 and 101 MHz. Methods for improving the diagnostic for future use are also discussed in detail. Orthogonal optical probes offer a low-noise alternative for direct simultaneous measurement of each vector field component.