On December 5th, 2022, controlled fusion ignition was demonstrated for the first time at the National Ignition Facility (NIF), a major achievement in the field of Inertial Confinement Fusion (ICF) requiring a multi-decadal effort involving broad national and international collaborations. To drive the fusion ignition reaction with the compressed fuel capsule, that yielded 3.15 MJ of nuclear energy [1], the NIF laser delivered a high-precision pulse shape with 2.05 MJ of ultra-violet (UV) laser energy and a peak power of 440 TW. This laser energy was an increase of similar to 8 % compared to that delivered on the previous "threshold of ignition" record yield experiment (1.37 MJ of yield for 1.89 MJ of laser energy) on August 8th, 2021 [2]. We explain how the results of our extensive research in laser technology and UV optics damage mitigation led to major improvements in the NIF laser, enabling this energy increase along with additional accuracy, precision, and power balance enhancements. Furthermore, we will discuss on-going efforts that have enabled operations at 2.2 MJ of UV energy as well as potential new initiatives to push the laser performance -accuracy and delivered energy- to even higher levels in the future as previously demonstrated on a small subset of NIF beams [3].

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

Lawrence Livermore National Security's (LLNS) National Ignition Facility (NIF) requires over 600 large crystals of KH2PO4 (KDP) and KH0.6D1.4PO4 (DKDP) for use in polarization rotation and frequency conversion assemblies. There are near term plans to increase the energy on NIF to 2.2 MJ and up to 3 MJ in future years. Managing optics' damage is one critical aspect of achieving these aggressive goals. Frequency conversion crystals are being examined for readiness to meet the goals for higher energy operations. All aspects of the crystal fabrication process are being studied including growth, processing, laser conditioning, and mitigation to strengthen the crystals against damage. THG crystals have an approximately 10× lower exchange rate than fused silica optics, so unlike NIF fused silica optics which are micromachined to mitigate surface laser damage between recycle loops, THG crystals are refinished to remove surface laser damage. Given the long time to grow NIF size DKDP crystals (3 years) and the impact of THG thickness on frequency conversion, a good understanding of the evolution of the inventory is necessary. The number of damage sites that require laser blocking dictates the exchange rate of THG crystals. Understanding the difference between bulk and surface damage, is important to limit the number of blocked sites. Also, in preparation for higher power operations on NIF, the current KDP polarization rotators on half of the NIF beams are being exchanged with DKDP due to an ~5% reduction in absorption at 1053 nm thus reducing beam contrast in the NIF amplifiers.

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. As recently reported, a burning plasma state, where the alpha-heating in the plasma is the primary source of heating, was achieved in laboratory experiments. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin ``burn propagation'' into surrounding cold fuel, enabling the possibility of high energy gain. While ``scientific breakeven'' (i.e. unity target gain) has not yet been achieved, this talk reports the first controlled fusion experiment on the National Ignition Facility to produce capsule gain greater than unity (here 5.8) and reach ignition by many different formulations of the Lawson criterion. In the talk, we will discuss some key basic physics inertial confinement fusion (ICF) principles behind the burning plasma and ignition results as well as discuss future challenges.

Sol–gel derived anti-reflective (AR) coatings, particularly those made by the Stöber silica process, have been used on glass and crystal optics in high power and energy laser systems for >30 years, employed on over 100,000 half-meter-scale optics. Sol–gel's advantages include an appropriate refractive index, which provides high-efficiency anti-reflection, high chemical purity, which leads to high laser-damage resistance, and relatively low cost synthesis and coating processes. This chapter reviews the requirements and design for AR coatings, the sol–gel synthesis process (including reaction rates, surface chemistry, and particle-size and porosity controls), coating methods and post treatments, coating stability, and laser-damage performance.

The final optics in the National Ignition Facility (NIF) are protected from target debris by sacrificial (disposable) debris shields (DDS) comprised of 3-mm thick Borofloat. While relatively inexpensive, Borofloat has been found to have bulk inclusions which, under UV illumination, damage, grow, and occasional erupt though the surface of the DDS. We have shown previously that debris generated from Input Surface Bulk Eruptions (ISBE) are a significant source of damage on NIF. Inclusion-free fused silica debris shield (FSDS) have been installed in between the DDS and the final optics on some NIF beam lines to test their efficacy in mitigating damage initiation. We will show results of the damage performance of the FSDS and its role in protecting the final optics. These results will help in our economic analysis of the potential benefits of using FSDS to protect NIF final optics.

Contaminants can severely limit the efficiency, laser damage threshold, and strength of photonic crystal fiber-based lasers. Such contamination can occur due to environmental exposure during the pulling or stacking of rods and tubes or improper handling and storage of these glass components. A preform made by the “stack and draw” process is susceptible to incorporating surface contaminants into the bulk laser glass. We have adapted cleaning and handling protocols originally developed for processing large fused silica optics for the National Ignition Facility. The etch cleaning process reported here mimics the “AMP” or “Advanced Mitigation Process” developed for NIF optics that see high fluence 351nm light. In addition, all cleaning, fixturing and assembly processes used to prep a stack for pulling into a fiber are done in a Class 100 cleanroom. Glass rods (1-3mm in diameter and 10” long) are assembled into a Teflon fixture that only contacts the rods at each end. The loaded fixture receives 120kHz ultrasonic cleaning in 10% sodium hydroxide at 45C and 3% Brulin 1696 detergent at 55C. Parts are thoroughly rinsed using ultrasonicated ultrapure water and spray rinses. A 200nm etch in buffered hydrofluoric acid (6:1 BOE diluted 2:1 in DI water) is followed by additional ultasonicated (120kHz-270kHz) ultrapure water and spray rinse. Finally, the components are allowed to fully dry inside the Teflon frame. The rods are cleaned, stacked, and assembled into a fused silica tube. The preform stack is then returned to a non-cleanroom area to be pulled into fiber using standard telecom fiber-based draw tower equipment and without clean air filters around the draw area. Four fibers were made to test independently the damage threshold and the background loss, two Yb core active fibers and two silica core (F clad) fibers. One of each was cleaned with the AMP process, and one of each with a methanol wipe cleaning process. The active fiber was coated with a dual acrylate coating, first with a low-index inner coating to provide a pump cladding, and then with a relatively hard coating to protect the relatively soft primary coating. The active fibers were pumped at 980nm in a double Fresnel cavity configuration and the power increased until the fiber was damaged up to 1kW. The passive fiber background loss was measured using a standard cut-back technique. Replacing the former methanol wipe clean process with this aqueous cleaning process improved the 1060nm damage threshold of a fiber laser by >30x to above the kW level in the laboratory and reduced the background attenuation by >18x. Early indications are that the acid etching also makes the tensile strength of the fiber consistently high. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

The primary sources of damage on the National Ignition Facility (NIF) Grating Debris Shield (GDS) are attributed to two independent types of laser-induced particulates. The first comes from the eruptions of bulk damage in a disposable debris shield downstream of the GDS. The second particle source comes from stray light focusing on absorbing glass armor at higher than expected fluences. We show that the composition of the particles is secondary to the energetics of their delivery, such that particles from either source are essentially benign if they arrive at the GDS with low temperatures and velocities.

High energy laser systems are ultimately limited by laser-induced damage to their critical components. This is especially true of damage to critical fused silica optics, which grows rapidly upon exposure to additional laser pulses. Much progress has been made in eliminating damage precursors in as-processed fused silica optics (the advanced mitigation process, AMP3), and very high damage resistance has been demonstrated in laboratory studies. However, the full potential of these improvements has not yet been realized in actual laser systems. In this work, we explore the importance of additional damage sources-in particular, particle contamination-for fused silica optics fielded in a high-performance laser environment, the National Ignition Facility (NIF) laser system. We demonstrate that the most dangerous sources of particle contamination in a system-level environment are laser-driven particle sources. In the specific case of the NIF laser, we have identified the two important particle sources which account for nearly all the damage observed on AMP3 optics during full laser operation and present mitigations for these particle sources. Finally, with the elimination of these laser-driven particle sources, we demonstrate essentially damage free operation of AMP3 fused silica for ten large optics (a total of 12,000 cm2 of beam area) for shots from 8.6 J/cm2 to 9.5 J/cm2 of 351 nm light (3 ns Gaussian pulse shapes). Potentially many other pulsed high energy laser systems have similar particle sources, and given the insight provided by this study, their identification and elimination should be possible. The mitigations demonstrated here are currently being employed for all large UV silica optics on the National Ignition Facility.

The use of any optical material is limited at high fluences by laser-induced damage to optical surfaces. In many optical materials, the damage results from a series of sources which initiate at a large range of fluences and intensities. Much progress has been made recently eliminating silica surface damage due to fracture-related precursors at relatively low fluences (i.e., less than 10 J/cm(2), when damaged by 355 nm, 5 ns pulses). At higher fluence, most materials are limited by other classes of damage precursors which exhibit a strong threshold behavior and high areal density (>10(5) cm(-2)); we refer to these collectively as high fluence precursors. Here, we show that a variety of nominally transparent materials in trace quantities can act as surface damage precursors. We show that by minimizing the presence of precipitates during chemical processing, we can reduce damage density in silica at high fluence by more than 100 times while shifting the fluence onset of observable damage by about 7 J/cm(2). A better understanding of the complex chemistry and physics of cleaning, rinsing, and drying will likely lead to even further improvements in the damage performance of silica and potentially other optical materials.

We describe the cleaning processes, treatment methods, facilities, and cleanliness verification techniques developed to achieve and maintain the demanding cleanliness requirements for both hardware and optics used in the National Ignition Facility (NIF).

A system of customized spatial light modulators has been installed onto the front end of the laser system at the National Ignition Facility (NIF). The devices are capable of shaping the beam profile at a low-fluence relay plane upstream of the amplifier chain. Their primary function is to introduce "blocker" obscurations at programmed locations within the beam profile. These obscurations are positioned to shadow small, isolated flaws on downstream optical components that might otherwise limit the system operating energy. The modulators were designed to enable a drop-in retrofit of each of the 48 existing Pre Amplifier Modules (PAMs) without compromising their original performance specifications. This was accomplished by use of transmissive Optically Addressable Light Valves (OALV) based on a Bismuth Silicon Oxide photoconductive layer in series with a twisted nematic liquid crystal (LC) layer. These Programmable Spatial Shaper packages in combination with a flaw inspection system and optic registration strategy have provided a robust approach for extending the operational lifetime of high fluence laser optics on NIF.

Current methods for the manufacture of optical components inevitably leaves a variety of sub-surface imperfections including scratches of varying lengths and widths on even the finest finishes. It has recently been determined that these finishing imperfections are responsible for the majority of laser-induced damage for fluences typically used in ICF class lasers. We have developed methods of engineering subscale parts with a distribution of scratches mimicking those found on full scale fused silica parts. This much higher density of scratches provides a platform to measure low damage initiation probabilities sufficient to describe damage on large scale optics. In this work, damage probability per unit scratch length was characterized as a function of initial scratch width and post fabrication processing including acidbased etch mitigation processes. The susceptibility of damage initiation density along scratches was found to be strongly affected by the post etching material removal and initial scratch width. We have developed an automated processing procedure to document the damage initiations per width and per length of theses scratches. We show here how these tools can be employed to provide predictions of the performance of full size optics in laser systems operating at 351 nm. In addition we use these tools to measure the growth rate of a damage site initiated along a scratch and compare this to the growth measured on an isolated damage site.

The effect of various HF‐based etching processes on the laser damage resistance of scratched fused silica surfaces has been investigated. Conventionally polished and subsequently scratched fused silica plates were treated by submerging in various HF‐based etchants (HF or NH4F:HF at various ratios and concentrations) under different process conditions (e.g., agitation frequencies, etch times, rinse conditions, and environmental cleanliness). Subsequently, the laser damage resistance (at 351 or 355 nm) of the treated surface was measured. The laser damage resistance was found to be strongly process dependent and scaled inversely with scratch width. The etching process was optimized to remove or prevent the presence of identified precursors (chemical impurities, fracture surfaces, and silica‐based redeposit) known to lead to laser damage initiation. The redeposit precursor was reduced (and hence the damage threshold was increased) by: (1) increasing the SiF62− solubility through reduction in the NH4F concentration and impurity cation impurities, and (2) improving the mass transport of reaction product (SiF62−) (using high‐frequency ultrasonic agitation and excessive spray rinsing) away from the etched surface. A 2D finite element crack‐etching and rinsing mass transport model (incorporating diffusion and advection) was used to predict reaction product concentration. The predictions are consistent with the experimentally observed process trends. The laser damage thresholds also increased with etched amount (up to ∼30 μm), which has been attributed to: (1) etching through lateral cracks where there is poor acid penetration, and (2) increasing the crack opening resulting in increased mass transport rates. With the optimized etch process, laser damage resistance increased dramatically; the average threshold fluence for damage initiation for 30 μm wide scratches increased from 7 to 41 J/cm2, and the statistical probability of damage initiation at 12 J/cm2 of an ensemble of scratches decreased from ∼100 mm−1 of scratch length to ∼0.001 mm−1.

Customized spatial light modulators have been designed and fabricated for use as precision beam shaping devices in fusion class laser systems. By inserting this device in a low-fluence relay plane upstream of the amplifier chain, "blocker" obscurations can be programmed into the beam profile to shadow small isolated flaws on downstream optical components that might otherwise limit the system operating energy. In this two stage system, 1920 x 1080 bitmap images are first imprinted on incoherent, 470 nm address beams via pixelated liquid crystal on silicon (LCoS) modulators. To realize defined masking functions with smooth apodized shapes and no pixelization artifacts, address beam images are projected onto custom fabricated optically-addressable light valves. Each valve consists of a large, single pixel liquid cell in series with a photoconductive Bismuth silicon Oxide (BSO) crystal. The BSO crystal enables bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in turn modulates the amplitude of the coherent beams at 1053 nm. Valves as large as 24 mm x 36 mm have been fabricated with low wavefront distortion (<0.5 waves) and antireflection coatings for high transmission (>90%) and etalon suppression to avoid spectral and temporal ripple. This device in combination with a flaw inspection system and optic registration strategy represents a new approach for extending the operational lifetime of high fluence laser optics.

In many high energy laser systems, optics with HMDS sol gel antireflective coatings are placed in close proximity to each other making them particularly susceptible to certain types of strong optical interactions. During the coating process, halo shaped coating flaws develop around surface digs and particles. Depending on the shape and size of the flaw, the extent of laser light intensity modulation and consequent probability of damaging downstream optics may increase significantly. To prevent these defects from causing damage, a coating flaw removal tool was developed that deploys a spot of decane with a syringe and dissolves away the coating flaw. The residual liquid is evacuated leaving an uncoated circular spot approximately 1mm in diameter. The resulting uncoated region causes little light intensity modulation and thus has a low probability of causing damage in optics downstream from the mitigated flaw site.