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.

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.

We have demonstrated the world's highest average power, fully diode-pumped, petawatt-class peak power laser, the High-repetition-rate Advanced Petawatt Laser System (HAPLS) [1-3]. These first commissioning results at 16J (stretched) at 3%Hz fully validate projected performance of 30J/30fs (>1PW) at 10Hz. The laser has been operated at this intermediate level at Lawrence Livermore National Laboratory to demonstrate integrated performance of all subsystems and provide benchmarking data to laser performance models before further increasing energy and peak power. Data was obtained during multiple campaigns, exceeding several hours of run time, and a snapshot of 60min of data is shown in Fig. 1. The average pump laser 1ω (1053nm) energy was 97J with an rms stability of 0.7%, 2ω (527nm) energy at the Ti:sapphire power amplifier was 62J, and the average stretched short pulse energy was 16J. A full-aperture diagnostic suite allows simultaneous, single-shot measurement of energy, spectrum, beam quality, and pulse duration at full repetition rate. Single-shot SPIDER retrieved pulse shapes (Fig. 1 inset) with an average pulse duration over 12000 consecutive shots of 28.6fs (rms=1.4fs). The mean pulse duration is consistent with the measured spectral bandwidth and is ∼1.2× the transform limit. All results shown are raw data without filtering or averaging, demonstrating the exceptional pulse characteristics, repeatability, and stability of the entire laser system.

Large laser systems that deliver optical pulses with peak powers exceeding one Petawatt (PW) have been constructed at dozens of research facilities worldwide and have fostered research in High-Energy-Density (HED) Science, High-Field and nonlinear physics [1]. Furthermore, the high intensities exceeding 1018W/cm2 allow for efficiently driving secondary sources that inherit some of the properties of the laser pulse, e.g. pulse duration, spatial and/or divergence characteristics. In the intervening decades since that first PW laser, single-shot proof-of-principle experiments have been successful in demonstrating new high-intensity laser-matter interactions and subsequent secondary particle and photon sources. These secondary sources include generation and acceleration of charged-particle (electron, proton, ion) and neutron beams, and x-ray and gamma-ray sources, generation of radioisotopes for positron emission tomography (PET), targeted cancer therapy, medical imaging, and the transmutation of radioactive waste [2, 3]. Each of these promising applications requires lasers with peak power of hundreds of terawatt (TW) to petawatt (PW) and with average power of tens to hundreds of kW to achieve the required secondary source flux.

The HAPLS laser system has been commissioned to its first integrated performance milestone, delivering laser pulses with 16J sub-30fs duration at a 3⅓Hz repetition rate. This first all-diode-pumped petawatt-class laser offers the average powers required for secondary source applications.

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.

Dip coating is used to apply sol-gel anti-reflective coatings to large optics for the National Ignition Facility (NIF). Recent process improvements in tooling and Quality Control have enabled production of coatings with improved damaged resistance.