14 ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2023 of the foam micro- and macroscopic properties (e.g., density, compositional, or structural gradients). Some IFE companies are also investigating advanced high-gain IFE concepts such as “fast ignition” that use composite targets, which incorporate mid-tohigh-Z structures such as focusing cones imbedded in polymer shells. These advanced targets will also require high quality manufacturing to ensure consistent ignition in an IFE plant with high energy gain (Fig. 2)[19]. Deuterated polymer precursor manufacturing. Materials other than polymers have been proposed for use as encapsulants in IFE targets, each having their own advantages and dis- advantages[18]. However, polymer-based capsules offer a relatively high tech- nology readiness level (TRL), lower cost, and simpler fabrication. Thus, they remain the leading candidate for current and future IFE facilities, especially IFE power plants, where vast quantities of targets per day will be needed. Despite the high TRL for existing polymer capsules, the current target fabrication process cannot be scaled to meet the unprecedented fueling demands of an IFE power plant. Microfluidic-based fabrication and low-density polymer foams are representative of some concepts proposed in the literature to meet the scaling challenges[16,20]. One particular challenge that polymer targets present for the fusion fuel cycle is that the hydrogen atoms in targets synthesized from normal have received less attention and this article seeks to highlight some of those research areas. IFE FUSION MATERIALS RESEARCH Inertial-confinement fusion energy (IFE) has been an increasing part of the discussion around fusion commercialization after an experiment, or “shot” at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California in December 2022 achieved ignition[4] and was recently repeated in July 2023 with greater yield. Both of these shots were preceded by another shot in August of 2021 that achieved near-unity gain and helped to guide experiments, with results published in peer-reviewed literature[11-13]. Ignition is a state when the energy produced by the fusion reaction exceeds the energy input into the reaction (Fig. 1)[15]. IFE was an area of research leading up to the early 2010s but efforts were reduced after NIF did not achieve ignition during the first few years of operation. Successful shots at NIF have reignited commercial interest in IFE and currently two of the eight companies within the FES Milestone Program are pursuing IFE concepts. The success in getting to ignition with IFE has created renewed interest in research topics that would enable IFE and that research includes many areas where advanced materials development and manufacturing are needed. Open questions exist about surrogacy of material lifetime studies as pulsed fusion schemes have unique temperature and incident particle or debris conditions for which the instantaneous damage rates can be up to six orders of magnitude higher than MFE reactors[14]. Meeting the strict tolerances and quantities for fusion targets remain a challenge where one design of IFE targets are polymer capsules having a diameter around 5 mm and which need sphericity > 99.9%[16]. Imperfections in targets are known to lead to asymmetries during implosions and other complications that can reduce energy gain during the fusion event. To operate an IFE plant, approximately 500,000 targets per day (assuming 6 Hz rate) could be needed[17]. Developing high throughput target manufacturing that can produce hundreds of millions of high-quality targets per year will be critical to the success of IFE. IFE target manufacturing. The target synthesis can be performed using multiple methods including multi- solvent synthesis and co-axial nozzle alignment with CO2 drying[18]. Incorporation of mid-to-high-Z material layers to improve direct laser coupling, mitigate beam imprint or x-ray/hot-electron preheat, or stabilize implosions can be performed by sputtering, doping, or other methods. Alternatively, next- generation target fabrication methods using microfluidics and/or additive manufacturing are also being actively explored. In particular, two-photon poly- merization (2PP)—a 3D printing method based on raster scanning a tightly focused femtosecond laser spot to polymerize complex parts with nanoscale resolution—is a promising approach to additively manufacture inertial confinement fusion (ICF) targets with high precision, superior reproducibility, and scalability than current fabrication techniques. Liquid deuterium- tritium (DT) wetted foam layer capsule designs are considered a critical aspect of most direct and indrect drive IFE concepts, but little is currently understood about the impact of the foam on the target performance; additive manufacturing uniquely offers the potential of unprecedented, architected control Fig. 2 — (a) Schematic of integrated cone-in-shell fast ignition target from initial experiments; and (b) photograph of gold re-entrant target from experiments on OMEGA at the University of Rochester Laboratory for Laser Energetics (LLE). Reproduced from Ref. 18.
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