Multiphoton Applications in Laser-Fusion Research—From Printing Fusion-Fuel Targets with Sub-150-nm Features to Acquiring Three-Dimensional Structural and Elemental Information of the Target

Laser-fusion experiments have demonstrated substantial progress this year, achieving thermonuclear burn and neutron yields (1.3 MJ) that are 70% of what is needed for ignition (more energy output than input).[1] One path to increasing the performance is to use new types of laser targets that may be manufacturable only if they are printed using the two-photon polymerization process (TPP). These targets (fuel pellets) are millimeter-size plastic shells with concentric foam structures on the outer and inner surfaces and possess an inner shell of solid hydrogen isotopes—deuterium and tritium. Characterization of these targets uses a multiphoton process, coherent anti-Stokes Raman spectroscopy (CARS), to provide 3-D tomographic information about the structure and elemental composition.<br/><br/>The specifications for these targets is at the limit of what the TPP process can achieve: the solid portion of the shell requires the mid-range spectral-mode (l ~ 20 to 300) roughness to be less than 100-nm rms, and the foam structure requires the fiber and void dimensions to be smaller than 0.3 <i>m</i>m and 1 <i>m</i>m, respectively. Furthermore, the foam structure should be isotropic, the density should vary radially; and the process needs to be deterministic and controlled so that the effect of the targets’ structural parameters can be decoupled from other hydrodynamic and laser–plasma interaction parameters that affect the fusion process. While a complete target with these specifications has not yet been produced, all the separate parts required to make the target have been demonstrated: foam structures with fibers &lt;300 nm that survive processing, adequately smooth shells, and graded foam densities. This presentation will describe how this was accomplished and the challenges that remain.<br/><br/>The second part of the presentation will demonstrate how Raman spectroscopy, using a high-intensity femtosecond laser, can provide 3-D tomographic analysis of a laser target at 18 K with submicrometer resolution—information that includes the target’s surface roughness, void content, and elemental composition of the solid hydrogen isotope fuel.<br/><br/>This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority.<br/><br/>1. J. Tollefson, “US Achieves Laser-Fusion Record: What it Means for Nuclear-Weapons Research,” Nature <b>597</b>, 163-164 (2021), https://doi.org/10.1038/d41586-021-02338-4; D. Clery, “Laser Fusion Reactor Approaches ‘Burning Plasma’ Milestone,” Science <b>370</b> (6520), 1019-1020 (2020), https://doi.org/10.1126/science.370.6520.1019.