Fabrication of Shells and Foams via Two-Photon Polymerization for Laser-Fusion Experiments

A novel approach to improving neutron yields and reaching ignition for future laser-fusion experiments will require fabrication of concentric foams on the outer and inner surfaces of spherical laser targets (thin-wall plastic capsules containing deuterium–tritium fuel) for laser imprint mitigation and fuel uniformity, respectively. Since the density, fiber, and pore sizes of these foams must be highly controllable, repeatable, and potentially tunable, additive manufacturing is a desirable approach over traditional, chemically synthesized foams. Given the added requirements for shell-surface roughness &lt;100-nm rms and uniform foam distribution of 0.3-<i>μ</i>m fiber diameters and 1-<i>μ</i>m pores, the ultrahigh resolution printing capability of two-photon polymerization (TPP) is ideal for this application.<br/><br/>The Laboratory for Laser Energetics (LLE) and University of Nebraska-Lincoln (UNL) are collaborating in a two-pronged approach, both using Nanoscribe Photonic Professional GT systems, to fabricate foam-on-shell targets via TPP. UNL’s approach, which will be highlighted in another presentation of this symposium by the UNL team, primarily focuses on printing full millimeter-scale shell and foam targets in a single, stitchless process. LLE’s approach is to utilize a Zeiss 63x NA1.4 immersion objective for highest feature resolution, at the cost of being limited to smaller print regions (&lt;200-<i>μ</i>m diameter) that will need to be stitched together to build a full target structure (&gt;1-mm diameter) without compromising the experimental requirements. LLE has conducted focused studies on optimizing print conditions for shells and foams separately with the end goal of combining the two in a single printing process to produce a full target. So far, LLE has used TPP to demonstrate acceptable surface roughness for individual print blocks and across print block stitching seams for thin-wall shells, good survivability of small stochastic foam volumes with 1-<i>μ</i>m pores and fibers &lt;0.3 <i>μ</i>m, as well as promising preliminary results printing hemispherical sections of foam-on-shell together, with radially-graded foam density and minimal stitching overlap. Printed samples are currently characterized at LLE with a combination of optical microscopy, atomic force microscopy (for shells), scanning electron microscopy, and 3D x-ray tomography.<br/><br/>This presentation will discuss how we accomplished our TPP shell and foam results at LLE, in addition to the challenges we face going forward to fabricate and characterize a fully printed viable target for the improvement of future laser-fusion experiments.<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.