Fusion Science and Technology / Volume 70 / Number 2 / August-September 2016 / Pages 377-386
Technical Paper / dx.doi.org/10.13182/FST15-220
High-density carbon (HDC) is being evaluated as an alternative to the current National Ignition Facility (NIF) point-design ablator material (glow discharge plasma, or GDP, plastic) due to its high density and optimal opacity, which leads to a higher implosion velocity. Chemical-vapor-deposition–coated HDC capsules have a near perfect surface figure but a microscopically rough surface. After polishing, the surface becomes smooth at nanometer scales but has numerous micron-sized surface pits, whose volumes, morphology, and distribution must be quantified to guide NIF target selection. Traditional metrology tools for GDP surface defects, such as the atomic force microscope (AFM) based Spheremapper and a phase-shifting differential interferometer, lack the resolution to characterize these localized features. In this paper, we describe how this metrology challenge is met by developing automated surface metrology solutions based on a high-density (HD) AFM and a Leica confocal microscope. These tools are complementary in nature. HD-AFM has a 0.1-μm spatial resolution and determines the overall shape distortion and pit statistics by tracing great circles on a capsule with high throughput. The Leica confocal microscope maps the two-dimensional (2-D) surface at low magnification to find all large defects that could be missed by HD-AFM. Then, a high magnification scan inspects at a 0.3-μm lateral resolution to characterize the defect volume. These 2-D maps provide an opportunity for modeling the shell performance at the peak implosion velocity, thereby aiding capsule selection. These new and improved metrology tools provide quantitative data for the continual refinement of the NIF specifications for HDC capsules. Finally, we report on the development of a laser ablation tool that, when combined with the Leica confocal microscope, can identify, quantify, and laser-ablate GDP domes that do not meet NIF specifications.