Cluster ion beam polishing for inertial confinement fusion target capsules

Targets for Inertial Confinement Fusion (ICF) typically consist of a hollow, spherical capsule filled with a mixture of hydrogen isotopes. Typically, these capsules are irradiated by short, intense pulses of either laser light (“direct drive”) or laser-generated X-rays (“indirect drive”), causing them to implode. This compresses and heats the fuel, leading to thermonuclear fusion. This process is highly sensitive to hydrodynamic (e.g., Rayleigh-Taylor) instabilities, which can be initiated by imperfections in the target. Thus, target capsules must be extremely spherical and smooth. One of the lead capsule designs for the National Ignition Facility, a 1.8 MJ laser being built at Livermore, calls for a 2-mm-diam capsule with a 150-μm-thick copper-doped beryllium wall. These capsules can be fabricated by sputter depositing the metal onto a spherical plastic mandrel. This results in surfaces with measured R_q´s of 50 to 150 nm, as measured with an atomic force microscope. For optimal performance the roughness should be below 10 nm rms. We have begun studying the use of ion cluster beam polishing as a means of improving the surface finish of as-deposited capsules. In this approach, a batch of capsules would be agitated in a bounce pan inside a vacuum chamber during exposure to the cluster beam. This would ensure a uniform beam dose around the capsule. We have performed preliminary experiments on both Be flats and on a stationary Be capsule. On the capsule, the measured R_q went from 64 nm before polishing to 15 nn after. This result was obtained without any effort at process optimization. Similar smoothing was observed on the planar samples