Optimizing target surfaces for inertial confinement fusion experiments using micro-electrical discharge machining

Cone guided fast-ignition laser fusion uses a re-entrant cone target to allow laser access through the plasma overlying the compressed core. Current experiments emulate that situation with a conical hole in an aluminum foil. It is critical that the 10 $\mu$m diameter laser spot be centered on the 30 $\mu$m diameter flat cone tip. Alignment is achieved by retro-imaging a low power beam from the cone tip. Micron scale roughness on the cone tip limits reflected light intensity making alignment difficult; an improved surface finish is required. The buried cone targets are fabricated by micro-electrical discharge machining (EDM). Various parameters are investigated to decrease the roughness and to improve the uniformity of the EDM cone tip surface: EDM wire material and shape, electrode voltage, machine capacitance, and machining speed. Results of surface roughness, uniformity, and material removal rate will be presented.