Capsule Shimming Developments for National Ignition Facility (NIF) Hohlraum Asymmetry Experiments

Capsule drive in National Ignition Facility indirect-drive implosions is generated by X-ray illumination from cylindrical hohlraums. The cylindrical hohlraum geometry is axially symmetric but not spherically symmetric, causing capsule-fuel drive asymmetries. It is hypothesized that fabricating capsules asymmetric in wall thickness (shimmed) may compensate for drive asymmetries and improve implosion symmetry. Simulations suggest that for high-compression implosions, Legendre mode P-4 hohlraum flux asymmetries are the most detrimental to implosion performance. General Atomics has developed a diamond-turning method to form a glow discharge polymer capsule outer surface to a Legendre mode P-4 profile. The P-4 shape requires full capsule surface coverage. As a result, in order to avoid tool-lathe interference, flipping the capsule part way through the machining process is required. This flipping process risks misalignment of the capsule, causing a vertical step feature on the capsule surface. Recent trials have proven this step feature height can be minimized to similar to 0.25 mu m.