Fusion Science and Technology / Volume 49 / Number 4 / May 2006 / Pages 750-756
Technical Paper / Target Fabrication
One of the current capsule designs for achieving ignition on the National Ignition Facility (NIF) is a 2 mm diameter graded Ge-doped CH shell that has a 160 m thick wall. The Ge doping is not uniform, but rather is in radial steps. This graded Ge-doped design allows rougher surface finish than the original undoped CH design, thus has a less stringent surface roughness requirement.
We selected quality mandrels by coating dozens of mandrel batches to ~70 m thickness to amplify submicrometer defects on the mandrels and successively removed inferior batches. The Ge-doped CH layers are made by introducing (CH3)4Ge to the gas stream. The doping concentrations were determined by performing trial runs and were characterized by X-ray fluorescence and quantitative radiographic analyses, with good agreement between the methods demonstrated.
The precise layer thickness and Ge concentrations were determined by a non-destructive quantitative contact radiograph. The as-deposited average layer thicknesses of the shells were 9.5 ± 1.1 m for inner undoped CH layer, followed by a 47.1 ± 0.5 m thick 0.83 ± 0.09 at. % Ge-doped CH, 10.0 ± 0.4 m thick 0.38 ± 0.04 at. % Gedoped CH and then 89.2 ± 0.5 m of undoped CH.
The atomic force microscope derived power spectrum of the shell meets the new NIF standard. The shells had a root-mean-square surface roughness of ~ 24 nm (modes 100-1000). The few surface flaws are isolated domes ~1 m tall and 20 m in diameter.
The PAMS mandrels were successfully removed by pyrolysis at 305°C for 10-20 h. After pyrolysis, the diameter and wall shrank 0.4% and 5.7%, respectively. Except for the outer undoped CH layer, which was 5.8 m less than the design specification, the average thicknesses of the three other layers met the NIF design specification after pyrolysis. The averages of the Ge doping concentrations were within the tolerance limits. The shell's inner surface has root-mean-square roughness of less than 6.5 nm.