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Gas Etching to Obtain Germanium Doped CHx Microshells Compatible with the Laser Megajoule Target Specifications

M. Theobald, C. Chicanne, J. Barnouin, E. Pêche, P. Baclet

Fusion Science and Technology

Volume 49 / Number 4 / May 2006 / Pages 757-763


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Several low atomic number materials as beryllium, polyimide or hydrocarbon can be suitable as an ablator for high energy class lasers. On CEA Laser "Megajoule" (LMJ) facility, amorphous hydrogenated carbon (a-C:H or CHx), is the nominal ablator used to realize inertial confinement fusion (ICF) experiments. These capsules contain the fusible deuterium-tritium mixture to achieve ignition. Coatings are prepared by glow discharge polymerization (GDP) with trans-2-butene and hydrogen and can be easily doped with germanium adding tetramethylgermanium. Laser fusion targets must have optimized characteristics: diameter of 2.4 mm for LMJ targets, thickness up to 167.5 m, sphericity and thickness concentricity better than 99% and an outer and an inner roughness of a few nanometers at high modes. The surface finish of these laser fusion targets must be extremely smooth in order to minimize hydrodynamic instabilities.

With GDP techniques, it is possible to obtain coatings without any growing structures and thus only with very small nodules at the surface by controlling the coating parameters and having low deposition rates (lower than 0.5 m/h). This allows to obtain high mode roughness lower than 10 nm. Nevertheless, this is not sufficient to obtain LMJ specifications especially at intermediate modes which are strongly degraded by local defects ("bumps"). In order to eliminate mostly of these singular points that could be the source of the defects, etching during the film growth can be useful. In this work, we show how we reduced the roughness of germanium doped CHx microshells by adding helium in the plasma or pulsing hydrogen in order to have an exacerbated etching effect and also a higher surface temperature and thus a better mobility of the adsorbed species.

By controlling these parameters, we obtained germanium doped CHx microshells compatible with the LMJ specifications. The RMS roughness from modes 10 to 1000 is lower than 20 nm and from modes 2 to 10 is around 160 nm.

New designs of graded germanium doped microshells improve the stability of the target to hydrodynamic instabilities. This allows relaxing the specification of the roughness. The first microshells were synthesized and the results are presented in this paper.

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