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Thickness, Doping Accuracy, and Roughness Control in Graded Germanium Doped CHX Microshells for LMJ

G. Legay, M. Theobald, J. Barnouin, E. P[^]eche, S. Bednarczyk, C. Hermerel, O. Legaie

Fusion Science and Technology / Volume 55 / Number 4 / May 2009 / Pages 438-445

Technical Paper / Eighteenth Target Fabrication Specialists' Meeting /

In the Commissariat à l'Energie Atomique Laser Megajoule (LMJ) facility, amorphous hydrogenated carbon (a-C:H or CHX) is the nominal ablator used to achieve inertial confinement fusion experiments. These targets are filled with a fusible mixture of deuterium-tritium in order to perform ignition. The a-C:H shell is deposited on a polyalphamethylstyrene (PAMS) mandrel by glow discharge polymerization with trans-2-butene, hydrogen, and helium. Graded germanium doped CHX microshells are supposed to be more stable regarding hydrodynamic instabilities. The shells are composed of four layers, for a total thickness of 180 m. The germanium gradient is obtained by doping the different a-C:H layers with the addition of tetramethylgermanium in the gas mixture.

As the achievement of ignition greatly depends on the physical properties of the shell, the thicknesses, doping concentration, and roughness must be precisely controlled.

Quartz microbalances were used to perform an in situ and real-time measurement of the thickness in order to reduce the variations - and so our fabrication tolerances - on each layer thickness. Ex situ control of the thickness of each layer was carried out, with both optical coherent tomography and interferometry (wallmapper).

High-quality PAMS and a rolling system have been used to lower the low-mode roughness [root-mean-square (rms) (mode 2) < 70 nm]. High modes were clearly reduced by coating the pan containing the shells with polyvinyl alcohol + CHX instead of polystyrene + CHX resulting in an rms (>mode 10) < 20 nm, which can be <15 nm for the best microshells.

The germanium concentration (0.4 and 0.75 at.%) in the a-CH layer is obtained by regulating the tetramethylgermanium flow. Low range mass flow controllers have been used to improve the doping accuracy.

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