Micron-Scaled Defects on Cryogenic Targets: An Assessment of Condensate Sources

Debris with footprints smaller than 40 mu m(2) on the outer and inner surfaces with heights of <10 mu m on outer surfaces and similar to 1 mu m on inner surfaces is present on cryogenic targets used for inertial confinement fusion studies on OMEGA. These features form during the gas-filling and cooling processes used to produce cryogenic deuterium (D-2) and deuterium tritium (DT) targets. The amount of debris on the surface has varied since the inception of the Laboratory for Laser Energetics' (LLE's) cryogenic program. The cause of the contamination is attributed to the cryogenic equipment high-vacuum and cleanliness limitations and to the radiolytic degradation of polymers. Empirical observations and a review of the processing conditions suggest that 1 mu mol of condensable contaminant is sufficient to account for the debris observed on a typical cryogenic target. This translates into a 3-ppm impurity content in the DT fuel. This paper focuses on condensed gases as one source of debris. It is postulated that methane, water, and nitrogen accompany the DT fuel transfer when it is transferred from the uranium storage beds that hold the DT fuel to the permeation cell where the targets are filled.