Fusion Science and Technology / Volume 45 / Number 2 / March 2004 / Pages 187-196
Technical Paper / Target Fabrication
Vapor-deposited polyimide thin films and shells have been developed for use in direct-drive-implosion experiments. The properties of these materials have been previously measured for different processing conditions, which have also been correlated with the material's microstructure. This paper addresses how the different material properties affect the subsequent stage of converting an empty capsule into a cryogenic fusion target containing solid hydrogen-isotope fuel. The advantages and limitations of these properties are defined in terms of (1) the time it takes to permeation-fill and cryogenically cool fusion targets, and (2) how the processing conditions used to realize these properties affect the capsules' specifications and the subsequent implosion. A paraxmetric comparison is presented.
A common limitation of all the processing conditions is that the roughness of the polyimide capsules is greater than is desirable. Efforts to improve the smoothness of the asdeposited polyamic acid shells (the precursor to polyimide) involve a combined theoretical and experimental approach. The internal components of the vacuum deposition chamber are theoretically modeled using two simulation codes to cover the pressure regime of interest: a Monte Carlo approach is used for the lowest pressure regime (<10-5 Torr) while a continuum fluid dynamics code (FLUENT) is used to calculate the higher pressure regime (>10-3 Torr). The experimentally measured evaporation mass flux of the monomers resulted in a calculated pressure that corresponded to the measured actual value. The resulting mass-flux distribution to, and around, a capsule quantified the uniformity of the deposition process. The mass flux uniformity varied by 50% over the surface of a capsule and varied by 80% over the surface of the bounced pan.