Fusion Science and Technology / Volume 45 / Number 2 / March 2004 / Pages 218-228
Technical Paper / Target Fabrication
Cooling thin-walled capsules with a high-pressure deuterium fill is a critical phase of operation for providing cryogenic direct-drive targets. During cooling to 20 K, buckling and burst forces develop due to transient thermal gradients, thermal expansion differences in the materials of the capsule and the permeation cell, and changing permeability of the plastic. This article presents the results of both a steady-state and a transient analysis of the pressure differences across the thin-walled capsule during the cooling process. The steady-state contribution to the pressure difference arises from two sources: (1) the different thermal contractions of the materials that comprise the permeation cell and capsule and (2) the room-temperature volume of gas in the line connecting the permeation cell to the isolation valve. The transient analysis considers the pressure differences across the capsule wall that arise from the changing temperature gradients within the gas during the cooling cycle. Both effects have been taken into account to determine an approach that produces fuel-filled, thin-walled cryogenic targets more rapidly. Currently, capsules are slowly cooled at a rate of 0.1 K/min to prevent their destruction. This process requires over 45 h to complete. The results of the present model suggest a faster cooling program that takes into consideration the induced pressure differences, the permeation occurring at higher temperatures, and the strength of the capsule. The time to cool a filled target can be reduced by 25% while maintaining capsule survival.