Optimization of thermal design for nitrogen shield of JET cryopump

Thermal analysis of the nitrogen shield of the JET (Joint European Torus) cryopump was done using a finite element computer program. In this analysis, a parallel flow arrangement and two series flow arrangements were compared for cooldown from 300 to about 80 K. In order to simplify the analysis, coolant was assumed to be a N₂ gas at an inlet temperature of 80 K. It is shown that all three flow arrangements have similar time for cooling down the shield from 300 to 80 K. This means that the heat exchange effect or radial conduction from the warm part of the shield to the cold part of the shield for series flow arrangements is not dominant. Due to small conduction effects, it will be feasible to modify the design to a more stable series flow arrangement. This flow arrangement will also have minimum cooling time. The inner stainless steel shield has small thermal conductivity and, hence, this part of the shield lags in cooling behind the rest of the shield. This could be remedied by adding about a 1-mm layer of copper in poloidal stripes to the stainless steel fin