THERMAL-ANALYSIS OF CRYOPUMP 1ST STAGE ARRAY

Detailed numerical analysis of the heat transfer in the first stage array of a model cryopump 20 cm dia was performed for both the steady state and the unsteady cooldown period, to find the effect on the temperature distribution and the cooldown time of several design parameters such as thickness and surface emissivity of the radiation shield, the use of thermal struts, shape of the thermal anchor, and the choice of shield material between aluminum and OFHC copper. The variation of thermal conductivity and heat capacity with temperature was taken into account. Some important results with typical parameters are: the aluminum shield cools down faster than the copper, especially for heavy thermal load; for copper shields thinner than 1 mm, a thicker shield gives shorter cooldown time; the use of thermal struts can shorten the cooldown time if only the mass of the struts is less than about 25% of that of the shield; there exists an optimum thickness for a thermal anchor which minimizes the temperature difference in the first stage array.