モンテカルロ法によるクライオポンプ排気速度の算出

概要

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The rate of removal of gas molecules in cryogenic pumping systems essentially depends on the configuration of radiation shields. In this paper, an attempt was made to analyze the shield configuration devised for the ETL space simulation chamber. Calculations were carried out by the Monte Carlo method on the basis of following assumptions : (1) The incident molecules are random both in direction and in position, (2) The flow is steady and molecular, (3) The molecule collided with the shield wall reflects according to the cosine law, and (4) The capture coefficient of gas molecules on the helium cryogenic pipes is unity. In actual, 3, 000 molecules were put in for each model of the shield configuration, differing in helium pipe diameter, pipe position, shield depth, fin angle, and entrance aperture. The pumping speed predicted from this calculation for the ETL space chamber cryopump was about 48, 700 <I>l</I>/s for nitrogen at a temperature of 90°K and was in good agreement with the observed value, 47, 500 <I>l</I>/s. A number of date available for designing cryopumping systems are given.