The response of the pressure in an elementary vacuum system

Abstract

A simple vacuum system consisting of a volume V (I), a wall, a real pump S (I. s −1 ) and a gas leak L is considered. The system gas is allowed to interact with the wall via a single adsorption state characterized by a sojourn time τ W and a wall pumping speed S W (I. s −1 ). Within this model, the response in time of the system pressure (i) to a pumpdown, (ii) to a sudden change of L , (iii) to a sudden change of S is evaluated. It is shown that these three cases can be described in terms of a single function containing all of the gas-wall interaction parameters and expressions for this function are derived for two situations of interest; i.e. S/S W ≪ 1, and τ S /τ W ≪ 1, where τ s = V/S . In addition, the change in d p /d t at t = 0 due to a sudden change in the real pumping speed by an amount Δ S (I. s −1 ) is shown to be P o Δ S/V, p o being the pressure at t = 0. It is noted that this result is in fact independent of the details of the gas-wall interaction processes.