A MARKOV-CHAIN MODEL FOR A TURBOMOLECULAR PUMP - THEORY AND EXPERIMENTS

The random flow of molecules in turbomolecular pump is proved to be of Markovian type stochastic process. Two new variables-mean fast passag e time and long range equilibrium probability-are related to the maxim um compression ratio and the maximum flow, theoretically. An enumerati on method to identify the flow pattern is adopted. The overall effecti ve compression ratio is proved to be equal to the partial pressure rat io due to the molecules taking any one of the first passage time flow pattern for either direction of flow. A unified concept to make a sele ction of flow events is suggested to be included in the calculation of the probability and for the comparison of results. The influence of t he backing pump and the flow at the ultimate vacuum level are quantifi ed for the design purpose and are experimentally verified. (C) 1997 El sevier Science Ltd. All rights reserved.