When designing a cryopump, it is desirable to have some method of predictin
g cryopump performance for the different cryopanel geometries that are bein
g considered. Monte Carlo simulation is a method that has been used for eva
luating the pumping speed of a;cryopump when it has not yet accumulated any
gas. The simulation of hydrogen pumping has its own challenges due to the
typical location of the hydrogen pumping sites. As gases accumulate on the
pumping surfaces, the passageways for hydrogen gas are restricted. In this
article we describe the work that was done, using Monte Carlo simulation, t
o study the hydrogen pumping speed and capacity of a 200 mm cryopump as it
accumulates hydrogen, water and argon. The hydrogen pumping speed is predic
ted as a function of the amount of other gases that have been accumulated.
(C) 1999 American Vacuum Society. [S0734-2201(99)16004-4].