Water vapor in vacuum systems

Abstract

A major concern for vacuum processing equipment designed to operate in the high and ultrahigh vacuum range, is the outgassing from the walls of the vacuum chamber. Whenever a vacuum system is opened to air, all the internal surfaces will be covered with layer upon layer of water molecules. The thickness of the layer will depend on the relative humidity of the air, the exposure history, etc. After a few hours of pumping down the system, water desorption from the walls will slow down and pressure will remain steady until the last layer of water is depleted. The economical way to reduce the desorption rate and remove large amounts of water vapor, is to use methods for conditioning the construction materials and transfer energy to the adsorbed gases to break their bonds. Traditional vacuum bakeout cannot be used with some applications such as accelerators, or fusion devices. Several other methods such as flushing with cold or hot neutral gas, glow discharge, or either electron, ion, or ultraviolet radiations have been successfully used. The problem of water vapor will be discussed in detail.