Pumping speed

As with any vacuum pump, the pumping speed is the major factor in determining the lowest (or "ultimate") pressure in a system and the time required to achieve this pressure. The speed at which a particular gas is pumped varies, mainly depending on the chemical and physical properties of the gas. This includes the chemical affinity to getter materials, the ionisation energy, as well as mass and size of the molecules.

In connection with ion getter pumps two types of gases are usually mentioned. There are reactive gases which refer to the above mentioned affinity for getter materials. Also, there are noble gases which can be described as inert in this context due to their high activation energies. The reactive gas ions have a tendency to chemically react with cathode materials and to form new solid compounds.

For example, an ionized oxygen molecule can borrow an electron from a titanium cathode atom and react with it. The newly formed titanium dioxide molecule is neutral in the solid state, a gas recombination to the system is excluded due to the ionization energies.

Noble gases, however, are inert and thus not reactive under UHV conditions. Therefore, they are not chemically bonded. Noble gases can only be bound to the surfaces by physisorption (physical bonding) or they can be bound in the surrounding walls of the pump by implantation.