The MODION® ion pump utilizes the familiar Penning discharge to ionize the residual gas molecules inside the pump.
The magnetic field confines the discharge to a small volume within the pump, and increases the efficiency of pumping by greatly increasing the number of collisions with electrons generated by the ionization process. Thus, a relatively dense cloud of spiraling electrons is formed within the dis-charge to ionize gas molecules more efficiently.
The ions formed are accelerated to the cathodes (titanium) where they impact with sufficient energy to become entrapped. This process, called “sputtering”, creates a continuously renewed surface of pure titanium molecules, which accomplish the active pumping.
J.B. Anderson uses a proprietary process to create a clean titanium surface that has a high chemical attraction (affinity for almost all chemically active gases (H2, N2, O2, etc.) When an active gas molecule strikes the fresh titanium surface, it is bound to that surface by strong chemical bonds. These gases are in effect, “pumped” by the titanium surface. The sputtering process also effectively buries gas molecules under layers of fresh titanium as the process continues over a long period of time.
The Inert gases (He, Ar, Kr, Xe) have little affinity for titanium, and these gases are held weakly to the surface by ionic bonds. The probability of these gas molecules adhering to the titanium surface is low, and the binding forces are much weaker than for other gases. When dealing with the noble(inert) gases a different cathode material such as tantalum can be used to more effectively pump these gases.
The “pumping” action of a titanium film surface can also be affected by ion bombardment at the cathodes. High energy ions striking the cathode not only sputter fresh titanium, but will release a small number of previously pumped molecules as well. For this reason one often observes an initial rise in pressure when the pump is first activated.
To learn more about ion pump theory of operation, please download the MODION Instruction Manual.