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Ionizer 2-9
Technical Reference
Principle of operation
The principle of operation of the ionizer is similar to the Bayard-Alpert gauge, except there is no central
wire collector, and the electron repeller has been added.
Figure 4 Ionizer Schematic
The filament is the source of the electrons used in ionizing the gas molecules. It operates at a negative
potential relative to ground and is resistively heated to incandescence with an electrical current from the
emission regulator. The thermionically emitted electrons are accelerated towards the anode grid which is
positively charged with reference to the filament and ground. Because of the open (i.e. wire mesh)
design of the anode grid cage, most electrons do not strike the anode immediately, but pass through the
cage where they create ions through electron impact ionization. Electrons which do not strike a grid wire
or ionize any molecules, pass through the grid volume into the region between the anode grid and the
repeller. There they decelerate and re-accelerate back toward the grid because of the electric field
created between the repeller and the anode grid. Electrons continue circulating in this fashion until they
are collected by the grid or are lost by recapture. The multiple passes increase the ionization efficiency
of the electron current relative to single pass configurations. Within the anode grid volume, ions are
attracted to the center by the negative potential produced by an increasing electron density toward the
center. Hence, the ions, once formed, tend to stay within the anode grid structure, and the ion
distribution is more localized along the axis. The ions formed within the anode grid volume, are
extracted from the ionizer by the electric field produced by the difference in voltage bias between the
anode grid and the focus plate. The focus plate is kept at a negative potential (relative to ground) and its
function is to draw the ions out of the anode cage and focus them into the filter section. The repeller,
which completely encloses the ionizer, is biased negative relative to the filament and prevents the loss of
electrons from the ion source.
The Repeller Grid and the Focus Plate are only biased while the
filament is emitting electrons.
The technique of electron impact ionization requires low pressure for the efficient production of
electrons and the subsequent formation of ions. The ionizer is best suited for operation in the high and
ultrahigh vacuum range (10
-4
to 10
-14
Torr). Non-linearities in the ion currents, attributed mostly to space
charge effects, are to be expected at pressures above 10
-5
Torr. Non-linearity effects can be minimized
operating at reduced electron emission current settings.