SECTION V
THEORY OF OPERATION
5.1 General Theory of Operation of Ion Gauge Tube and Controller
FILAMENT
GRID
FIG. 5-1 SCHEMA TIC OF COMMON IG TUBE
The pressure indication of a Bayard-Alpert gauge is based on the ionization of gas
molecules by a constant flow of electrons. The functional parts of the gauge, shown
schematically in Fig. 5-1, are the filament, the grid, and the collector. In ionization gauge
tubes as well as some electronic vacuum tubes the filament serves as the cathode (hot
cathode) or emitter. It's purpose is to emit a stream of electrons. This is accomplished by
heating and thereby imparting energy to the molecules thus causing electrons to bet.boiled off.
The rate at which electrons move from the cathode is dependent on the filament temperature
and grid potential which must both be highly controlled by the filament power supply and the
grid bias supply.
The grid (anode) serves to control the electron beam or more accurately the speed of
the electron. In the ion gauge tube the grid is placed at a positive voltage such that most of
the electrons emitted at the filament are accelerated through the grid structure and into a
drift or interaction region which is the volume surrounding the collector, enclosed by the grid.
Since the ion gauge is open to the vacuum system and since vacuum is the measure of
gas particles absence or presence there will be a varying density of gas particles in the
electron interaction region depending on the vacuum pressure. The electrons emitted from
the cathode will now collide with the gas particles dislodging electrons and producing ions in
numbers that are proportional to the gas density. The collector is at a negative potential and
thus the positive ions are attracted to it and a current (I
+
) is produced also being proportional
to gas density. The electrons being negative particles are attracted to the grid structure, this
current is noted as 1-. It is these two currents I
+
and
r
which allows the precise measurement
of the vacuum. Through electronic detection and control, currents are monitored and vacuum
pressure indicated.
5-1
