Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 49 / 362

Ionizer 3-3
SRS Residual Gas Analyzer
Ionizer
Positive ions are produced in the ionizer by bombarding residual gas molecules with
electrons derived from a heated filament. The ions are then directed toward the entrance
of the ion filter where they are separated based on their mass-to-charge ratio.
Description
The SRS RGA ionizer is of an open design (wire mesh construction) with cylindrical
symmetry and mounted co-axially with the filter assembly. The open configuration
provides unrestricted communication between the vacuum environment and the ionizing
region. The entire assembly is built out of type 304 stainless steel for complete vacuum
compatibility. The simple mesh design is very clean (low outgassing) and easy to service.
The principle parts of the ionizer are: the repeller, the anode grid, the filament and the
focus plate.
Focus Plate
Anode Grid
Filament
Repeller
Figure 3 Ionizer Components
The filament is made out of oxidation-resistant thoria coated iridium wire, which operates
at low temperature and can be exposed to atmosphere without the risk of burn-out. The
circular construction consisting of two ThO
2
/Ir wires surrounding the anode grid
preserves the cylindrical symmetry of the ionizer. Both filaments operate simultaneously
under normal operation, and service is not interrupted if a single filament burns out (i.e.
the emission regulator automatically readjusts the temperature of the remaining filament
to restore the electron emission current). Once they wear out, the filaments can easily be
replaced in the field (Please see the RGA Maintenance chapter for details). A firmware
driven filament-protection feature constantly monitors (675 Hz) for overpressure. If
overpressure is detected the filament is immediately shut off, preserving its life.
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.