SRS Labs QMS 100 Series, Руководство пользователя

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2-8 Ionizer
SRS QMS Gas Analyzer
Ionizer
Positive ions are produced in the ionizer by bombarding 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.
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 Service 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.