Stanford Research Systems RGA100, Operating Manual And Programming Reference

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3-6 Ionizer
SRS Residual Gas Analyzer
at low mass settings (1 to 10 amu) and can easily be eliminated biasing the focus plate at
least 30V more negative than the repeller.
The electron emission current is the electron current from the filament to the grid. The
available emission current range is 0 to 3.5 mA.
When an electron emission current is requested, the RGA biases the ionizer’’s electrodes
and activates the filament’’s heater until the desired emission current is achieved. A
““
Background Filament Protection”” Mode is automatically enabled to monitor the
performance of the filament while it is emitting electrons. The Filament LED remains on
as long as the filament is emitting electrons. If a problem is detected in the operation of
the filament (i.e. burnt filament or overpressure), the heater is immediately shut down and
the problem is prompted by the LED’’s. If a current of 0 mA is requested, the filament’’s
heater is shut down, the repeller and focus plate are biased to ground, the CDEM is
turned off (if necessary), and the Filament LED is turned off to indicate the absence of
emission in the ionizer.
The electron emission current is very tightly regulated by a feedback control loop which
dynamically adjusts the operating temperature of the filaments to keep the total emission
current constant. The emission of electrons from the filaments is limited by space-charge
effects (Child’’s Law limitations). The maximum current attainable is a function of the
grid-to-repeller spacing and its potentials. Ideally, the rate of formation of ions should be
proportional to the electron emission current. In practice, the exact dependence between
ion signal and electron emission current in small ion sources is complicated by space-
charge effects and only general trends can be predicted. Greater ionization efficiency is
to be expected as the electron emission current increases.