990 CLD Autoline™ Leak Detector
1-16
DRAFT 2/2/04
Ion Source
The ion source consists of two filaments, two halves of an ion
chamber, a pair of focus plates, and a grounded exit slit (the exit slit
is a reZmovable part of the spectrometer tube). The top half of the
ion chamber (the repeller plate) is held at a positive potential
(repeller voltage) with respect to the bottom half of the ion cham-
ber. The bottom half of the ion chamber is held at a positive poten-
tial (ion voltage) with respect to the grounded exit slit. Two focus
plates are also held positive (variable focus and fixed focus) with
respect to the bottom half of the ion chamber.
When a filament is electrically heated, electrons are directed into
the ion chamber with the help of the enhancement magnets. Elec-
trons colliding with molecules produce positive ions. These ions
are forced through the bottom of the ion chamber, the grounded
potential exit slit, and enter the analyzing magnetic field. This mag-
netic field separates and allows only the helium ions to reach the
preamplifier. The variable focus and ion chamber voltages require
adjustment when ion sources are changed. This fine-tuning proce-
dure produces an efficient, helium-sensing spectrometer tube.
Preamplifier Assembly The preamplifier assembly consists of an ion collector assembly
and a sensitive electrometer amplifier. The ion collector assembly
includes:
❑
Ground potential electrodes to guide the beam of
helium ions.
❑
A suppressor electrode to exclude any other ions.
❑
An ion collector electrode mounted on a low-leakage
feed-through.
Analyzing Magnet
The magnetic field is generated by two blocks of Alnico V magnet
material which are bonded to a mild steel yoke. The yoke connects
the two pole pieces that define the magnetic field which separates
the helium ions from the other ions.
Enhancement Magnets External magnetic pole pieces on each side of the ion source
enclosure direct the electron beam for maximum ionization and
sensitivity.
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