Adixen ASM 142, Руководство пользователя

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A 300
Analyzer cell operating principle
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Description (continued)
These ionized particles are accelerated by an electrical field.
The entire analyzer cell is subject to a magnetic field which has the
property of deflecting the trajectories of the ions along different curves
according to the masses of those ions (to be more precised, according
to their m/e ratios). Thus the ions beam, which contained ions with
different masses, is divided into several beams, each containing
only ions with the same m/e ratio. The helium ions (m/e = 4) are
separated from the lighter (H2+ or H1+, smaller beams) or heavier
ions (N2+ or O2+, small beams).
Because there is a constant magnetic field (permanent magnet), the
accelerator electrical field is adjusted so that the helium ions (m/e
= 4) follow a pre-determined trajectory (passing through diaphragms)
and arrive on the target at the input to a direct current amplifier.
The current of helium ions is proportional to the partial pressure of
helium in the installation and by measuring it we can find the flow
rate of the leak that has been detected.
It is essential that the total pressure in the analyzer cell is less than
10 -4 mbar, so that the trajectories of the electrons and the ions are not
disturbed by residual molecules.
Around 10 -3 mbar there is a risk of damaging the heated filament.
In order to separate the helium ions from «noise» caused by
«stray ions», an electrode located in front of the target eliminates
the secondary ions with low energies. This electrode is called the
«braking electrode».
There is an auxiliary electrode at the top of the cell, shaped like
a plate, which collects the ions that are heavier than helium. This
electrode thus measures the total pressure in the analyzer. This
electrode serves as the plate for a triode gauge, hence its name of
«triode electrode».
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