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».
2
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