Alcatel Vacuum Technology France - ASM 180 TD/TD+ - ASM 181 TD+ User’s Manual
Analyzer cell operating principle
1/1
Edition 03 - May 97
A 30
Analyzer cell operating principle
The mass spectrometry
analyzer cell
is used for helium
partial pressure measurements.
The molecules of the gas being analyzed are bombarded
by an electron beam from a heated tungsten
filament (1)
in
an
ionization chamber (3)
.
A large proportion of the molecules are transformed into
ions. These ionized particles are accelerated by an electrical
field: the acceleration voltage. A magnetic field deflects the
ion beam by a radius propotional to the mass of the ions.
The
acceleration voltage
directs the Helium ions to
the
target
at the entrance of an amplifier, an electron multiplier
based system,
developed and patented by ALCATEL.
The stream of Helium ions is proportional to the partial
pressure of helium in the installation and its measurement is
used to find the value of the flow rate of the detected leak.
It is essential for the total pressure in the analyzer cell to be
less than 10
-4
mbar so that the paths of the electrons and
ions are not disturbed by residual molecules.
In order to separate the helium ions from the "noise" due to
"dispersed ions", a
"braking electrode" (6)
, placed in front
of the target, eliminates secondary, low-energy ions.
The top of the cell contains an auxiliary electrode which
collects ions that have a higher mass than that of helium.
This electrode, the
triode electrode (4)
, is used to measure
the total pressure inside the analyzer.
He leak signal
Electron
multiplier
Target
He
He
2.5V
3 A
He
He
Amplifier
80 V
+ 200 V
adjustable
" "
Electronic
current
0.2 - 2.0 mA
adjustable
3
2
1
4
"Heavy" ions
Electron beam
"Light" ions
Helium ions
5
6
Acceleration
voltage
N
magnetic
field
Ie
+
(Sensitivity)
(Helium
peak)
1
Filament
2
Electron collector
3
Ionization chamber
4
Triode electrode
5
Diaphragm
6
Braking electrode
Cell principle
Magnetic deflection
spectrometry
Leak flow rate
Vacuum operation
Separation of He ions
from "noise"
Total pressure