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decisive factor for the quality of the entire analysis. Errors in this part of the process can largely not be
corrected in the later steps. Choosing the correct ion source is crucial. Closed (gas-tight) ion sources
allow gas analysis with minimal contribution of the residual vacuum. They exhibit practically no de-mix-
ing, have a high signal-to-ground ratio, low gas consumption and a low time constant.
3.2.1 Axial ion source
By focusing the ions in axial direction, the axial ion source supplies ions with a narrow energy distribu-
tion and a small speed component transversely to the axis. Excellent resolution, high sensitivity, and
good linearity are thus achieved. The open design allows recording of rapid changes in the partial pres-
sure, with minimum distortion due to self-outgassing and surface reactions.
Standard filament
W, with YO
Application examples
● General gas analysis
● Residual gas analysis
● Desorption measurements
Function
The electrons emitted by the cathode are accelerated toward the grid of the ionization area. The Weh-
nelt electrode, which is negative to the cathode, focuses the electrons. Most of the electrons pass
through the grid and reach the lens (focus) or return to the grid. The ions produced by electron impact
are drawn out by the focus and entrance orifice and focused to the mass filter. The grid prevents the
ions from being drawn to the cathode. The field axis potential is a few volts below the potential of the
ionization area, so that mainly ions from the ionization area enter the mass filter. Ions emitted outside
the ionization area have a lower kinetic energy when they enter the mass filter. They therefore remain in
the mass filter significantly longer. Such ions are separated out almost completely.
Degas
With degassing, the outgassing rate of the axial ion source can be reduced.
● Pressure: ≤ 10
● Emission: ≤ 10 mA (at 550 V)
● Time: ≤ 5 minutes
100 V
0
Fig. 2:
1 Cathode
2 Wehnelt
3 Ionization area
-Ir also being available.
x
-8
hPa
1 2
3
4
Electrode arrangement and potential curve of the axial ion source
4
Focus
5
Entrance orifice
6
Rod system
5
6
Product description
17/52
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