Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Page 76
Vacuum Technology
Cold cathode ionization vacuum gauges
Cold cathode ionization vacuum gauges essentially consist of only two electrodes, a cathode
and an anode, between which a high voltage is applied via a series resistor. Negatively charged
electrons leave the cathode because of the high voltage, moving at high velocity from the
cathode toward the anode. As they travel this path, they ionize neutral gas molecules, which
ignites a gas discharge. The measured gas discharge current (Figure 3.5) is a parameter for
pressure. However only few molecules are ionized with straight electron trajectories, which
results in lower sensitivity and interruption of the gas discharge at approximately 1 mbar.
A design that avoids this disadvantage is the inverted magnetron after Hobsen and Redhead.
A metal pin (anode) is surrounded by a rotationally symmetrical measurement chamber
(cathode) (Figure 3.5). An axially magnetized, cylindrical, permanent-magnet ring is placed on
the exterior of the measurement chamber to generate a magnetic field within the chamber.
3 kV
mbar
Figure 3.5: Design of an inverted magnetron
The electrons travel through the magnetic field on spiral trajectories (Figure 3.6). The electron
paths extended in this manner increase the probability of collisions with the gas molecules
and ensure that sufficient ions are generated to maintain the gas discharge, even at pressures
of less than 1 mbar. The pressure reading will depend upon the type of gas in question
due to the different ionization probabilities of the various gases. For example, a lower
pressure will be indicated for helium than for air.
Cold cathode vacuum gauges can be easily contaminated under the following conditions:
If the device is activated at pressures p of more than 10 - 1 mbar
Argon is often used for applications in sputtering systems. This results in sputtering of
the cathode, as well, which can cause short circuits and thus failures of the gauges
Gases are also gettered on the surfaces of the cathode. This produces a pumping effect
that will falsify the measurement signal.
N
S
R
Cathode Anode
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