937B Controller Instruction Manual
pg. 91
11
Maintenance and Service of MKS Vacuum
Sensors
11.1 422, 423 and 431 Cold Cathode Sensors
Cold Cathode Theory
Ambient gas molecules are ionized by a high voltage discharge in Cold Cathode sensors and sensitivity is
enhanced by the presence of a magnetic field. MKS Cold Cathode sensors utilize an inverted magnetron
design that includes an isolated collector, as shown in Figure 11-1. This makes the sensor less
susceptible to contamination and allows a wider range of pressure measurement.
Figure 11-1 A comparison of Inverted Magnetron Cold Cathode Geometries. Isolated collector
design is shown on the left.
Cold Cathode ionization sensors have inherent advantages over Hot Cathode sensors. These include:
•
No filament to break or burn out, which makes the gauge immune to inrushes of air. It is also
relatively insensitive to damage due to vibration.
•
No X-ray limit for lower pressure measurements.
•
No adjustment for emission current or filament voltage is needed.
•
Degassing is not needed.
•
Sensor tubes can often be cleaned and reused almost indefinitely.
•
The control circuit is simple and reliable, having only one current loop, as compared with a Hot
Cathode sensor which has three.
•
Less power consumption enables the use of significantly longer cables between the controller
and the sensor.
A Cold Cathode sensor consists of a cathode and an anode with a potential difference of several kilovolts
between them. The electrodes are surrounded by a magnet, arranged so the magnetic field is
perpendicular to the electric field. The crossed electric and magnetic fields cause electrons to follow long
spiral trajectories within the sensor, increasing the chance of collisions with gas molecules, thereby
providing a significant increase in ionization efficiency over a Hot Cathode sensor.
In operation, a near constant circulating electrical current is trapped by the crossed fields in which the
collisions between electrons and residual gas molecules produce ions that are collected by the cathode.
The relationship between sensor current and pressure is
n
kP
i
, where
i
is the sensor ion current,
k
is