MKS HPS 903 Series, Operation And Maintenance Manual

Page: 31 / 36

A.1
Cold Cathode Ionization Vacuum Sensor
Appendix A: How the Series
903 (IMT) Works
Theory of a Cold Cathode Ionization Sensor
A cold cathode ionization sensor has a number of inherent
advantages over a hot cathode sensor. These include:
No filament to burn out, which makes it immune to inrushes
of air, and it is relatively insensitive to vibration damage
No x-ray limit for lower pressure measurement
No adjustment for emission current or filament voltage is needed
Since heating or degassing is not needed, there is little
effect on the system
Properly designed sensor tubes can be cleaned and
reused almost indefinitely
With only one current loop, the control circuit is simple
and quite reliable, as opposed to a hot cathode sensor,
which has three.
The cold cathode magnetic discharge sensor consists of a cathode
and anode with a potential difference of several kilovolts. The
electrodes are surrounded by a magnetic field, arranged so that the
magnetic field is essentially perpendicular to the electric field. The
crossed electric and magnetic fields cause the electrons to follow long
spiral trajectories increasing the chance of collisions with gas
molecules, thereby providing a significant increase in ionization
efficiency relative to a hot cathode sensor.
In operation, a near constant circulating electron current is trapped by
the crossed fields. Collisions of electrons with residual gas molecules
produce ions which are collected by the cathode. The sensor current
i
as a function of pressure
P obeys the relationship:
i = kP
n
where,
k is a constant
P is in Torr
n is a constant, usually in the range of 1.00 to 1.15.