MKS 109030111, Инструкция по эксплуатации

Страница: 24 / 40

5 Operation
24
Series 903 Cold Cathode Vacuum Sensor
Instruction Manual - #109030026
5.2 Theory of a Cold Cathode Ionization Sensor
A cold cathode ionization sensor has a number of inherent advantages over a hot cathode sensor,
including:
• 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
This equation is valid for the pressure range from 10
-3
Torr down to 10
-8
Torr depending
upon the series resistor used. Sensitivities of 1 to 10 amperes/Torr are not unusual.
Starting a cold cathode sensor depends upon some chance event such as field emission or a cosmic
ray producing the first electron. This produces additional electron/ion pairs during its transit
between the electrodes, and the discharge soon builds up to a stable value. Start of the discharge
normally requires a very short time at 10
-6
Torr or above, a few minutes at 10
-8
Torr, and longer
times at lower pressures.
If the series resistor is small, e.g. 1M, the current at high pressures increases, and sputtering of the
cathode can become a problem. A larger series resistor reduces sputtering, and the voltage across
the tube is pressure dependent between 10
-4
and 10
-2
Torr. This extends the measuring range of the
cold cathode to 10
-2
Torr.
Because of the difficulty in maintaining the discharge at low pressures, sensors of the loop anode
design do not work well below 10
-6
Torr. To reduce this problem, a cylindrical anode, cathode
plates at each end, and a cylindrical magnet are used. During the 1950s, the inverted magnetron
sensor was developed, which uses auxiliary cathodes and is able to measure pressures below 10
-12
Torr.
Many electrode arrangements have been used in cold cathode sensors. Single feedthrough cold
cathode sensors often suffer from spurious currents due to insulator leakage and field emission,
which mask the small pressure dependent ionization currents. The 903 Sensor uses an inverted