MKS HPS 903 Series, Руководство по эксплуатации и обслуживанию

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A.2
Cold Cathode Ionization Vacuum Sensor
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. 1 MW, 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 (IMT) uses an inverted magnetron to reduce
these problems by using separate feedthroughs for the anode
high voltage and the cathode current. This geometry uses a
cylindrical cathode, a central wire anode, and external
cylindrical magnet which provides an axial field. The cathode is
insulated from the grounded metal housing.
The inverted magnetron geometry has a characteristic electrical
conductance vs. pressure curve which is more reproducible than
other arrangements, and also works well to low pressures without
risk of the discharge going out. This gives the (IMT) a measuring
range from 5 x 10
-3
to 3 x 10
-10
Torr.