Penning and Ion Gauges Operation
Penning (cold cathode) and ion (hot cathode)
gauges are classified as indirect gauges that
typically operate from one Pascal (1x10-2 TORR)
to 1x10-10 Pascals (1x10-12 TORR). Penning
(cold cathode) and Ion (hot cathode) gauges
differ only a small amount, depending on the
manufacturer, but fundamentally they function
in the same manner.
Penning and ion gauges operate by ionizing
gas within a magnetically confined cathode
discharge. The combination of a strong magnetic
field and the high voltage applied to the
electrodes creates a directed plasma discharge
of electrons. Ions are formed by the electron
bombardment of the gases inside the confined
cathode. The ions are accelerated by the
magnetic field towards the charged cathode. The
impact of the ions with the cathode is measured
and reported by the gauge’s electronics.
Penning and ion gauges are significantly
affected by the gas composition in the vacuum
system and cannot be used with reactive gases.
Some ion gauges are constructed from glass
and once the gauges glow discharge starts, the
heat can cause the gauge to become very brittle
(and fragile). The glow discharge that is required
for the gauge to operate is also the reason
that these gauges cannot operate at pressures
above one Pascal (1x10-2 TORR).