Ion Gauge Heads

Below the operating range of the Pirani gauge, an Ion Gauge is used to measure pressure. There are a range of gauge heads and filament materials to cover specific pressure ranges and vacuum requirements in this region.

The Ion Gauge heads all operate using the same principle. Subtle differences in design and construction determine the pressure range and robustness of the gauge in question.

Principle of Operation

The Ion Gauge consists of three distinct parts, the filament, the grid, and the collector. The filament is used for the production of electrons by thermonic emission. A +ve charge on the grid attracts the electrons away from the filament; they circulate around the grid passing through the fine structure many times until eventually they collide with the grid. Gas molecules inside the grid may collide with circulating electrons. The collision can result in the gas molecule being ionised. The collector inside the grid is -ve charged and attracts these +ve charged ions. Likewise they are repelled away from the +ve grid at the same time. The number of ions collected by the collector is directly proportional to the number of molecules inside the vacuum system. By this method, measuring the collected ion current gives a direct reading of the pressure.

The above paragraph is a simplification of what happens. The design of the gauge head effects how efficiently electrons are produced, how long they survive, and how likely they are to collide with a molecule. Combining these factors together gives the gauge a sensitivity. As a general rule, the higher the sensitivity, the more efficient the operation of the gauge.

There are other factors which determine the lowest pressure that a gauge head can measure. One of these limiting factors is the X-Ray Limit. When an electron collides with the grid, there is a probability of a photoelectron being produced. Once generated, there is also a chance that the photoelectron will hit the collector and produce an electron. Unfortunately, the collector does not know the electrical difference between collecting a +ve charge or losing a -ve charge. This means that every time an electron is knocked off the collector, the electronics measure it as receiving a +ve ion instead. This effect is very small and depends on the design of the gauge head. It normally generates a current measured in the picoamp range. At 10^-10 to 10^-11 mbar, however, this is also the current produced by the gauge head itself. If pressure is plotted against current, the graph can be seen to tail off as this x-ray current becomes the dominant effect. The x-ray current therefore limits the lowest pressure that the ion gauge can measure.