Ionization gauge

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An ionization gauge, or ion gauge, is a vacuum gauge that is used to measure the residual pressure of vacuum in the high vacuum and ultra-high vacuum (UHV) ranges. There are two main types of ionization gauges, called the hot cathode and cold cathode types. A third type which is more sensitive and expensive is a spinning rotor gauge.

High vacuum is usually measured in units of Torr. (1 Torr = 133 Pa) Most ion gauges cannot measure the higher pressures of the medium vacuum range, where Pirani gauges, thermocouple gauges, and convection gauges are used instead. Instrumentation measurements of a hot cathode ionization gauge are always logarithmic. The calibration of an ion gauge is unstable and dependent on the nature of the gases being measured, which is not always known. They can be calibrated against a McLeod gauge which is much more stable and independent of chemistry.

[edit] Hot cathode

A hot cathode ionization gauge is mainly composed of three electrodes all acting as a triode, where the cathode is the filament. The three electrodes are a collector or plate, a filament, and a grid. The collector current is measured in picoamps by an electrometer. The filament voltage to ground is usually at a potential of 30 volts while the grid voltage at 180–210 volts DC, unless there is an optional electron bombardment feature, by heating the grid which may have a high potential of approximately 565 volts. The most common ion gauge is the hot cathode Bayard-Alpert gauge, with a small collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the Nude Gauge is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber. Hot cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot.

Electrons emitted from the filament move several times in back and forth movements around the grid before finally entering the grid. During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (Electron ionization). The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current.

The low pressure sensitivity of hot cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce x-rays that produce photoelectric noise in the ion collector. This limits the range of older hot cathode gauges to 10^-8 Torr and the Bayard-Alpert to about 10^-10 Torr. Additional wires at cathode potenial in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire, but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a

Faraday cup
Microchannel plate detector with Faraday cup
Quadrupole mass analyzer with Faraday cup
Microchannel plate detector with Faraday cup
Quadrupole mass analyzer with Microchannel plate detector Faraday cup
ion lens and accelartion voltage and directed at a target to form a sputter gun. In this case a valve lets gas into the grid-cage.

[edit] Cold cathode

There are two subtypes of cold cathode ionization gauges: the Penning gauge, and the Inverted magnetron, also called a Redhead gauge. The major difference between the two is the position of the anode with repect to the cathode. Neither has a filament, and each may require a DC potential of about 4 kV for operation. Inverted magnetrons can measure down to 1x10^-12 Torr.

Such gauges cannot operate if the ions generated by the cathode recombine before reaching the anodes. If the mean-free path of the gas within the gauge is smaller than the gauge's dimensions, then the electrode current will be vanishingly small. A practical upper-bound to the detectable pressure is, for a Penning gauge, of the order of 10^-3 Torr.

Similarly, cold cathode gauges may be reulctant to start at very low pressures, in that the near-absence of a gas makes it difficult to establish an electrode current - particularly in Penning gauges which use an axially symmetric magnetic field to create path lengths for ions which are of the order of metres. In ambient air suitable ion-pairs are ubiquitously formed by cosmic radiation; in a Penning gauge design features are used to ease the set-up of a discharge path. For example, the electrode of a Penning gauge is usually finely tapered to facilitate the field emission of electrons.

Maintenance cycles of cold cathode gauges is generally measured in years, depending on the gas type and pressure that they are operated in. Using a cold cathode gauge in gases with substantial organic components, such as pump oil fractions, can result in the grown of delicate carbon films and shards within the gauge which eventually either short-circuit the electrodes of the gauge, or impeded the generation of a discharge path.