Ionisationsgauge

Ionisationsgauge, also known as an ionization gauge, is a vacuum pressure sensor used to measure very low pressures. It is a standard instrument in vacuum technology and is employed in fields such as physics, surface science, semiconductor fabrication, and electron microscopy. The term Ionisationsgauge reflects the German spelling, but the device is widely described in English literature as an ionization gauge or Bayard–Alpert gauge in its common variant.

The basic principle relies on ionizing residual gas molecules inside a vacuum chamber. In a hot-cathode ionization gauge, a heated filament emits electrons that collide with gas molecules, creating positive ions. These ions are collected by an electrode at a known bias, generating an ion current. The ion current is proportional to the number of gas molecules and thus to the pressure in the regime of high and ultra-high vacuum. A widely used version is the Bayard–Alpert gauge, which employs a grid geometry to suppress secondary electrons and enables reliable measurements down to about 10^-11 mbar.

Gauges can be categorized as hot-cathode and cold-cathode (Penning) types. Cold-cathode designs use a glow discharge in a strong magnetic field and have no hot filament, making them rugged and suitable for some environments but with different starting pressures and sensitivity characteristics. Sensitivity of ionization gauges depends on the gas species, gauge geometry, and operating conditions, and readings require calibration against a known standard. In practice, the ion current must be translated into a pressure value, and readings can be influenced by outgassing, desorption, and background currents.

Applications include maintaining and monitoring ultra-high vacuum in scientific experiments, thin-film deposition, and materials research. Proper conditioning, bake-out, and periodic calibration help ensure accuracy and stability.