Restgases

Restgases, or residual gases, are the gas species that remain in a vacuum system after pumping. They originate from outgassing of chamber walls and components, permeation through seals, leaks, desorption of adsorbed films, and, in some cases, pump oil decomposition or getter activity. The partial pressures of these gases set the background pressure and can influence experimental conditions.

In ultra-high vacuum, the composition is typically dominated by hydrogen, water vapor, carbon monoxide, carbon dioxide, and a range of hydrocarbons. The exact mix depends on materials, surface treatments, temperature, and the type of vacuum pump used. Hydrogen commonly arises from diffusion through metal walls; water from adsorbed moisture; oxygen-bearing species from hydrocarbons and oxide films.

Residual gas analysis (RGA) is used to monitor restgases. A mass spectrometer detects desorbing gas species and yields a spectrum indicating partial pressures. Outgassing rates are often reported in mbar·l/s per piece; bake-out reduces them by desorbing surface layers.

Impact: Restgases contribute to surface contamination, chemical reactions, and unwanted sputtering in vacuum devices. In accelerators and storage rings, interactions between beam particles and residual gas molecules can cause scattering, energy loss, and shortened beam lifetimes.

Control and mitigation: Bake-out at elevated temperatures; selection of low-outgassing materials and clean assembly; use of non-evaporable getters or cryopumping; maintaining tight seals and avoiding leaks; periodic RGA monitoring to track composition and pressure.