diffusionspump

A diffusion pump is a type of vacuum pump used to achieve relatively high vacuum by employing a stream of hot vapor, usually oil, to entrain gas molecules from the chamber and eject them through the pump outlet. Gas molecules in the chamber are captured by the momentum transfer from the hot oil vapor and are carried toward the exhaust, where they are pumped away by a backing pump and subsequently condensed or trapped.

Construction and principle: The pump contains a reservoir of diffusion oil that is heated to produce a large flow of oil vapor. The vapor is directed through a diffuser or jet assembly, creating a turbulent plume inside the pump. Gas molecules entering the pump are entrained by this vapor flow and driven toward the exhaust path. The exhaust typically passes through a cold trap or condenser and then to a foreline pump, which maintains the required backing pressure. Oil returning to the reservoir may condense and be recycled.

Oil choice and maintenance: Common diffusion oils are mineral, silicone, or fluorinated oils, chosen for thermal stability and vapor pressure characteristics. The pump requires a separate cooling system for the condenser and a mechanism to minimize oil backstreaming into the chamber. Regular oil changes and maintenance of seals, traps, and baffles are part of routine service.

Performance and limitations: Diffusion pumps provide high pumping speeds over a broad range of gases but are less effective for light gases such as hydrogen and helium and can backstream oil into the chamber without proper baffles and traps. They are often used with a backing pump and may be combined with other technologies (such as turbomolecular pumps) to reach lower pressures.

Applications: They are commonly used in vacuum coating, thin-film deposition, mass spectrometry interfaces, and other laboratory processes requiring moderate to high vacuum. They are valued for robustness, simplicity, and relatively low cost per pumping speed.