Diffusion pumps are a type of vacuum pump designed to achieve better vacuums than possible by use of mechanical pumps alone. They use a high speed jet of fluid to direct residual gas molecules in the pump throat down into the bottom of the pump and out the exhaust. The high speed jet is generated by boiling the fluid (typically silicone oil) and directing the vapor through a multistage jet assembly. Often several jets are used in series to enhance the pumping action.

Unlike mechanical pumps, diffusion pumps have no moving parts and as a result are quite durable and reliable. They can function over pressures ranging from about 10^-10 torr (10 nPa) to about 10^-2 torr (1 Pa). Diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around 0.1 torr (10 Pa).

One major disadvantage of diffusion pumps is the tendency to backstream oil into the vacuum chamber. This oil can contaminate surfaces inside the chamber or upon contact with hot filaments or electrical discharges may result in carbonaceous or siliceous deposits. Due to backstreaming, diffusion pumps are not suitable for use with highly sensitive analytical equipment or other applications which require an extremely clean vacuum environment. Often cold traps and baffles are used to minimize backstreaming, although this results in some loss of pumping ability.

See also: vacuum pump