Diffusion pump

They were the first type of high vacuum pumps operating in the regime of free molecular flow, where the movement of the gas molecules can be better understood as diffusion than by conventional fluid dynamics.

Most modern diffusion pumps use silicone oil or polyphenyl ethers as the working fluid.

Diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around 0.1 mbar (10 Pa).

The outside of the diffusion pump is cooled using either air flow, water lines or a water-filled jacket.

As the vapor jet hits the outer cooled shell of the diffusion pump, the working fluid condenses and is recovered and directed back to the boiler.

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, oil diffusion pumps are not suitable for use with highly sensitive analytical equipment or other applications which require an extremely clean vacuum environment, but mercury diffusion pumps may be in the case of ultra high vacuum chambers used for metal deposition.

Often cold traps and baffles are used to minimize backstreaming, although this results in some loss of pumping speed.

Also, diffusion pumps are capable of much higher compression ratios, and cannot discharge directly to atmosphere.