diffusionpump

A diffusion pump is a type of vacuum pump that uses a jet of hot oil vapor to entrain and transport gas molecules from a vacuum chamber to the pump’s exhaust. It is designed to achieve high vacuum without any moving parts in contact with the process gases, relying on the momentum transfer between the oil vapor and gas molecules.

Operation and design: Oil is heated to produce a continuous stream of vapor that passes through a diffuser assembly. The diffuser creates a high-velocity, directed flow that sweeps gas molecules from the chamber into the oil vapor stream, carrying them upward toward the exhaust. As the oil vapor cools, it condenses and returns to the reservoir, while the captured gases are discharged through the pump’s foreline to a roughing pump. Because the pump relies on hot oil vapor rather than mechanical components in the gas path, it can provide relatively high pumping speeds at very low pressures.

Advantages and limitations: Diffusion pumps offer high pumping speeds at high vacuum levels and have no moving parts in contact with the gases, contributing to simplicity and reliability in certain applications. Limitations include the potential for oil backstreaming into the chamber, oil contamination of sensitive processes, and the need for a continuous oil supply and oil management (including cold traps or baffles to control backflow). They also require a suitable roughing pump to establish the initial vacuum.

Applications and oils: Diffusion pumps are commonly used in vacuum deposition, coating, electron microscopy, and mass spectrometry setups. They typically use silicone-based or high-boiling mineral oils; oil choice affects cleanliness and backstreaming behavior.

Overall, diffusion pumps remain a foundational technology for achieving high vacuum in systems where oil-based pumping is acceptable and where moving parts in the gas stream are undesirable.