MATTER LogoIntroduction to Electron Microscopes

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Transmission Electron Microscope (TEM) > Vacuum Pumps

Rotary Pumps (RP)
Used for preliminary evacuation and to back up the other pumps.

It is a simple mechanical pump that sweeps gas out using rotating vanes immersed in an oil tank.

This reduces air pressure to about 10-1 to 10-3 mbars.

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Diffusion Pump (DP)
Works by heating oil or mercury until it gives off vapour.

Gas molecules from the microscope diffuse into a fast moving stream of vapour and are 'swept away'.

This reduces air pressure to about 10-4 to 10-7 mbars.

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Sputter Ion Pump (SIP)
Ion pumps act by attracting ionised gases towards an electrode.

They generate an oil-free, ultra high vacuum of < 10-7 mbars.

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Diagram of vacuum pumps Diagram of Sputter Ion Pump Diagram of Rotary Pump Diagram of Diffusion Pump

The electron beam must be generated and used in a high vacuum of 10-4 mbars or less.

This ensures that:

Three different pumps are shown in the diagram. Click on each of them to find out how they work.

Question

Question
Why is a diffusion pump usually backed by a rotary pump?

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