Part 1: Vacuum Systems - G. Jensen

Summary

This video explains the process of evacuating a microscope column to achieve ultra-high vacuum, detailing various types of vacuum pumps used in sequence. It starts with mechanical rotary pumps, followed by oil diffusion pumps, turbomolecular pumps, and finishes with ion getter pumps. Each pump serves a specific function depending on the pressure range, with the mechanical pump being the noisiest and the ion getter pump reaching the highest vacuum levels. The process is complex, requiring careful sequencing and maintenance to ensure high-quality imaging without interference from pump noise or failure.

Takeaways

• 😀 Different types of vacuum pumps are used to achieve various pressure ranges, and they work sequentially from atmospheric pressure to ultra-high vacuum levels in a microscope column.
• 😀 The mechanical or rotary pump is the first stage of evacuation, using a rotor and veins to gradually scoop gases from the column and push them into the exhaust valve, where they are expelled into the room.
• 😀 Mechanical pumps create vibrations and noise, so they should be turned off before taking high-resolution images to avoid interference.
• 😀 Maintenance of mechanical pumps includes periodic oil changes, and they are only effective in reducing pressure from atmospheric to lower levels.
• 😀 The oil diffusion pump is the next stage and operates by heating oil in a reservoir, where oil molecules interact with gas molecules to push them out of the pump.
• 😀 Oil diffusion pumps are quieter than mechanical pumps, but occasional 'pinging' sounds may occur when oil molecules collide with the pump's walls.
• 😀 Turbomolecular pumps, with a propeller-like design, are highly effective at reducing pressure further and are capable of starting at atmospheric pressure and working down to ultra-high vacuum levels.
• 😀 Ion getter pumps, utilizing electrodes to capture ions, are the final stage of evacuation and achieve extremely high vacuums, though they work slowly and may eventually need replacement after years of use.
• 😀 Ion getter pumps can be damaged if turned on immediately after venting the column to atmosphere, so it is crucial to wait for the column to reach a high vacuum before activating them.
• 😀 The full vacuum system in an electron microscope includes a complex arrangement of pumps (mechanical, oil diffusion, turbomolecular, and ion getter pumps) working in concert to maintain the required vacuum conditions for imaging.
• 😀 Proper procedure for venting the microscope includes shutting off all pumps, allowing atmospheric pressure to enter, and then gradually evacuating the system in stages, with the ion getter pumps activated only once a very high vacuum is achieved.

Q & A

What is the primary function of a mechanical or rotary pump in a vacuum system?

The mechanical or rotary pump is used to reduce the pressure from atmospheric levels to a lower vacuum range. It gradually evacuates gases from the electron microscope column into the room.

How does a mechanical rotary pump work?

In a mechanical rotary pump, a rotating rotor with attached veins moves gases through the pump. As the rotor turns, the veins extend or contract, pushing the gases towards the exhaust valve, where they are expelled into the room after passing through an oil mist trap.

What is a significant drawback of mechanical pumps during electron microscopy imaging?

Mechanical pumps create vibrations that can interfere with high-resolution imaging, so it is important to turn off the pump before taking images.

What is the role of the oil diffusion pump in a vacuum system?

The oil diffusion pump further reduces the vacuum level after the mechanical pump by using heated oil to capture gas molecules. As the oil molecules interact with gas molecules, they knock the gas down further, achieving a lower pressure in the system.

How does an oil diffusion pump work, and what causes the 'pinging' sound sometimes heard in the microscope?

The oil diffusion pump uses a reservoir of heated oil to capture gas molecules, which are deflected by baffles and condensed on cold walls. The 'pinging' sound occurs when oil molecules hit the walls of the pump while carrying gas molecules down.

What is the advantage of a turbomolecular pump over a mechanical or oil diffusion pump?

The turbomolecular pump is capable of working over a wider pressure range, starting from atmospheric pressure and reducing it to even lower vacuum levels. It operates by using spinning propellers to push gas molecules out of the system.

What is the main principle behind the operation of an ion getter pump?

An ion getter pump works by using charged electrodes to accelerate and capture ions created when molecules are split by cosmic rays. The ions are embedded into the electrodes, effectively removing gases from the vacuum system.

Why do ion getter pumps require periodic replacement?

Ion getter pumps can become exhausted over time because they accumulate too much material from the gases they capture, which may require replacement after several years of use.

How does the vacuum system work during sample insertion or film exchange in electron microscopy?

Before sample insertion or film exchange, the ion getter pumps are turned off to avoid damage. After the new sample is inserted, the vacuum is re-established, and the ion getter pumps are turned back on once the pressure is low enough.

What is the role of sensors and valves in managing the pressure levels in a vacuum system?

Sensors monitor the pressure levels in the system and control valves to maintain the appropriate vacuum levels. When pressure is too high, the mechanical pump is activated, and when it's low enough, the oil diffusion or ion getter pumps are used to achieve higher vacuums.