Vacuum Flashcards
Collective vs Molecular Regime
Collective: high pressures, gas molecules collide with each other and behave collectively like a gas.
Molecular: Below 10^-4 mbar. Mean free path exceeds the dimensions of the vacuum system and molecules behave independently.
What pumps work in collective regime?
Gas displacement pumps (roughing pump) rotary vane pump.
What pumps work in the molecular regime?
Kinetic pumps e.g. diffusion pump, turbomolecular pump.
What pumps are used for ultra-high-vacuum?
Gas-binding pump: Getter pumps e.g ion getter pump
Explain how a rotary van pump works and its issues
- Uses compression of gas to use pumping.
- An eccentric rotor draws in air from the vacuum system by creating an expanding chamber
- Aliquot of gas is isolated and compressed out of a valve air
- Rotating vanes spring loaded and oil lubricated
- Pump causes hydrocarbon contaminations so its often employed with a trap containing high surface area ceramics
- spring-loaded rotors sweep out successive trapped volumes of gas and compresses them through an oil-sealed valve.
Explain how a turbomolecular pump works and its issues
Imparts downward momentum to a gas molecule through repeated collisions with a solid surface
- Not good at pumping very light gases
v = root(8kT/pim)
Explain how a oil vapour diffusion pump works and its issues
Imparts downward momentum to gas molecules.
- Oil heated until it vaporises
- Vapor forced through a jet
- Vapor hits cooled walls causing it to condense
- Oil flows back to bottom to be heated again
- Oil vapour creates a pressure difference forcing gas molecules downward
70% efficient
Silicone oils, SVP limits at 10^-7 Pa
Not good at pumping very light gases
What does a sputter ion pump comprise of?
- Two titanium plates (cathode, -ve charge)
- Large permanent magnet
- Stainless steel cell (anode)
How does a sputter ion pump work?
- Electrons spiral in the magnetic field until they interact with a gas molecule and ionise it
- Ion is attracted to titanium cathode and sputters titanium onto the anode
- Pumping is by “gettering”
- Titanium film reacts with gas molecules that strike it
- Also burying the inert gas molecules into the cathodes
- Very clean pump
What does a titanium sublimation pump consist of?
- Titanium filament
- surrounded by chamber walls
- Filament heated
How does a titanium sublimation pump work?
- Titanium sublimed from a filament made of titanium or molybdenum-titanium alloy
- Heated resistively for 90 seconds
- Titanium deposits onto the walls of chamber
- Coating is reactive and it getters reactive gases
- It is re-evaporated every 8 hours
Where does Qin come from?
Leaks
Desorption (outgassing)
Pump back-streaming
Virtual leaks (blind bolt holes)
How to reduce outgassing (Qin)
Wear gloves when handling
Use clean materials: stainless steel, glass
Clean components before attaching
Bake out: remove water vapour by heating to 150 - 250 C for 8-48 hours
What gauges are used in each regime?
Pirani gauge (collective).
Ionisation gauge (molecular).
How does a Pirani gauge work?
- Heated wire by current
- Temperature measure by resistance
- Wire loses heat by molecular impingement then desorption: from of convection
- Heat loss changes with pressure
- Less molecules, less heat loss
