The key working principles of High and Ultra-High vacuum pumps
January 15, 2021
High Vacuum (HV) and Ultra-High Vacuum (UHV) levels can only be effectively and efficiently obtained by using a main pump that has the functional capabilities. Choosing which pump to use depends on a number of factors, such as noise/vibration, cost (initial and on-going), tolerance to contamination, footprint, maintenance schedules, and resilience to shock.
In this blog post, we’ll examine the working principles of HV and UHV pumps to help you make an informed decision.
Turbomolecular pumps: working principles
Turbomolecular pumps (TMPs) are kinetic units that use a high-speed spinning rotor (usually between 24,000 and 90,000 rpm). Their working parts are similar to a multi-bladed turbine, with pairs of rotor/stator stages along the shaft.
TMPs transfer the high-speed impact of their blades directly onto gas molecules, which changes the motion of these molecules and “pushes” them towards the “exit” of the pump. As the name indicates, TMPs typically operate in the molecular flow range between 10-3 and 10-11 mbar. When coupled with a drag pumping mechanism, this range can be extended to 10-2 mbar. Since they cannot compress against atmospheric pressure, all TMPs require appropriate backing pumps. Common backing pumps are rotary vane pumps or dry pumps like scroll or multistage roots.
There are several bearing concepts for TMPs, with the most common being:
Fully (5-axis) active magnetic levitated bearing design
All mechanical bearing design
Combination of passive magnetic and mechanical bearing design
Ion getter pumps: working principles
Ion getter pumps (also known as sputter ion pumps or ion pumps) produce UHV without the help of moving parts or valves. Initial pumping — usually managed by a turbomolecular pump combination — is used to remove the bulk gas until the vacuum drops to approximately 10-4 mbar or lower.
After removing the bulk gas, a high voltage (between 4,000 and 7,000 volts) is applied through the element assembly. This “pulls” electrons into the cylindrical anode-tube assembly. The electrons are bound into tight spiral paths by a permanent magnet (of 0.12 Tesla field strength) located outside the vacuum chamber, thus forming a plasma discharge.
The ions created then bombard the titanium cathode plate, and pumping of molecular/gas ions can occur through implantation (physisorption). The bombardment causes the sputtering of Titanium atoms from the cathode lattice, resulting in deposits on surrounding surfaces of sputtered film. This film produces pumping via gettering (i.e. chemisorption of gas molecules).
Cryo pumps: working principles
Cryo pumps work by either condensing or absorbing gases on cold surfaces. The required low temperatures are typically provided by a dual-stage cold head, where the first stage usually attains temperatures of between 50 and 80K at the cryopanels, and about 10K at the second stage.
A thermal radiation shield with the baffle is closely linked to the first stage of the cold head, where mostly H20 and CO2 are condensed. The remaining gases penetrate the baffle, where gases like N2, O2 or Ar will condense at the second stage. H2, He and Ne cannot be pumped by the cryopanels but will be adsorbed by the activated charcoal coated on the inside of the cryopanels attached to the second stage. A cryo pump's main advantage is its high pumping efficiency, which accelerates the pumping speed for water vapor.
Diffusion pumps: working principles
Diffusion pumps use a high-speed jet of vapor to direct gas molecules from the pump throat towards the bottom of the pump and out of the exhaust. Diffusion pumps produce pressures of < 10-7 mbar, which makes them ideal for both industrial and research usage.
Diffusion pumps operate with an oil of low vapor pressure, usually silicone oil or polyphenyl ethers. A high-speed jet is generated by boiling this oil and directing the vapor through a jet nozzle, where the gaseous flow changes from laminar to supersonic-and-molecular, with several jets frequently being employed in series. The outside of the diffusion pump is cooled using either air-flow or a water jacket. As the vapor jet hits the outer, cooled chamber of the pump, the vapor condenses and is recovered before being directed back to the boiler.
Diffusion pumps have no moving parts and are durable and reliable. However, a major disadvantage of diffusion pumps is a tendency to backstream oil into the vacuum chamber. This can result in carbonaceous or siliceous deposits. Due to this backstreaming, oil diffusion pumps are not suitable for highly sensitive analytical equipment or other applications that require an extremely clean vacuum environment (although baffles can be employed to mitigate this effect).
Choosing the right HV or UHV pump for your application
