Turbomolecular pump

Vahid Bahrami YektaMBE LAB

UVIC

Pressure

1 atmosphere ~ 1 bar ~ 760 mm Hg ~ 760 torr ~ 100,000 Pa

Ion gauges read in mbar i.e. 1x10-10 mbar = 1x10-13 atm.

Rough Vacuum

1x10-4 mbar

High Vacuum Ultra High Vacuum

1x10-8 mbar

Lower Pressure

Viscous vs. Molecular Flow Regimes

The gas in a vacuum system can be in a viscous state, in a molecular state or in a state which is intermediate between these two.

When a system is brought from the atmospheric pressure to "high vacuum", the gas in the system goes through all these states. The mean free path of the gas molecules is very small at atmospheric pressure so that the flow of the gas is limited by its viscosity.

At low pressures where the mean free path of the mole cules is similar to the dimensions of the vacuum enclosure, the flow of the gas is governed by viscosity as well as by molecular phenomena; this is the intermediate flow.

At very low pressures where the mean free path is much larger than the dimensions of the vacuum enclosure, the flow is molecular.

Viscous > 10-4 Molecular < 10-6

Inlet

Outlet

Rot

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Non

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blad

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As the gas molecules enter through the inlet, the rotor, which has a number of angled blades, hits the molecules. Thus the mechanical energy of the blades is transferred to the gas molecules. With this newly acquired momentum, the gas molecules enter into the gas transfer holes in the stator. This leads them to the next stage where they again collide with the rotor surface, and this process is continued, finally leading them outwards through the exhaust.

Turbo pumps have advantages over diffusion pumps:

-no back-stream oil into the vacuum system at any time

-can be started and stopped in a few minuets

- can be directly connected to the chamber without a high vacuum valve.

Turbomolecular Pumps Potential Problems

– Very high speed rotor blades have close-mating stator blades.• Slight imbalances can cause vibration and bearing wear

problems.• Sudden blast of atmospheric pressure can bend the blades

down, causing catastrophic failure, “crashing the pump.”– Lubrication of the high speed rotor is an engineering problem.• Circulating oil is most reliable, but pump must be right-side-up.• Grease-lubricated bearings are less reliable, but allow pump to

be placed at any orientation.– Too high of a pressure will cause aerodynamic lift and drag.• Aerodynamic lift can bend blades, causing catastrophic failure.

TURBOVAC MAG line

• Turbomolecular pumps with magnetic rotor suspension

Thanx