turbo molecular pump


A turbomolecular pump is the typical choice for the second stage of a pumping system, once pre-vacuum is achieved by a roughing pump. It is therefore mounted between the roughing and the UVH recipient.

As shown in the picture below, the pump consists of a motor-driven rotor with different stages of turbine blades (this particular pump is actually cut in half for demonstration purposes). The big flange on the top of the pump is connected to the UHV recipient. A small flange on the side (cut off) is connected to the roughing line. The working principle of the pump is that the rest gas molecules which collide with the rotor blades attain some momentum deeper into the stack of blades (either to the left or to the right) and are thereby removed from the vacuum system. In order to achieve efficient pumping the speed of the rotor has to be very high. Typical values are up to 80.000 revolutions per minute. This principle of mechanical momentum transfer to the rest gas works less well for light rest gas atoms. In particular hydrogen is not well-pumped by a turbomolecular pump. For better hydrogen pumping, the combination with a titanium sublimation pump can be appropriate.

The picture below shows a fraction of a turbine blade

The fast rotation puts high mechanical demands on the bearings of the pump. Some newer pumps have magnetic bearings, such that the entire rotor is lifted and rotating in a magnetic field. The controllers are even clever enough to cope with a power failure: the pump is then operated as an electricity generator, generating the power needed to keep the rotor levitated. The only way to induce catastrophic failure is to drop something into the rotor or to take off the cable between pump and controller while it is running.


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