Magnetic bearings for high speed turbo molecular pumps

A turbo molecular pump (TMP) is a multi-stage axial-flow turbine in which high-speed rotating blades work against similar static blades to provide compression by momentum transfer to the gas molecules. The TMP is optimised for molecular flow conditions and requires a backing pump to exhaust to atmosphere achieving ultimate vacuums up to 10/sup -10/ millibars. Turbine speeds of 30,000 to 100,000 RPM are typical, depending on the design flow rate. Pumps such as these are used extensively in semiconductor FABs where clean ultra-high vacuums are required. Magnetic bearings have become the preferred option for use in such applications where the extra cost and complexity are outweighed by the many advantages which are summarised in the paper.