Development of a turbo-molecular pump with a magnetic shield function

To safely use turbo-molecular pumps (TMPs) in a magnetic field, it is necessary to reduce the eddy current induced in a rotating rotor. Instead of adding a magnetic shield facility to the TMPs available in markets, we developed a TMP with a magnetic shield function by replacing the housing material of the TMP with a ferromagnetic substrate SUS430. Before and after machining, the SUS430 was vacuum-fired at 700 °C for 10 h in order to attain a good vacuum quality and recover its high magnetic permeability. The magnetic shield efficiency of the TMP with SUS430 housing was then examined. When a perpendicular magnetic field of 9 mT was applied, the field inside the TMP was reduced to less than 0.3 mT. Next, we confirmed that the developed TMP shows good performance in achieving an ultrahigh vacuum in magnetic fields of up to 9 mT. The limit of the magnetic shield efficiency for the developed TMP is also discussed.