Influence of magnetic field on turbomolecular pumps

Because Magnetically suspended turbo-molecular pumps (TMPs) with dry backing pumps are very preferable to achieve and maintain UHV without oil contamination, recently they have been widely used in nuclear fusion devices and sometimes in particle accelerators. In order to use them in these fields some researches have been published into the influence of the magnetic fields on TMPs. However, these evaluations were carried out under some ambiguous magnetic fields and only about the special type of TMPs. Thus, we have intended to establish the standard for dealing with the influence of magnetic field on TMPs. First of all, the influence has been exactly investigated for two parts: one is due to the vertical magnetic field, and the other due to the parallel component. A solenoid of diameter 500 mm and length 1500 mm has been newly constructed to examine the TMPs in the uniform magnetic fields. This solenoid has 1493 turns per m, therefore, letting a current of about 9.0A flow leads to generate the axial magnetic field of 100 gauss in the central region. The axial magnetic field is uniform with the deviation less than 0.5% in the cylindrical region of 400 mm in a diameter and 280 mm long at the center. The radial component is only about 0.5% of the axial one. We have studied the influence of the magnetic field on the TMPs of Osaka vacuum Products Co., Ltd. This TMP has not only the magnetic suspension bearing but the spirally grooved rotor, which endures such high backing pressures as 500 Pa. The TMP is set on the table made of Al and the direction to the axis of solenoid was adjusted by the support attached to the table. We have measured the rotating speed, power for driving the rotor, and the rotor temperature as a function of the magnetic fields. In the field vertical to the rotating axis, both the rotor temperature and the driving power clearly increased even at 10 Gauss. On the other hand, they had no changes at 100 Gauss in a parallel field. In this paper the experimental results are shown in detail. (author)