Influence of magnetic fields on turbo-molucular pumps

Ogiwara, Norio; Kanazawa, Kenichiro; Inohara, Takashi*; Wada, Kaoru*

Recently turbo-molecular pumps (TMPs) have been widely used in nuclear fusion devices and sometimes in particle accelerators. Therefore, in order to use them in a quasi-static magnetic field the influence of the fields on TMPs has been exactly investigated. First of all, the influence for the two special magnetic fields has been examined: one is due to the vertical magnetic field, and the other due to the parallel component. The parallel magnetic field has no influence of the operation of the TMP. On the contrary, the driving power and the rotor temperature were largely affected by the vertical magnetic field. The eddy current loss in the vertical field increases rapidly with the field strength. It is proportional to the square of the field strength. The eddy current loss also increases as the revolution frequency, however, it has quite different dependence from the magnetic field strength. These dependences are well explained by the skin depth, which referred to the region where a localized magnetic field and current flow exists. Then we have successfully evaluated the influence of the magnetic field with an arbitrary direction by means of adding the contributions from the two magnetic components. Furthermore, from the demonstration experiment, it is generally found that the magnetic field with the symmetry around the rotor axis does not affect the operation of the TMP.