Influence of magnetic fields on turbo-molecular pumps

Recently, turbo-molecular pumps (TMPs) have been widely used in nuclear fusion devices and sometimes used in particle accelerators. In order to use them in the quasi-static magnetic field often employed in these devices, the influence of these fields on TMPs was investigated in detail.First of all, the influence of two special magnetic fields was examined, a magnetic field orthogonal to the TMP, and one which was parallel. The parallel magnetic field had no influence on the operation of the TMP. On the other hand, the driving power and the rotor temperature were greatly affected by the vertical magnetic field. The eddy current loss in the vertical field increased rapidly with the field strength, proportionally to the square of the field strength. The eddy current loss also increased with the rotation frequency; however, it was roughly proportional to the square root of the frequency at higher frequencies. These dependences are well explained by the skin depth, which indicates the region beneath the surface of the rotor where a localized magnetic field and current flow exist.Then, we successfully calculated the influence of a magnetic field in an arbitrary direction by adding the contributions from the above two magnetic components. Furthermore, experiment revealed that a magnetic field with symmetry around the rotor axis does not affect the operation of the TMP.