Design and test of magnetic shields for turbomolecular pumps

The use of turbomolecular pumps in plasma experiments with magnetic field confinement presents troublesome problems because of the presence of the static magnetic fields. The utmost consequence of this interaction can result in failure of the turbomolecular pump. In order to protect these pumps from the applied magnetic field, different magnetic shield layouts have been analyzed and tested: simple cylindrical tube, tube with partial enclosures, and tube with an enclosure plus a reinforced collar. All these shields have been made by cheap and easy to find material such as iron Fe-37. This work presents the study of the expected shielding performance of the three shielding configurations carried out by finite element analysis. Measurements to evaluate the effectiveness of the shielding structure have also been made. The principal aim of the present work is to indicate a possible approach to the problem and how to obtain a practical and inexpensive solution. The general guidelines described in this work can be applied also for vacuum devices exposed to the stray magnetic field of tokamaks such as ITER.