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.