Design and Test of Magnetic Shields for Quadrupole Mass Spectrometers

The use of quadrupole mass spectrometers in plasma experiments with magnetic field confinement set troublesome problems because of the presence of static magnetic fields. In mass spectrometers, the Lorentz force associated to the magnetic field deflects the ion trajectory in the analyzer section with a consequent loss of the instrument gas sensitivity. In order to reduce the effect of the externally applied magnetic field in the quadrupole, different magnetic materials (iron Fe-37, soft iron Armco, Amumetal sheets) have been used as shielding structure of the quadrupole head. To prove the efficiency of the shielding system it has been monitored the hydrogen signal (which is the gas mostly affected by the presence of external magnetic field); the intensity and the direction of the magnetic field respect to the quadrupole axes have been varied. The principal aim of the present work is to indicate a possible approach to the problem and how to obtain a practical solution with a worthy compromise between costs and performances. The general guidelines described in this work can be applied also for vacuum devices exposed to the stray magnetic field of large tokamaks as ITER.