Quadrupole Magnets
Quadrupole magnets have pole tips at a common distance away from a
central
axis and at angles of 45, 135, 225 and 315 degrees to the horizontal.
The poles are alternating positive and negative, with the result that there is
no magnetic field along the central axis but the field strength increases in
proportion to the distance away from the axis. Ions which pass through a
quadrupole magnet experience a focusing action from such a field, similar the
the way a simple lens can focus light. A single quadrupole magnet
which focuses in the horizontal direction will defocus in the vertical (and
vice versa); however, a combination of two or three magnets with alternating
signs are able to produce net focusing action in both the horizontal and
vertical directions.