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.