A Paul trap uses electric fields oscillating at a radio frequency (rf)
to confine charged particles in space. The graphic shows the full path (in blue, only when "excess micromotion" is false) and a low-order approximation (in orange) of the motion of a single ion in a Paul trap. One of the simplest Paul traps is implemented with an oscillating quadrupole electric field. A static quadrupole vector field is shown for reference.
A linear Paul trap features translational symmetry along one of its axes (
, not shown), while the quadrupole extends along the transversal plane
. The motion of the ion in this plane is shown for selected conditions
. This motion is described by what is normally a fast oscillation of small amplitude at the rf, which is called micromotion, on top of an averaged, slower oscillation with larger amplitude, called the secular motion (in green). The
parameter characterizes the strength of the trap and is part of the definition of the micromotion amplitude and the secular frequency. The
parameter controls the total length of the ion path in time. Micromotion can be minimized, which occurs when the ion oscillates around the center of the quadrupole field or rf-null. You can select "excess micromotion" to offset the location of the ion away from the origin by
. It will experience excess micromotion [1, equation (15)].