English: Classical motion of a trapped ion in a radiofrequency (rf) quadrupole (Paul) trap. A quadrupole electric field is displayed for reference. The blue line represents the ion path in the transversal (or radial) direction of a linear trap. The orange line is the secular motion. A linear or a circular secular motion can be generated depending on the initial conditions. Micromotion is the fast oscillation around the secular motion, which is enhanced when a stray dc electric field pushes the ion away from the center of the trap, located at the intersection of the axes. [Berkeland's et al., J Appl Phys, Vol 83, N 10, Page 5025 (1998)].