The graph depicts a mechanical realization of pseudo potentials used in mass spectrometers and radio frequency ion traps.
A rapidly oscillating electric potential can confine and even trap atomic ions and other charged particles. In part A, a static quadrupole potential is shown. A particle placed at the center of the potential is at an unstable equilibrium point, where any small disturbance will make it roll off. If the potential is rotated around the z-axis as indicated, at a suitable frequency, the particle will feel an average potential well. This is the pseudo-potential. If the shown quadrupole potential is built in e.g. epoxy resin with a diameter of 10cm, and is rotated a few times per second, then small metal balls can stay trapped. A trapped particle can move around in the pseudopotential, but such motion will include small oscillations with the rotation frequency. An example of such motion is shown in part B as a black line, where a circular motion in the pseudopotential has an additional oscillation with small amplitude. This graph was submitted by Kristian Moelhave, graduate student, Institute of Physics and Astronomy, University of Aarhus, Denmark.