The top image shows a cloud of particles loaded into the Linear Trap.  Several different forces on the trapped ions are present: 1) The AC trapping forces push the particles toward the center of the trap, which for the Linear Trap is a horizontal line; 2) Coulomb replusion keep the like-charged ions from getting to close together; 3) endcaps keep the particles weakly trapped in the horizontal direction; 4) gravity pulls the particles down; and 5) static (DC) electric fields pull the particles up, approximately balancing gravity.

To produce the lower image, the AC trapping fields were first reduced, producing a much weaker trap. Particles with lower-than-average charge were then pulled down and out of the trap by gravity, while particles with higher-than-average charge were pulled up and out of the trap by the DC electric fields.  After letting most of the particles escape, the AC fields were then turned back up again. The remaining ions were then held at the trap center, organizing themselves into a linear Coulomb crystal.

Electrodynamic Ion Traps