Scheme of a Quadrupole ion trap of classical setup with a particle of positive charge (red), surrounded by a cloud of similarly charged particles (red). The electric field E (blue) is generated by a quadrupole of endcaps (a, positive) and a ring electrode (b). Picture 1 and 2 show two states during an AC cycle.
Click this LINK to visit the original image and attribution information. Right click on the image to save the 800px teaching JPEG.