Penning Trap Question: Causes of Axial Oscillation

[jonlg_uk]

Hi I realize that this is probably not going to be a well known topic. However does anyone know what actually causes the axial oscillation of a charged object (ion) in a Penning trap. I know that it is a result of the potential at the end caps. But the 2 end caps have a zero potential between them. Why does it oscillate ? Is it because it is getting attracted to one end cap then the other, which creates an oscillation?

 

[Gerenuk]

I think someone explains the concept here
http://tinyurl.com/32jjeyw
:)