This is from the _BUNCHER example in SIMION 7.0. Below is the description from the README file:
The files in this directory provide an example of having user programs model an ion buncher. The problem is to slow a packet of ions so that they converge in time. The trick is to have them decelerate in a linear field and then (when they are in the middle of it) chop the field to zero Thus the leading ions will be going slower than the trailing ions and they will bunch together at some point down stream.
Ions in flight, along with a trace of times of ion splats against the electrode. Notice that ion splat times are near identical, as desired in an ion buncher.