Once created in the notch collimator, FACET's two electron or positron bunches must be positioned one in front of the other, creating the tightly-focused leading "drive" bunch and trailing "witness" bunch that will enable plasma wakefield acceleration. The final bunch positioning and focusing will take place in a new lattice of magnets, to be installed in Sector 20 of the SLAC linac.

A schematic view of the magnet layout is shown below. The lattice consists of the chicane section, the Final Focus section, and the experiment section with a beam dump. The beam interaction point, labeled IP, is located 2 meters downstream from the last Final Focus quadrupole magnet. The lower chicane will be built first and be used to compress either electrons or positrons. All initial experiments can be carried out with this single line. At a later date the upper chicane will be added to allow both electron and positron bunches to be placed one behind the other, to allow a drive bunch and witness bunch arrangement. The optics are arranged so that both bunches are compressed and focused the same way at the final focus point. The scheme for both bunches entails a small-charged positron bunch accelerated ahead of a larger electron bunch in the linac. The electron bunch is sent through the lower chicane and the positrons through the upper chicane. Since the path lengths are carefully made to be different, the positron bunch comes out just behind the electron bunch at a position to be accelerated by the electron plasma wake fields. The distance of the positrons behind the electron bunch can be adjusted by slight adjustments of the length of the upper chicane.