FACET will use an instrument called a notch collimator, installed in the SLAC linac's Sector 10 bunch compressor to split each of the linac's single electron bunches into two. The notch collimator will take advantage of the distribution of different energies of the incoming electrons to separate them into a leading "drive" bunch and following "witness" bunch. The drive bunch will knock electrons to the side as it passes through plasma inside the beam tube. The witness bunch will ride the electron wake created as the displaced plasma electrons spring back into place just behind it.

As they enter the notch collimator, the bunch of electrons—or later, positrons—necessarily has a distribution of energies that are highly correlated with position along the bunch. In the middle of the magnetic bunch compressor, the beam is dispersed in energy and in time. Thus, "collimating" a different portion of the bunch based on position will collimate a different portion of the bunch in time, and lead to different schemes for acceleration and bunch shapes. By placing a collimator of appropriate, variable geometry at different locations along the bunch, a wide variety of drive/witness bunch configurations can be created. One example that produces two bunches with a charge ratio, spacing and bunch length similar to that envisioned for a plasma wakefield linear collider is shown below. This technique can be used to generate an electron drive bunch with an electron witness bunch or a positron drive bunch along with a positron witness bunch.