  The Project X Injector Experiment (PXIE) will serve as a prototype for the
Project X front end. The aim is to validate the Project-X design and to
decrease technical risks, known to be mainly related to the front end. PXIE
will accelerate a 1 mA CW beam to about 25 MeV. It will consist of an ion
source, LEBT, CW RFQ, MEBT, two SC cryomodules, a diagnostic section and a beam
dump. A bunch-by-bunch chopper located in the MEBT section will allow formation
of an arbitrary bunch structure. PXIE deviates somewhat from the current
Project-X front end concept in that it provides additional instrumentation and
relies on a reduced number of kickers for bunch chopping; the diagnostic
section also include an RF separator to allow studying extinction of removed
bunches. The paper discusses the main requirements and constraints motivating
the facility layout and optics. Final adjustments to the Project X front end
design, if needed, will be based on operational experience gained with PXIE.
