  The Planet Formation Imager (PFI) Project has formed a Technical Working
Group (TWG) to explore possible facility architectures to meet the primary PFI
science goal of imaging planet formation in situ in nearby star- forming
regions. The goals of being sensitive to dust emission on solar system scales
and resolving the Hill-sphere around forming giant planets can best be
accomplished through sub-milliarcsecond imaging in the thermal infrared.
Exploiting the 8-13 micron atmospheric window, a ground-based long-baseline
interferometer with approximately 20 apertures including 10km baselines will
have the necessary resolution to image structure down 0.1 milliarcseconds
(0.014 AU) for T Tauri disks in Taurus. Even with large telescopes, this array
will not have the sensitivity to directly track fringes in the mid-infrared for
our prime targets and a fringe tracking system will be necessary in the
near-infrared. While a heterodyne architecture using modern mid-IR laser comb
technology remains a competitive option (especially for the intriguing 24 and
40{\mu}m atmospheric windows), the prioritization of 3-5{\mu}m observations of
CO/H2O vibrotational levels by the PFI-Science Working Group (SWG) pushes the
TWG to require vacuum pipe beam transport with potentially cooled optics. We
present here a preliminary study of simulated L- and N-band PFI observations of
a realistic 4-planet disk simulation, finding 21x2.5m PFI can easily detect the
accreting protoplanets in both L and N-band but can see non-accreting planets
only in L band. (abridged -- see PDF for full abstract)
