A high power (10 MW), ultra-short pulse (10 ps to 1 ns), millimeter-wave (35 GHz to 100 GHz) Free-Electron Laser (FEL) ex- periment is currently underway at UCLA. The device is energized by a 11/2 cell S-band photocathode RF linac which produceselectron bunches in the 3-4 MeV energy range, with pulse FWHMs ranging between 3 and 10 ps and charges up to 1 nC. Thecopper photocathode is driven by a 1 ps HVVHM frequency-quadrupled Nd:YAG laser. The electron bunch is transversally ac- celerated by a 8.4 cm-period, helically polarized wiggler. Because the electron bunch is much shorter than the FEL wavelength,all electrons coherently radiate spontaneously in the free-space limit, where slippage is important. In the TE11 grazing limit,where the axial bunch velocity matches the electromagnetic wave group velocity, the device behaves as a superradiant high- gain Compton FEL amplifier, seeded by the coherent spontaneous radiation produced at the wiggler entrance. Since the beamis prebunched, efficiencies are very high. Detailed studies of the temporal, spectral, power and phase characteristics of the radi- ation produced in both regimes are planned.