  Terahertz (THz) radiation occupies a very large portion of the
electromagnetic spectrum and has generated much recent interest due to its
ability to penetrate deep into many organic materials without the damage
associated with ionizing radiation such as x-rays. One path for generating
copious amount of tunable narrow-band THz radiation is based on the
Smith-Purcell free-electron laser (SPFEL) effect. In this Letter we propose a
simple concept for a compact two-stage tunable SPFEL operating in the
superradiant regime capable of radiating at the grating's fundamental bunching
frequency. We demonstrate its capabilities and performances via computer
simulation using the conformal finite-difference time-domain electromagnetic
solver {\sc vorpal}.
