Application of Volume diffraction grating for TeraHertz lasing in Volume   FEL (VFEL)
  The generation of induced radiation in volume resonator formed by metal threads is considered. It is shown that using of such volume diffraction grating allows increasing of lasing efficiency in terahertz range. The requirements on beam and grating parameters are obtained. 
V.G. Baryshevsky, K.G. Batrakov, V.I. Stolyarsky