Progress of the Volume FEL (VFEL) experiments in millimeter range
  Use of non one-dimensional distributed feedback in VFEL gives possibility of frequency tuning in wide range. In present work dependence of lasing process on the angle between resonant diffraction grating grooves and direction of electron beam velocity is discussed. 
V. Baryshevsky, K. Batrakov, A. Gurinovich, I. Ilienko, A. Lobko, V.   Moroz, P. Molchanov, P. Sofronov, V. Stolyarsky