Threshold for FELWI operation
  It is shown that the possibility to create the free electron laser without inversion (FELWI) has a threshold behavior on the field of intensity of amplified wave. In the collective approach, the description of threshold conditions is given. It is shown that the threshold of observation of amplification without inversion is sufficiently high, which hampers essentially the possibility of experimental realization of the FELWI. 
Karen Badikyan
physics.acc-ph