Small signal gain analysis for a wiggler with noncollinear laser wave   and electron beam
  The collective and single-electron amplification regimes of a non-collinear free electron laser are analyzed within the framework of dispersion equations. The small-signal gain and the conditions for self-amplified excitations are found. The collective excitations in a free electron laser are shown to be favored by the non-collinear arrangement of the relativistic electrons and the laser wave. Implications for free-electron lasing without inversion are discussed. 
A.I. Artemiev, D.N. Klochkov, G. Kurizki, N.P. Poluektov, N.Yu. Shubin