Free-Electron Lasers Without Inversion: Design of Two-Magnet Drift   Region
  We propose a two-magnet design of a drift region for a free-electron laser without inversion (FELWI). By performing direct calculations of the phase shifts for electrons passing the drift region, we prove that the small-signal gain integrated over the detuning is positive and is inversely proportional to the energy spread of the ``hot'' electron beam. The dispersion and the geometry of the drift region are specified, and the requirements to the electron beam quality, including the transverse size and the angular spread, are found. 
A. I. Artemyev (1,2,3), Yu. V. Rostovtsev (2), S. Trendafilov (2), K.   Kapale (2), M. V. Fedorov (3), G. Kurizki (1), M. O....