Physics issues for high-power free-electron laser amplifiers

Light from a high-gain single-pass FEL amplifier is expected, from theoretical and numerical analysis, to be of very high optical quality The light is also anticipated to be guided by the electron beam in the FEL, permitting operation of amplifiers with wigglers that are many Rayleigh ranges long, removal of a restriction to 2 or 3 Rayleigh ranges makes possible much higher extraction efficiencies in the FEL. Neither good optical beam quality nor guiding has been experimentally verified but represents critical uncertainties in the physics of FEL amplifiers.