P. Sprangle, B. Hafizi, C.W. Roberson and D.F. Gordon
Beam Physics Branch, Plasma Physics Division, Naval Research Laboratory
A laser pumped free electron laser (FEL) can in principle generate coherent, polarized, high power x-rays using a much lower energy electron beam than would be required by a conventional FEL. This is due to the dramatic reduction in the wiggler period, which in the case of a laser pumped FEL is the laser wavelength. However, a number of practical obstacles could prevent the realization of a laser pumped FEL in the laboratory. Foremost amongst these is the requirement that the electron beam be cold. In order for the beam to be regarded as cold, the axial velocity spread must be less than the difference between the average beam velocity and the beat wave (ponderomotive) velocity. This requirement places a severe limitation on both the beam emittance and energy spread. Conditions are given on the emittance, energy spread, current density, and laser intensity needed to achieve lasing in the x-ray regime.
