Design of a compact, optically guided, pinched, megawatt class free-electron laser

A conceptual design for a compact megawatt class free-electron laser (FEL) operating at 1 μm is presented. The proposed FEL consists of an optically guided, pinched amplifier configuration driven by a RF linac. The gain length, efficiency, electron pulse slippage, and the distance between the wiggler and first relay mirror are determined for a megawatt class design. Of particular concern in the design is the overall length of the optical system, i.e., the wiggler length and distance to the first relay mirror. In the present design, the wiggler length is ∼1 m and the distance between the first relay mirror and the wiggler is determined by the average intensity damage threshold on the mirror. By focusing the electron beam, the optical beam can be pinched upon exiting the wiggler. The pinched optical beam has a reduced Rayleigh length which permits the first relay mirror to be relatively close to the wiggler. By pinching the optical beam and employing grazing incidence the first relay mirror can be located within ∼3 m of the wiggler. It is shown that frequency detuning can more than double the FEL efficiency. In the present design with frequency detuning and uniform wiggler the efficiency is ∼1.8%. In addition, electron-pulse slippage is shown to be substantially reduced in a high-gain amplifier.