Electron-beam quality requirements for tapered-wiggler free-electron lasers

A general approach to optimization of the free-electron laser interaction is used to develop the scaling of emittance and energy spread requirements for tapered wigglers optimized for highest optical gain at fixed e-beam energy extraction. The required e-beam properties for a high extraction oscillator are found to be quite stringent at visible wavelengths, but state-of-the art accelerators should be sufficient. The applicability of various methods of emittance acceptance enhancement is examined. One very promising option is a magnet canting scheme for providing two-plane focusing in planar wigglers. Two-plane focusing relaxes the severe emittance requirement resulting from the need to maintain spatial overlap between the optical beam and the free-expanding e-beam. In addition, options for adjusting various system parameters for enhanced emittance acceptance, at reduced gain per unit current, are explored.