Wavelength and emittance requirements in free-electron lasers

Improvements in electron beam quality are required for operation of free-electron lasers (FELs) at short wavelengths and low voltages. The authors investigated this scaling for a FEL amplifier in the exponential regime of operation wherein the optical beam is tied to the electron beam as a result of gain focusing, subject to the requirements of (1) good geometrical overlap between the radiation beam and the matched electron beam and (2) cold-electron-beam interaction. The scaling is given by simple analytical formulas. >