Free-Electron Laser Amplifier Driven by an Induction Linac

Abstract

A free-electron laser (FEL) directly converts the kinetic energy of a high-brightness, relativistic electron beam into coherent radiation. It is a classical device, much like a traveling wave tube. In an FEL amplifier the conversion takes place in a single pass through a wiggler magnet, therefore, the fraction of kinetic energy converted must be high if the device is to be efficient. Since the conversion is a rapidly increasing function of the electron beam current, a current of 1 kA or greater is desired. The particle energy required depends on the wavelength of coherent radiation. For a given wiggler wavelength, λW, wiggler magnetic field B, and radiation wavelength, λS, there is a resonance condition that determines the proper electron energy. This condition will be derived later in this paper and is $$ {\lambda_S} = \frac{{{\lambda_W}}}{{2{\gamma^2}}}\left[ {1 + \frac{1}{2}{{\left( {\frac{{eB{\lambda_W}}}{{2\pi m{c^2}}}} \right)}^2}} \right] $$ (1) in which γ is the electron energy in units of the rest energy, mc2. If B and λW do not vary with position as the particle’s energy is converted and γ decreases, the condition is no longer satisfied at some position and saturation results. However, if B or λW or some combination varies with axial position, the resonance condition can be maintained. The conversion efficiency (also called extraction efficiency) is greatly enhanced and the output radiation energy increased.