Theory And Simulation Of The Harmonic Amplifier Free-Electron Laser (HARP/TEL)

We investigate the advantages of a device named the HARP/FEL (for HARmonic amPlifier/Free-Electron Laser) which may be described as a two-element, optical-klystron FEL with the prebuncher stage oscillating at a frequency different form the output-stage frequency. In analysis based on the single particle treatment of Harvey and Palmer, where a one-dimensional, free-space is examined, it is found that when the prebuncher-wiggler period (λ
_w1
) differs from the output-coupler-wiggler period (λ
_w2
), then the gain and saturation power of the output coupler are at a strong maximum when the ratio, λ
_w1
/ λ
_w2
, is an integer. Physically, this synchronism condition arises when the ratio of the bunching wavenumbers is also an integer, a condition that insures that both FEL modes are resonant and coherently and coupled via the electron-beam bunching. The harmonic condition on the wavenumbers holds regardless of resonator or waveguide structure and details of the prebuncher device. The gain-enhancement mechanism is precipitated by injecting electron bunches into the output coupler with a period that is a subharmonic of the output coupler's ponderomotive potential. If the bunches are sufficiently localized, then each one will be confined to a single well in the ponderomotive potential and efficient energy coupling occurs between the electrons and the electromagnetic wave.