Use of inverse tapering to optimize efficiency and suppress energy spread in an RF-linac free-electron laser oscillator

We have studied the operation of tapered undulator free-electron lasers using a realistic numerical model which accurately accounts for short-pulse effects, mode pulling, and coupled electron-optical beam instabilities. Our simulations are based on the Maxwell-Lorentz equations of motion, incorporating realistic optical resonator modes and electron density fluctuations, and accurately track the phase and energy of the electrons throughout their entire interaction with the optical pulse. The studies assume a 2-m taperable undulator with a normalized vector potential of roughly unity, driven by an electron beam from either a thermionic or photocathode microwave gun. Inverse tapering was found to provide greater extraction efficiency and optical power than conventional tapering in moderate gain systems using thermionic injector technology, and yielded over four times the extraction efficiency of an untapered undulator with minimal effect on the energy spread of the electron beam. In contrast, little improvement in efficiency or power output was observed using a photocathode injector due to loss of coherence at high gain. The remarkable spectral stability, laser power output, and reduced energy spread achievable using inverse tapering in moderate gain systems are discussed with respect to applications in remote sensing and spectroscopy.