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

摘要

We have studied the operation of tapered undulator free-electronnlasers using a realistic numerical model which accurately accounts fornshort-pulse effects, mode pulling, and coupled electron-optical beamninstabilities. Our simulations are based on the Maxwell-Lorentznequations of motion, incorporating realistic optical resonator modes andnelectron density fluctuations, and accurately track the phase and energynof the electrons throughout their entire interaction with the opticalnpulse. The studies assume a 2-m taperable undulator with a normalizednvector potential of roughly unity, driven by an electron beam fromneither a thermionic or photocathode microwave gun. Inverse tapering wasnfound to provide greater extraction efficiency and optical power thannconventional tapering in moderate gain systems using thermionic injectorntechnology, and yielded over four times the extraction efficiency of annuntapered undulator with minimal effect on the energy spread of thenelectron beam. In contrast, little improvement in efficiency or powernoutput was observed using a photocathode injector due to loss ofncoherence at high gain. The remarkable spectral stability, laser powernoutput, and reduced energy spread achievable using inverse tapering innmoderate gain systems are discussed with respect to applications innremote sensing and spectroscopy