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 usi ng a realistic numerical model which accurately accounts for short-pulse ef fects, mode pulling, and coupled electron-optical beam instabilities. Our s imulations are based on the Maxwell-Lorentz equations of motion, incorporat ing realistic optical resonator modes and electron density fluctuations, an d accurately track the phase and energy of the electrons throughout their e ntire interaction with the optical pulse. The studies assume a 2-m taperabl e undulator with a normalized vector potential of roughly unity, driven by an electron beam from either a thermionic or photocathode microwave gun. In verse tapering was found to provide greater extraction efficiency and optic al power than conventional tapering in moderate gain systems using thermion ic injector technology, and yielded over four times the extraction efficien cy of an untapered undulator with minimal effect on the energy spread of th e electron beam. In contrast, little improvement in efficiency or power out put was observed using a photocathode injector due to loss of coherence at high gain. The remarkable spectral stability, laser power output, and reduc ed energy spread achievable using inverse tapering in moderate gain systems are discussed with respect to applications in remote sensing and spectrosc opy.