Design of an efficiency-enhanced Raman FEL oscillator

The paper reports numerical calculations appropriate for the design of an 'efficiency-enhanced' Raman free electron laser (FEL) oscillator. A helical undulator is used which consists of a constant period section followed by a section where the period is decreased roughly 10 percent. Simulation of the FEL is done by following the radiation dynamics in both the smalland large-signal regimes, using the 'generalized pendulum equation' with self-consistent radiation field. It is found that the efficiency of the oscillator may be enhanced from about 8 (no taper) to about 15 percent (with taper). The configuration of an experimental oscillator is described; this device should produce about 25 MW at a wavelength about 1.8 mm using an 800-kV electron beam.