Three-dimensional, time-dependent simulation of a regenerative amplifier free-electron laser

Free-electron lasers have been designed to operate over virtually the entire electromagnetic spectrum from microwaves through x rays and in a variety of configurations including amplifiers and oscillators. Oscillators typically operate in the low-gain regime where the full spectral width is ð !=!Þ 1=Nw and the efficiency 1=ð2:4Nw Þ. Further, since a low-gain oscillator saturates when the gain compensates for losses in the resonator G ¼ L=ð1 LÞ, this implies that the losses must be relatively small and the cavity Q must be relatively large. This imposes problems for high power oscillators because the high Q can result in mirror loading above the damage threshold, and in short-wavelength oscillators because sufficiently low loss resonators may not be possible at x-ray wavelengths. In contrast, regenerative amplifier FELs (RAFELs) employ high-gain wigglers that reach exponential gain and can operate with high loss (i.e., low Q) resonators. As such, RAFELs may be able to function at either high power levels or short wavelengths. In this paper, we describe a three-dimensional, time-dependent simulation of a RAFEL operating at a 2:2- m wavelength, and show that its behavior differs substantially from that of low-gain oscillators, and is closer to that of self-amplified spontaneous radiation FELs in regard to spectral linewidth and extraction efficiency.