Abstract
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.
Highlights
Free-electron lasers [1] 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
We present a three-dimensional, timedependent simulation of a 2:2-m regenerative amplifier Free-electron laser (FEL) (RAFELs) using the MEDUSA/optics propagation code (OPC) simulation code(s) and use the results to discuss the properties of RAFELs, and how RAFELs differ from low-gain oscillators
We have described a simulation procedure for a RAFEL, and discussed the application of the procedure for a specific example that employed a concentric resonator with a hole outcoupler in the downstream mirror
Summary
Free-electron lasers [1] 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. An oscillator reaches saturation when the single-pass gain, G, is balanced by the loss, L, and this occurs when G 1⁄4 L=ð1 À LÞ or alternately L 1⁄4 G=ð1 þ GÞ This implies that the loss, which is typically dominated by the fraction of the radiation that is coupled out of the resonator and includes losses due to absorption in the resonator optics, can impose difficulties if the mirror absorption causes excessive heating in the mirror, which could lead to mirror distortion. This affects the slippage of the radiation pulse relative to the electrons
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