Efficiency Enhancement in Regenerative Amplifier Free-Electron Lasers Using a Tapered Undulator

Recent progress in short-wavelength free-electron lasers (FELs) from the extreme ultraviolet through x rays has opened new avenues for industrial and research applications. Most such FELs rely on self-amplified spontaneous emission (SASE) in which the optical pulse grows from noise in a single pass through the undulator. However, this results in significant shot-to-shot fluctuations in power and spectrum. Oscillators are under consideration to improve the properties of the light generated by the FEL. In particular, a high-gain/low-Q oscillator, i.e., a regenerative amplifier free-electron laser (RAFEL), is one possible concept. In this paper, we present the first analysis of efficiency enhancement in a RAFEL with a long, tapered undulator line and demonstrate substantial enhancements in the brightness of the optical output. Here, we consider a high average power EUV RAFEL at 13.5 nm and show an increase in output power of factors of 4 and 1.5 over a comparable single-pass non-tapered and tapered SASE FEL, respectively. This points the way to high-power, tapered-undulator short-wavelength RAFELs.