Optical guiding in a free-electron laser

Refractive Guide Switching a Regenerative Amplifier Free-Electron Laser for High Peak and Average Power Hard X Rays

We present an x-ray regenerative amplifier free-electron laser design capable of producing fully coherent hard x-ray pulses across a broad tuning range at a high steady state repetition rate. The scheme leverages a strong undulator taper and an apertured diamond output-coupling cavity crystal to produce both high peak and average spectral brightness radiation that is 2 to 3 orders of magnitude greater than conventional single-pass self-amplified spontaneous emission free-electron laser amplifiers. Refractive guiding in the postsaturation regime is found to play a key role in passively controlling the stored cavity power. The scheme is explored both analytically and numerically in the context of the Linac Coherent Light Source II High Energy upgrade.

Ultrashort free-electron laser pulse

Three-dimensional characteristics of short free-electron laser pulses are analyzed. When the optical pulse length is short, the growth rate and optical guiding will vary among the Fourier components comprising the pulse. Matched beam solutions of the wave equation, including diffraction and nonparaxial effects, are discussed. In certain limits a front to back asymmetry develops along the pulse as well as a frequency spread across it. In these limits the asymmetry and the frequency spread are relatively small unless the number of optical cycles in the pulse approaches unity.