Echo-Enabled Harmonic Generation Studies for the FERMI Free-Electron Laser

Abstract

Studying ultrafast processes on the nanoscale with element specificity requires a powerful femtosecond source of tunable extreme-ultraviolet (XUV) or x-ray radiation, such as a free-electron laser (FEL). Current efforts in FEL development are aimed at improving the wavelength tunability and multicolor operation, which will potentially lead to the development of new characterization techniques offering a higher chemical sensitivity and improved spatial resolution. One of the most promising approaches is the echo-enabled harmonic generation (EEHG), where two external seed lasers are used to precisely control the spectro-temporal properties of the FEL pulse. Here, we study the expected performance of EEHG at the FERMI FEL, using numerical simulations. We show that, by employing the existing FERMI layout with minor modifications, the EEHG scheme will be able to produce gigawatt peak-power pulses at wavelengths as short as 5 nm. We discuss some possible detrimental effects that may affect the performance of EEHG and compare the results to the existing double-stage FEL cascade, currently in operation at FERMI. Finally, our simulations show that, after substantial machine upgrades, EEHG has the potential to deliver coherent multicolor pulses reaching wavelengths as short as 3 nm, enabling x-ray pump-x-ray probe experiments in the water window.

Coherent soft X-ray pulses from an echo-enabled harmonic generation free-electron laser

“…However, up to now neither FEL amplification nor coherent emission was reported in the soft-X-ray spectral region. In this regime, seed laser phase errors 30 as well as electron-beam instabilities 31 can strongly affect the emission process and may limit the effectiveness of the EEHG scheme at very high harmonics.…”

Editorial: Lasers in Accelerator Science and Secondary Emission Light Source Technology

“…Future laser-based or laser-enabled accelerator and light source technologies could tap into an incredibly broad range of methodologies, including but not limited to well-underway approaches like laser wakefield acceleration [15][16][17][18] and seeded [9] and echo-enabled harmonic generation in FELs [19,20] as well as more exploratory compact accelerator concepts such as free-space [21][22][23], terahertz [24][25][26], and on-chip [27][28][29] accelerators, among many others. Rather than a competition for the best possible future technology, these approaches answer to vastly differing scientific and technological needs, from large-scale colliders for exploration of fundamental physics [16,30] to compact systems for radiation therapy [27], to name a few.…”

Bunching phase and constraints on echo enabled harmonic generation

“…The responsible quadratic beam chirp can be largely removed by phase space linearization, or used intentionally to produce compressible FEL pulses if the linear frequency chirp is maintained during amplification. The sinusoid exemplifies a type of nonlinear energy structure that could be used beneficially to produce coherent bunching sidebands [8,10,28], or conversely may compromise the FEL output if uncontrolled.…”

“…As with most harmonic up-conversion schemes, initial errors can be multiplied and spoil the final output. The sensitivity of EEHG to initial laser phase, noise, and energy distortions in the electron beam has been examined previously [2][3][4][5][6][7][8]. EEHG uses two laser modulators and two dispersive chicanes to perform the harmonic upshift, and the final bunching spectrum is relatively insensitive to small distortions on the beam or in the laser upstream of the first chicane [9,10].…”