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

Abstract

X-ray free electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths.

Probability of radiation of twisted photons by cold relativistic particle bunches

“…The leading terms of the expansion are in agreement with general formula (56). d) Generalized exponential profile…”

“…Notice that, in the case at issue, the assumptions that were used to derive general formulas (55), (56) for the asymptotic expansion of the interference factor at large x are not satisfied. Therefore, (63) does not coincide with (55), (56). General formulas (55), (56) reproduce only the powerlike part of the asymptotics.…”

“…General formulas (55), (56) reproduce only the powerlike part of the asymptotics. Nevertheless, just as for (55), (56), the contribution of the odd Fourier harmonics is suppressed in comparison with the contribution of the even Fourier harmonics at large…”

“…(83) The Macdonald functions entering into this expression are expressed through elementary functions. According to (56), (82), the asymptotics for x max(1, |m|, |k|), m 0, reads…”

Purifying Electron Spectra from Noisy Pulses with Machine Learning Using Synthetic Hamilton Matrices

“…Hence, it should also work on experimental spectra, which will be slightly different to the extent to which many-electron effects show up in photoelectron spectra as compared to the present 3D single-active-electron calculations. To measure reference spectra in a proof-ofprinciple experiment one could either use seeded FEL pulses [29][30][31][32] or set up an experiment at a coherent (highharmonic) source and generate noisy pulses artificially.…”