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Figure 6
Use of the third-harmonic spectrometer as a barometer of machine performance during setup at a 2.1 keV fundamental. All panels show data recorded at the third harmonic (6.3 keV); the horizontal axis is plotted as the corresponding fundamental-equivalent photon energy (measured third-harmonic energy divided by three). (a) Contour plot of a 100-shot average 2D spectrum during the initial setup. The horizontal axis is the dispersed photon energy and the vertical axis is the horizontal transverse profile of the FEL beam, so that contour lines trace regions of constant shot-averaged photon density on the detector. A bimodal structure is visible: a main peak around 2105 eV and a low-energy tail between 2090 and 2100 eV, with a transverse extent and intensity distribution distinct from the main peak. (b) One-dimensional spectrum obtained by integrating panel (a) along the transverse coordinate, showing the bimodal profile but not the underlying spatial inhomogeneity. (c) Contour plot of an equivalent 100-shot average after optimization of the machine settings, displaying a more uniform single-mode distribution. (d) One-dimensional spectrum obtained by integrating panel (c) along the transverse coordinate, which is close to Gaussian. Together, these panels show how the 2D spectrometer images provide rapid on-line feedback on both spectral and transverse coherence properties, enabling efficient optimization of the FEL.

Journal logoJOURNAL OF
SYNCHROTRON
RADIATION
ISSN: 1600-5775
Volume 33| Part 5| September 2026|
https://doi.org/10.1107/S1600577526006739