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Figure 5
Example single-shot two-dimensional image of the third-harmonic spectrum. Photon energy is dispersed along the x axis, and the horizontal FEL beam profile is mapped onto the y axis. The image shows a non-uniform spatial structure across the full spectrum, characteristic of the multimode nature of third-harmonic SASE radiation. The low-energy part of the spectrum has a different vertical extent and mode density than the central peak, indicating emission from different sections of the electron bunch with distinct lasing conditions. The white curve shows the sum of the image over the energy axis (projection onto the transverse coordinate).

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