Observation of harmonic lasing in the Angstrom regime at European X-ray Free Electron Laser

Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned x-ray free electron laser (FEL) user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is harmonic lasing self-seeding that allows to improve the longitudinal coherence and spectral power of a self-amplified spontaneous emission FEL. This concept was tested at the soft x-ray FEL user facility FLASH in the range of 4.5–15 nm and at Pohang accelerator laboratory X-ray Free Electron Laser (XFEL) at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both third and fifth harmonic lasing and, for the first time, operated a harmonic lasing cascade (fifth-third-first harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV.

Figure 1

Conceptual scheme of a SASE FEL (a), a harmonic lasing self-seeded FEL (b) and a harmonic lasing cascade (c). Green arrows correspond to the wavelength of interest $\lambda$, amplified through the whole undulator. Red and yellow arrows correspond to the fundamental wavelengths of the corresponding parts of the undulator, tuned to the subharmonic of $\lambda$.

Figure 2

Scan of the K value of the first part of the undulator consisting of five undulator segments, tuned to 700 eV. Mean pulse energy and its standard deviation are measured after the second part of the undulator tuned to 2.1 keV. FEL operates in “$5+7$ segments HLSS” mode.

Figure 3

Synchronous scan of phase shifters after first four undulator segments. FEL pulse energy is measured after the second part of the undulator tuned to 2.1 keV. Mean pulse energy and its standard deviation are plotted versus a delay measured in units of the fundamental wavelength in the first part (tuned to 700 eV). FEL operates in “$5+6$ segments HLSS” mode.

Figure 4

Single-shot spectra for SASE (left column) and HLSS (right column). Plots show spectral intensity (in arbitrary units) versus photon energy (in eV). HLSS operates in “$5+7$ segments” configuration. One can see a reduction of number of spikes in the HLSS spectra.

Figure 5

Scan of the K value of the first part of the undulator consisting of five undulator segments, tuned to 900 eV. Mean pulse energy and its standard deviation are measured after the second part of the undulator tuned to 4.5 keV. FEL operates in “$5+11$ segments HLSS” mode.

Figure 6

Synchronous scan of phase shifters after first four undulator segments. The FEL pulse energy is measured after the third part of the undulator tuned to 4.5 keV. Mean pulse energy and its standard deviation are plotted versus a delay measured in units of the fundamental wavelength in the first part (tuned to 900 eV). The FEL operates as a harmonic lasing cascade (five segments at 900 eV, six segments at 1.5 keV, and six segments at 4.5 keV).