A high-harmonic generation source for seeding a free-electron laser at 38 nm

Abstract

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the selfamplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.

Modal analysis of a seeded free-electron laser

“…We have used the procedure described above to analyze the 38 nm HHG seed pulse at the sFLASH direct seeding experiment [5,6]. The transverse intensity distribution at the waist is shown in Fig.…”

“…This beam line rotates the beam by about 11 , thus introducing coupling between x and y. The second is an astigmatism which comes directly from the source as it has been experimentally studied in [6]. Third, the near-infrared laser beam used for the production of the harmonics is also astigmatic.…”

“…The simulation results are compared with experimental data from the seeding experiment sFLASH [5]. The M 2 of the sFLASH seed [6] has been estimated using a technique based on the modal decomposition proposed in [7]. A study on the quality of the photon pulse generated by an FEL emitting self-amplified spontaneous emission (SASE) using modal decomposition can be found in [8].…”

Toward spatial and spectral control of waveguided high-harmonic generation

“…This is of importance for unrestricted access, other than in large-scale facilities. The excellent coherence in both spatial and temporal domain and the possibility of wavelength tunability [48,49] make the HHG source a promising candidate for direct seeding of XUV FELs in order to achieve fully coherent FEL output [50].…”

“…In geometries based on free propagation of the drive laser beam, i.e., in gas cells and gas jets, fluctuations in HHG have been characterized extensively, via recording the directional fluctuations [50,87], fluctuations of the pulse energy [87,88], and also spectral fluctuations [89]. In contrast to free propagation, one might expect a lower degree of output fluctuations and a higher degree of spatial coherence, when controlling the drive laser propagation through the gas sample via waveguiding.…”