Upon shifting of the seed laser timing the output radiation properties in the two cases could be compared. Among others, this time two electron bunches were distributed to the FLASH1 beamline - mimicking the shortest possible bunch-train. While both seeded operation modes provide clean and reproducible spectra in contrast to SASE, the benefit of smallest spectral bandwidth and highest spectral intensity for EEHG is evident.Ī week after the first demonstration the Xseed team was able to timely re-establish EEHG seeding conditions after accelerator maintenance in 4 hours.
Here the difference between EEHG and the two operation modes High-Gain Harmonic Generation, overlapping a seed laser with the electron beam in just a single stage, are especially highlighted. A few selected results taken with the 12th harmonic at 22.1 nm are displayed in Figure 1. Compared to standard SASE operation, the spectral quality and the longitudinal coherence are drastically improved and will ultimately allow for the next generation of user experiments.ĮEHG is a seeding technique based on the interaction of the electron beam with two lasers to generate fully coherent and stable FEL pulses at harmonic wavelengths of the 266 nm seed laser (tripled Ti:Sa). This is a major step towards FLASH2020+ external seeding, achieved by a major effort of the Xseed team with the support of many FLASH experts. In addition the transverse mode of FLASH2 lasing in SASE mode is shown in the bottom centre is depicted.Įxternal seeding via Echo-Enabled Harmonic Generation (EEHG) has been established for the first time at FLASH. Also the plots on the righthand side demonstrate that upon variation of the strength of the second (bunching) chicane EEHG has an increased wavelength stability (right centre) and rather broad peak ins spectral intensity (right top). Evident are for example the reduced spectral bandwidth (left bottom) and high spectral reproducibility (left centre). Figure 1: The graphs on the lefthand side illustrates the spectral properties of EEHG and HGHG at 22.1 nm, the 12th harmonic of the seed lasers fundamental of 266 nm.
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