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Description
FLASH is an XUV and soft X-ray free-electron laser user facility at DESY. In the FLASH2 main undulator beamline radiation down to 3.2 nm wavelength in the fundamental has been demonstrated. In order to efficiently generate soft X-ray radiation at the third harmonic of the FLASH2 main undulator wavelength with tunable polarization, an additional APPLE-III type undulator has been installed downstream of the FLASH2 SASE undulator section. This so-called afterburner allows to reach wavelength down to 1.33 nm with selectable polarization. The afterburner undulator also serves as a full-scale prototype for the FLASH1 seeding radiators to be installed in 2025. This paper describes the successful commissioning with beam, the demonstration of the amplification of the third harmonic and the generation of different polarizations and operation of the afterburner undulator.
