More and more high-gain SASE FELs operate at high repetition rates, either in burst or in continuous wave mode of operation, offering an unprecedented number of electron bunches per second. External seeding techniques provide high quality FEL pulses of full coherence and shot-to-shot stability but cannot keep up with MHz repetition rates of such FELs due to their dependence on the seed laser repetition rate. One attractive solution to overcome this limitation is to employ an optical cavity to store radiation that acts as a seed for the electron bunches arriving at high repetition rates. Such a scheme not only allows seeded operation at multi-MHz repetition rates but also introduces the possibility to achieve seeded radiation at shorter wavelengths, overcoming the hurdle of insufficient power availability of seed laser systems in the vacuum ultraviolet (VUV) wavelength range. Here, we present different optical-cavity-based schemes and we give an overview of their unique capabilities together with simulation results.

A tunable and multicolor light source with near Fourier-limited pulses, controlled delay, and fully coherent beam with precisely adjustable phase profiles enables state-of-the-art measurements and studies of femtosecond dynamic processes with high elemental sensitivity and contrast. The start-to-end simulations efforts aim to take advantage of the available global pool of software and past and present extensive efforts to provide realistic simulations, particularly for cases where precise and fine manipulation of the beam phase space is concerned. Since, for such cases, tracking of beams with billions of particles through magnetic structures and handover between multiple codes are required, extensive realistic studies for such cases are limited. Here we will describe a workflow that reduces the needed computational resources and share studies of the EEHG seed section for the FLASH2020+ [1] project.

The FLASH facility houses a superconducting linac powering two FEL beamlines with MHz repetition rate in 10 Hz bursts. Within the FLASH2020+ project, which is taking care of facility development, one major aspect is the transformation of one of the two FEL beam lines to deliver externally seeded fully coherent FEL pulses to photon user experiments. At the same time the second beam line will use the SASE principle to provide photon pulses of different properties to users. Since the electron beam phase space conducive for SASE or seeded operation is drastically different, here a proof-of-principle experiment using the existing experimental seeding hardware has been performed demonstrating the possibility of simultaneous operation. In this contribution we will describe the setup of the experiment and accelerator, and discuss the chances and limitations of the experimental seeding hardware. Finally, we will discuss the results and their implications also for the FLASH2020+ project.

In the framework of the FLASH2020+ project, the FLASH1 beamline will be upgraded to deliver seeded FEL pulses for users. This upgrade will be achieved by combining high gain harmonic generation and echo-enabled harmonic generation with a wide-range wavelength-tunable seed laser, to efficiently cover the 60-4 nm wavelength range. The undulator chain will also be refurbished entirely using new radiators based on the APPLE-III design, allowing for polarization control of the generated light beams. With the superconducting linac of FLASH delivering electron beams at MHz repetition rate in burst mode, laser systems are being developed to seed at full repetition rates. In the contribution, we will report about the progress of the project.

The external seeding scheme Echo-Enabled Harmonic Generation (EEHG) utilizes two modulators and two chicanes to manipulate the longitudinal phase space of an electron beam to achieve bunching at higher harmonics of the seed laser wavelength. Different combinations of energy modulation and longitudinal dispersion can result in the same amount of bunching at a certain harmonic. This study investigates the impact of the choice of the energy modulation amplitudes on the bunching properties and the spectro-temporal characteristics of the free-electron laser (FEL) radiation. Finally, a comparison between EEHG and the single modulator-chicane seeding scheme High-Gain Harmonic Generation (HGHG) at the 15th harmonic of the seed laser wavelength is presented. The corresponding numerical modelling and simulations are performed within the parameter range of the future upgrade of the FEL user facility FLASH at DESY.

The external seeding technique Echo-Enabled Harmonic Generation (EEHG) consists of two undulators which are used to imprint energy modulations to an electron bunch via interaction with a seed laser. Each of these so-called modulators is followed by a chicane introducing longitudinal dispersion. Proper adjustment of the amplitudes of the energy modulations and dispersive strengths allows to achieve bunching at high harmonics of the seed laser wavelength. In the near future, this seeding scheme will be utilized in one of the beamlines of the free-electron laser (FEL) user facility FLASH at DESY to provide stable seeded radiation down to the soft X-ray regime at high repetition rate. Dedicated numerical simulations are carried out within the foreseen parameter space to investigate how variations of the energy modulations due to power fluctuations of the two seed lasers affect the bunching properties and the stability of the generated FEL radiation.