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The advent of x-ray free-electron lasers (FELs) drastically enhanced the capabilities of several analytical techniques, for which the degree of transverse (spatial) coherence of the source is essential. FELs can be operated in self-amplified spontaneous emission (SASE) or seeded configurations, which rely on a qualitatively different initialization of the amplification process leading to light emission. The degree of transverse coherence of SASE and seeded FELs has been characterized in the past, both experimentally and theoretically. However, a direct experimental comparison between the two regimes in similar operating conditions is missing, as well as an accurate study of the sensitivity of transverse coherence to key working parameters. In this paper, we carry out such a comparison, focusing in particular on the evolution of coherence during the light amplification process.

We have demonstrated the generation of circularly polarized coherent light pulses at 66 nm wavelength by combining laser seeding at 263 nm of a 375 MeV relativistic electron bunch with subsequent coherent harmonic generation from an elliptical undulator of APPLE-II type. Coherent pulses at higher harmonics in linear polarization have been produced and recorded up to the sixth order (44 nm). The duration of the generated pulses depends on the temporal overlap of the initial seed laser pulse and the electron bunch and was on the order of 200 fs. Currently, this setup is the only source worldwide producing coherent fs-light pulses with variable polarization in the vacuum ultraviolet.

This paper shows how the MAX linac injector and transport system can be efficiently retuned to suit free electron laser (FEL) performance. In a collaboration between MAX-lab and BESSY, a seeded harmonic generation free electron laser is being constructed at MAX-lab. The setup uses the existing MAX-lab facility upgraded with a new low emittance photocathode gun, a Ti∶Sa 266 nm laser system used for both the gun and seeding and an FEL undulator system. To produce the high quality electron beam needed, it is shown how the magnet optics in an achromatic dogleg can be tuned to create an optimum bunch compression and how a good quality beam can be maintained through the beam transport and delivered to the FEL undulators. In extensive start-to-end simulations from the cathode of the gun to the generation of photons in the undulators, FEL performance and stability has been calculated using simulation tools like ASTRA, ELEGANT, and GENESIS. This has been done for both the third and fifth harmonic of the seed laser. The results from the calculation are 30 fs light pulses with a power of 11 MW at 88 nm and 1.4 MW at 53 nm.