We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation.

The external seed of the high-gain harmonic generation (HGHG) free-electron laser (FEL) determines the wavelength of the output radiation. Therefore, the tunability of such a laser depends upon the tunability of the seed. In this paper, we present and discuss an alternative scheme for the tunable HGHG FEL wherein the seed's wavelength is fixed and the variations in the wavelength of radiation are achieved by tuning the accelerator. As an illustration, we apply our proposed scheme to the deep ultraviolet free electron laser (DUV FEL) at Brookhaven National Laboratory demonstrating the ability to attain about a +/-10% variation in the wavelength's tuning range.

We report on an experimental investigation characterizing the output of a high-gain harmonic-generation (HGHG) free-electron laser (FEL) at saturation. A seed CO2 laser at a wavelength of 10.6 microm was used to generate amplified FEL output at 5.3 microm. Measurement of the frequency spectrum, pulse duration, and correlation length of the 5.3 microm output verified that the light is longitudinally coherent. Investigation of the electron energy distribution and output harmonic energies provides evidence for saturated HGHG FEL operation.

We present the experimental investigation of a collective effect driving strong modulation in the longitudinal phase space of a high-brightness electron beam. The measured beam energy spectrum was analyzed in order to reveal the main parameters of modulation. The experimental results were compared with a model of space-charge oscillations in the beam longitudinal phase space. The measurements and analysis allowed us to determine the range of the parameters of the observed effect on the modulation dynamics and illustrate its potential impact on short-wavelength free-electron laser performance.

Prebunching via echo-enabled harmonic generation (EEHG) is an efficient way to reduce the radiator length and improve the longitudinal coherence as well as output stability in storage-ring-based free-electron lasers. We propose a conceptual design, which uses two straight sections to seed coherent extreme-ultraviolet (EUV) and soft X-ray emission with nearly MHz repetition rate. To take the large energy spread (10−3) of a storage ring into account and utilize the existing bending magnets between the two straight sections as the first chicane, we implement a special modeling tool, named EEHG optimizer. This tool has been successfully applied to maximize the prebunching with a reasonably low energy modulation, thereby generating intense coherent X-ray pulses within a short undulator length (a few meters) limited by the available space of a storage ring. Numerical simulations confirm that the optimized EEHG parameters can be directly applied to generate a 10 MW scale peak power with fully coherent ultrafast EUV to soft X-ray pulses based on the NSLS-II parameters. This method can be easily extended to other types of diffraction-limited storage rings.